investigation of a measurement technique to determine led

HSEHealth & Safety

Executive

Investigation of a measurement technique to determinethe apparent source size for light emitting diodes

Prepared by National Physical Laboratory and Europtics Ltdfor the Health and Safety Executive 2005

RESEARCH REPORT 345

HSEHealth & Safety

Executive

Investigation of a measurement technique to determinethe apparent source size for light emitting diodes

Simon HallLaura CraneDavid Gibbs

National Physical LaboratoryHampton Road

TeddingtonMiddlesexTW11 0LW

Brooke WardEuroptics Ltd

Current ocular safety standards associated with the application of light emitting diodes (LED), andother intermediate sources, cite the angular subtense of the apparent source as an essential quantityfor optical hazard assessment. Under these standards, the angular subtense parameter is calculatedfrom the apparent source size of the LED package and the specified most hazardous viewing distance.However, an international standard for the measurement of the apparent source size parameter doesnot yet exist.

This report describes the results of a study that provide rigorous practical support for a techniqueproposed for the measurement of apparent source size when observed from the most hazardousviewing distance. The results of this study allow, for the first time, an estimate of the potential opticalhazard of LEDs and other intermediate sources, in accordance with current safety standards. This is asignificant step in reducing the ambiguity that currently exists in the application of these optical safetystandards. The results also verify earlier numerical modelling of an improved method for the estimationof the critical angular subtense parameter for extended sources, such as LEDs and intermediatesources.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Itscontents, including any opinions and/or conclusions expressed, are those of the authors alone and donot necessarily reflect HSE policy.

HSE BOOKS

ii

© Crown copyright 2005

First published 2005

ISBN 0 7176 6108 3

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

Applications for reproduction should be made in writing to:Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected]

ACKNOWLEDGEMENTS

We would like to acknowledge the software development expertise provided by Oxford

Framestore Applications Ltd.

iii

CONTENTS

Executive Summary......................................................................................................... v ii

1 Introduction .............................................................................................................. 1

2 Theory....................................................................................................................... 2

2.1 Angular subtense .............................................................................................. 2

2.2 Beam measurements ......................................................................................... 5

2.3 The optical system............................................................................................ 6

3 Description of Apparatus.......................................................................................... 7

3.1 Initial System Design ....................................................................................... 7

3.2 8-Bit System Design......................................................................................... 7

3.3 12-Bit System Design....................................................................................... 7

4 Measurement Procedure ......................................................................................... 11

4.1 Preparation for measurement.......................................................................... 11

4.2 Calibration of CCD array and associated equipment ..................................... 11

4.3 LED beam width measurement ...................................................................... 11

4.4 Transform Validation Experiment.................................................................. 14

5 Results .................................................................................................................... 15

5.1 Initial results ................................................................................................... 15

5.2 8-Bit Transform Validation Experiment results ............................................. 17

5.3 12-Bit Transform Validation Experiment results ........................................... 20

5.4 Yellow LED - Ligitek LUY 3833/A29 .......................................................... 25

5.5 Blue LED - Nichia NSPB500 Rank WS ........................................................ 27

5.6 Green LED - Nichia NSPG500 Rank GS....................................................... 29

5.7 Red LED - Kingbright L-53SRC/E ................................................................ 31

5.8 White LED - Nichia NSPW500 Rank BS ...................................................... 33

5.9 Orange LED - Toshiba TLOH190P................................................................ 35

5.10 High Power Blue LED - Luxeon Star............................................................. 37

6 Uncertainty Analysis .............................................................................................. 39

7 Conclusions ............................................................................................................ 46

7.1 Future directions............................................................................................. 50

Appendix 1: Second moment, azimuth and principle width derivation ......................... 51

Appendix 2: Design And Technical Specification For A Facility To Determine The

Apparent Source Size Of Light Emitting Diodes ........................................................... 53

Appendix 3: LED techinical data sheets......................................................................... 58

References ...................................................................................................................... 59

Glossary.......................................................................................................................... 61

v

EXECUTIVE SUMMARY

The work detailed in this report was commissioned to allow the optical hazard level of light

emitting diodes (LEDs), and more laser-like intermediate sources, to be quantified. The

dramatic increase in the use of superbright LEDs for consumer, medical and industrial

applications necessitates a responsible assessment of the hazard presented by these devices.

The International Electrotechnical Committee (IEC) and Commission Internationale de

l'Eclairage (CIE) cite that the angular subtense of the apparent source is an essential quantity for

the assessment of optical hazard. Under current optical hazard safety standards the angular

subtense parameter is calculated from the apparent source size and a specified most hazardous

viewing distance. However an international standard for the measurement of the apparent

source size parameter does not exist.

The aim of this current study is to provide rigorous practical support for a technique proposed

for the measurement of apparent source size when observed from the most hazardous viewing

distance. Development of the practical technique required the recognition of the apparatus

limitations and the development of strategies to overcome these limiting factors. Both an 8-bit

and 12-bit system were tested. The 12-bit systems’ superior dynamic range and cooled array

highlighted the effect of stray light and noise. This demonstrated the need for a large dynamic

range in the measurement facility to measure second moment beam diameters effectively.

A validation experiment suggested by the International Standards Organisation (ISO/TC 172/SC

9) comprehensively verified the suitability of the technique. It is therefore proposed that the

results of this work should be used to underpin the adoption of this methodology within

international standards for the assessment of the optical hazard potential of LEDs and other

intermediate sources.

The report highlights the following:

Technical specification of the critical components and the design of a facility for the

measurement of apparent source size of LEDs and intermediate sources.

Verification of an 8-bit and a 12-bit apparent source size measurement facility. This was

achieved by computerized processing of images of spatial beam profile using a

converging second moment method.

High level of agreement between the propagation parameters derived through the 8-bit

and 12-bit methods using the IR LED. This was an unexpectedly good correlation

between results, considering the dynamic range limitations of the 8-bit camera.

Evaluation of the astigmatic state of the beam by analysis of the change of azimuth as

the beam propagates. This was carried out by azimuth determination of the beam by the

comparison of the second moment widths in perpendicular axes.

Measurement of a selection of 8 LEDs with differing peak emission wavelengths,

construction and beam propagation characteristics.

Visualisation of real beam propagation using a montage of beam images and spatial

profiles related to the propagation envelope for one of the LEDs.

Effective demonstration that the point in the beam envelope where a sharp image of the

electronic structure of the LED is obtained does not necessarily correspond to the beam

waist or location of the apparent source.

vii

Identification of general astigmatism (as opposed to simple astigmatism) in the output

beam from one of the LEDs.

Populated angular subtense contour plot with results from this work. This plot enables

the easy estimation of the angular subtense of real LEDs and intermediate sources from

the measured beam propagation characteristics.

Verification of the technique using a test suggested by the International Standards

Organisation (ISO/TC 172/SC 9) identifying that this method can be applied

successfully to the analysis of beam propagation parameters and hence the apparent

source size determination for stigmatic and simple astigmatic beams from LEDs.

Development of this technique would allow the assessment of generally astigmatic

beams in line with ISO 11146-2 ‘Lasers and laser-related equipment. Test methods for

laser beam widths, divergence angle and beam propagation ratio. Part 2: General

astigmatic beams’.

The results of this study allow, for the first time, the effective characterisation of the optical

hazard of LEDs and other intermediate sources, in accordance with the IEC and CIE standards.

This is a significant step in reducing the ambiguity that currently exists in the application

of these optical safety standards. The results also verify earlier numerical modelling of an

improved method for the estimation of the critical angular subtense parameter for extended

sources, such as LEDs and intermediate sources.

viii

1 INTRODUCTION

The assessment of the optical hazard associated with beams from sources of light intermediate

in quality between a laser and light emitting diodes (LED)1 has been a challenging problem for

the international standards community for a large number of years.

This report has been produced to contribute to the international debate regarding the optical

hazard due to LEDs. The current requirements for the classification of LEDs follows IEC 608253 and requires a measurement of “apparent source size and its location”. The CIE publication

CIE S 009/E:2002 “Photobiological Safety of Lamps and Lamp Systems” cites apparent source

size as part of the methodology to calculate angular subtense and hence Retinal Hazard.

However a procedure for establishing apparent source size and location is not described.

The apparent source size of an LED is a critical parameter used in the assessment of the ocular

viewing hazard of these devices under ISO 60825-1 ‘Safety of laser products. Equipment

classification, requirements and user’s guide’. Under the committee draft IEC 60825-13

‘Measurements for the classification of laser products’ a proposed measurement method is

described to determine the apparent source size of LEDs. The validity of this method has been

questioned at a national and international level and continues to be debated within the various

standards bodies such as IEC, ISO and CIE. Specifically, the applicability of propagation

models to low divergence beams from LEDs has been challenged.

Previously the validity of these models has not been demonstrated through physical

measurement of LED devices. This project aimed to resolve this situation through the

construction of a suitable measurement facility and by performing an assessment of a range of

commercially available LED sources.

1

2 THEORY

Figure 1 is a schematic diagram of the proposed measurement method for the determination of

the apparent source size and beam characteristics of LEDs. A CCD diode array camera system

is placed on a movable carriage in front of the LED source. The relay lens of the camera system

allows the CCD to capture a spatial intensity profile of the beam at a particular plane. The beam

width is then calculated using a modified second moment technique. It is necessary to ensure

that enough of the beam power has been captured to allow an accurate determination of the

beam width. To address this problem a self-converging width measurement technique is used to

estimate the beam width at each measurement plane and represent the true value to an

acceptable level of uncertainty. This measurement is repeated at a number of locations along the

test beam axis until sufficient data points have been obtained to allow the fitting of a maximum

likelihood hyperbola using a least squares fitting technique. The coefficients of the fitted

hyperbola allow the derivation of the beam propagation parameters of the source.

LEDCCD

A

A’

u v

B’

B

do

Figure 1 Proposed methodology to determine apparent source size of LEDs

AA’ – plane of beam waist

BB’ – plane of transformed beam waist

u - distance from beam waist to lens

v – distance from lens to ransformed beam waistt

do – beam waist diameter

If the beam waist is not accessible for direct measurement then using an aberration-free

focussing system, or transform lens can create an artificial waist. This may be necessary if, for

example, the beam waist is formed within the LED package or there is insufficient space to

perform the required number of measurements either side of the waist. The position and

diameter of this artificial waist can then be used, along with the known properties of the

transform lens, to calculate the location and size of the original beam waist. The equations used

to calculate the location and size of the original beam waist using this procedure are given in

Section 6 as part of the uncertainty derivation process.

2.1 ANGULAR SUBTENSE

The angular subtense of an apparent source of radiation in the 400 nm to 1400 nm wavelength

range is required by current laser safety standards 3 to permit calculation of the relaxation factor

C6, for thermal retinal damage from extended sources. It is the ratio of the angular subtense of

the source in question to that of a source that would form the realistic minimum spot size on the

retina (1.5 mrad). Classification or assessment of the thermal hazard from a source requires that

both the angular subtense (see Figure 2) and location of an extended source be known before

there can be a relaxation of the maximum permissible exposure (MPE). The location of the

source is required so that the angular subtense can be calculated for viewing this from the

2

minimum conceivable eye accommodation distance of 100 mm (in IEC standards) 3. It should be

noted that this latter assumption may not describe the full range of potential hazards. It is

possible that some large divergence sources, when held closer than 100 mm from the eye, might

produce a significant thermal hazard in a blurred retinal spot even though the eye cannot

achieve a sharp focus.

Optical

Source

Image of

Optical

Source

Angular

SubtenseEye

Figure 2 Classical representation of Angular Subtense

It is a simple matter to measure the physical size of the chip of a LED that has a Lambertian

radiation pattern but it is more difficult to know or measure the location or size of the apparent

source with low divergence beams from a LED. Such beams can have a near planar wavefront,

which would imply that the apparent source is located at infinity with an unknown angular

subtense. However, recent advances in the characterization of optical beams, both coherent and

incoherent, enable prediction of their propagation envelopes 2,16,17. It is now possible to assess

the intrabeam-viewing hazard by using known beam characteristics to estimate the angular

subtense of an extended source that would present the greatest hazard to a retina 3.

The level of the thermal hazard to the retina is defined here as the power or energy per

millimeter of beam diameter falling on the retina 19. The process of calculation of the size of the

beam formed on a retina and the fraction of incident power passing through the pupil has been

performed for a wide range of feasible conditions. The calculations assume that the beam has a

divergence of less than 30° and has a power density profile that produces the greatest peak

irradiance on the retina (i.e. a Gaussian profile).

Measurements of the enclosed power envelope of beams from lasers have confirmed that they

propagate with a hyperbolic profile, the constants of which are modified when passing through a

lens. The new constants can be used to estimate the location of the waist of the new hyperbola

and its Gaussian beam diameter as a function of propagation distance. In this way it is possible

to determine the spot size on the retina formed by a beam after passing through the lens of the

eye.

d01 – beam waist diameter of input beam

L1 – waist to lens distance

Zr1 – Rayleigh length of input beam

fe – focal length of lens

L2 – lens to transformed waist distance

Zr1 – Rayleigh length of output beam

d02 – beam waist diameter of output beam

dr – beam diameter on retina

Lr –transformed waist to retina distance

Figure 3 Calculation of spot size (dr) on the retina of the eye.

3

For a given set of beam propagation constants (waist diameter and divergence say) it is possible

to predict the hazard level (P/d) at the retina. The hazard level results from calculations of the

fraction of beam power that passes through the 7mm iris of the eye as a function of both the

strength of the eye lens (assumed to vary anywhere between 14.5 mm and 17 mm) and the

distance of the incident beam waist from the eye. The maximum hazard occurs when the eye

accommodates itself at the most hazardous viewing distance. In the interests of simplicity, IEC

60825-1 assumes that this most hazardous viewing distance is 100 mm but this is not always

found to be the case.

Previous calculations (numerically verified by workers in Austria and the UK) have

concentrated on determining the spot size on the retina at the most hazardous viewing condition

as a function of the two beam propagation parameters, beam waist diameter and far-field

divergence. Knowing the spot size at the retina and by assuming the eye to be 17 mm "long",

the artifact of the angular subtense of the apparent source has been estimated over the most

relevant range of incident beam parameters. The values of angular subtense can be displayed as

contours in the two-dimensional map of waist diameter and divergence. Further calculations

based on the measured values of these parameters will also reveal the location of the apparent

source. If the Rayleigh length of the beam is significantly less than 50 mm then the source can

be assumed to coincide with the measured beam waist location.

While some rather extreme conditions have been assumed when modeling the beam (e.g. a

Gaussian beam profile), the procedure for estimating angular subtense from beam parameter

measurements is thought to offer an unambiguous and non-subjective result. While the

procedure may over-estimate the hazard level it can permit a greater relaxation of the MPE level

than simply assuming that C6=1.

A CCmap showing the range of angular subtense values as contours against the Beam waist

width and the beam divergence was produced from these calculations (Fig 4)2.

Contour ofequal angularsubtense in mrad

Beam waistdiameter and divergence of LED

Angular subtense,

, of LED

Figure 4 Theoretical plot of beam waist diameter (width) vs. beam divergenceshowing contours of angular subtense

4

The contours show equal values of angular subtense in milliradian. To use the contour plot, the

he objective of this investigation was to demonstrate that it is possible to determine the

.2 BEAM MEASUREMENTS

easurement of the optical constants of the propagation envelope of a beam has been the

here are a number of methods available for measurement of the diameter of a beam as well as

o accurately measure the second moment beam diameter both the number of pixels and the

he methods leading to estimates of the diameter of a beam use a procedure known as the

waist diameter and the divergence of the LED beam are measured. The results are plotted on the

graph and the value of the angular subtense, , is then read from the contour just below the

measured point.

T

propagation characteristics of the beam produced by a LED. This information could then be

used to estimate the size of the image formed on the retina and from this the angular subtense of

the apparent source at the eye at a given distance. These results then enable the population of a

theoretical contour map of the computed angular subtense as a function of the measured beam

characteristics of LEDs. The angular subtense for all beam types can then be determined by

measuring the beam waist diameter and the divergence.

2

M

subject of considerable research over the last ten years. A consequence of this work is the

evolution of ISO standards for the measurement of the diameter and divergence of a beam. ISO

11146:1999. “Test methods for laser beam parameters: beam widths, divergence angle and

beam propagation factor” 6 is the current draft standard being reviewed by ISO. The procedures

and techniques that are described here for the determination of the diameter and location of the

apparent source of a beam are based on the principles underlying the ISO standards7 for

stigmatic and simple astigmatic beams. The proposed methods are applicable to beams whose

full divergence angle is less that 30°. Relaxation of the laser safety criteria should not be applied

to a beam displaying general astigmatism.

T

its far-field divergence. The basic principles for those methods have been established in an ISO

standard. They are applicable to laser beams with a relatively small beam propagation ratio, M2.

Recent research has demonstrated that adequate steps have to be taken to counter the effects of

noise and offset errors when measuring the transverse irradiance distribution of a beam. When

these steps are taken, the propagation behaviour of incoherent broadband beams as well as high-

quality laser beams can be predicted reproducibly with considerable precision.

T

level of digitisation of the signal received on each pixel has to be considered. For beams with a

rapidly changing beam diameter the number of bits in the digitisation process becomes more

critical. Noise on the image acquired by the camera both from electrical and optical sources

must be removed by setting a discrimination level. This effectively reduces the dynamic range

of the camera and this favours cameras with an inherently large dynamic range due to a larger

number of bits available on the digitisation electronics.

T

Converging Second Moment diameter or width measurement (CSM). The schematic of this

method is shown in Figure 5. These methods are being defined in the revision of ISO 11146 that

is currently in preparation.

5

Figure 5 Schematic of converging second moment iteration

The preferred method for measuring all the propagation characteristics of a beam is to perform

CSM diameter measurements at a number of locations either side of the beam waist. The

calculation of second moment width is described in Appendix 1.

2.3 THE OPTICAL SYSTEM

The beam measurement process consists of using a CCD sensor to image the irradiance profile

at a minimum of ten measurement locations either side of the beam waist. The proposed optical

system contains variable magnifying optics that are designed to facilitate imaging the transverse

irradiance profiles to occupy approximately one quarter of the sensor screen height. Other

components are included in the system to attenuate the beam power to avoid sensor saturation

and to provide spatial calibration of the pixel array of the sensor.

6

3 DESCRIPTION OF APPARATUS

3.1 INITIAL SYSTEM DESIGN

An initial specification of the 12-bit measurement system was written and can been found in

Appendix 2. This specification details the required elements to measure apparent source size of

LEDs.

3.2 8-BIT SYSTEM DESIGN

Both an 8-bit and 12-bit camera systems were used for measurement. The final system design

for the 8-bit system was identical to that described in Section 3.3, except for the camera and

zoom lens. The details of the 8-bit camera are given in Section 3.2.1. The details of the

associated zoom lens are presented in Section 3.2.2.

3.2.1 8-Bit Camera System

The 8-bit system consisted of a analogue CCD interline transfer camera connected to an 8 bit

frame grabber card. A Leica Monozoom optic was used to adjust the size of the image of the

propagating beam from the test LED. An 8-bit system would imply a digitised dynamic range of

28 =256 bits. This takes no account of noise or camera processing. The dynamic range in these

measurements was assumed at the start to be one of the greatest limiting factors of the

measurement. This assumption was later shown to be true by adjusting discrimination levels and

plotting the effect against the measured second moment values for identical camera frames.

3.2.2 Leica Monozoom 7

The camera zoom lens used for the 8-bit system was a 1:7 par-focal microscope zoom. During

zooming the focus could be maintained, whilst providing a wide field of view and a long

working distance. The zoom did not include an integral iris and the zoom setting could not be

locked. The latter meant that special care was required to ensure that the zoom was not

disturbed during measurements, otherwise the dimensional calibration would be lost. The

shortcomings of this zoom prompted the acquisition of a higher specification zoom system to

form part of the 12-bit set-up.

3.3 12-BIT SYSTEM DESIGN

3.3.1 Electrical Measurements

The LED sources were operated at a constant current using a power supply stabilised to 0.02%.

Setting a constant voltage is also possible, although this is more likely to be affected by

differences in contact potential. To measure the current to the LED, a standard resistor was

placed in series with the power supply and the LED source. The potential across the standard

resistor was measured using a calibrated digital voltmeter. Using this value, the current to the

LED was calculated and recorded. This ensured that the same electrical conditions were used

for each LED measurement.

7

3.3.2 Transform Lens

Following a survey of commercially available products, it was identified that a single large

diameter achromatic lens of sufficient power and quality for the measurements was not

available. Two high quality achromats were combined to provide an equivalent effect. The large

diameter was required to provide effective coupling of the LED output to the camera input to

reduce vignetting. Optics of large diameter also allows the inner portion of the lens to be used

which introduces less aberration to the measurement process. It is critical that the geometry of

the lens is known accurately, so that the lens transformation properties can be calculated (see

Figure 6 and Figure 7).

y 2) shoul

uality lens mounts with yaw and tilt adjustments were purchased to allow uniaxial alignment of

e measurement system.

The distance between the two lenses and the distance from the LED required calculation to

ensure that the beam would not overfill the aperture of the camera zoom. In addition, the

transformed waist diameter must not be too small as to cause measurement problems due to the

camera resolution. Additionally, the Raleigh length (distance for the beam diameter to increase

b d be long enough to allow accurate distance measurement to be carried out. High

q

th

Fi ram of achromat showing critical measurements needed to allow

Figure 7 Scale drawing of the two transform achromats showing some of

gure 6 Schematic diagthe lens transformation properties to be calculated (all dimensions in mm).

A description of the parameters used can be found in the glossary.

the calculated measurement distances, definitions of parameters are given in section 6 (all dimensions in mm)

8

3.3.3 Optical Rail

2-metre cast iron optical rail was used as the primary bench for the mounting of the optical

.3.4 LED Mount

stable LED clamp which could in turn be mounted on a 3 axis gimbal mount with height and

.3.5 Beam Attenuation

eutral Density (ND) optical filters were used to attenuate the light input to the camera. Critical

.3.6 Rotating diffuser

was found to be necessary to use a rotating frosted scatter screen to allow visualisation of the

.3.7 Graticule

photoetched transparent graticule with traceable calibration was used to calibrate the imaging

.3.8 12-Bit Camera System

he 12-bit system utilized a superior zoom system that had a larger input optic and a greater

alculations were performed to ensure that the LED and lens(es) were located so that the beam

A

components. A second, machined, aluminium rail was used to mount the LED and the achromat

lenses. This secondary rail was mounted on a roller carriage on the primary bench. All carriages

and benches carried vernier scales to ensure accurate measurement readings.

3

A

transverse adjustment was required. A commercially available solution was unavailable so a

clamp was designed and produced by the NPL workshop. This was mounted on a high stability

goniometric mount with height and transverse adjustment provided by two other stages. This

provided a low vibration mount with high resolution and repeatable displacement.

3

N

attributes were spectral neutrality and spatial uniformity. Tests were made on NPL’s primary

Zygo Fizeau interferometer to inform the purchase of a high quality set of filters with low

wavefront aberration. The filters were placed in a mount that allowed stacking of filters with an

adjustment for variable tilt to reduce inter-reflection. The proposed initial system did not include

a scatter screen and thus the use of an iris with the zoom system would have caused vignetting.

Later adoption of the scatter screen allowed this option.

3

It

beam profile at the focal point of the zoom system. Measurements made without this system

caused vignetting problems. The rotating diffuser had the added advantage that it allowed the

use of the integral iris in the zoom lens to attenuate the LED light. Several measurements were

made to ensure that the real beam diameter was not greater than the diameter obtained by the

use of the screen.

3

A

system (zoom in combination with the CCD array and analysis software).

3

T

magnification range. The CCD detector used produced a digital 12 bit output and incorporated a

two stage peltier cooler to both reduce the temperature of the array and the level of noise

acquired. The dynamic range of the system was 212=4096 bits, the software used a

“discriminator level” which allowed the baseline for detection to be raised above the ambient

noise level. The zoom system incorporated an iris which allowed the light levels to be reduced

within the range afforded by the detector integration time adjustment.

C

could converge to a waist and re-expand within the travel range of the optical bench system. At

the same time the anticipated diameter of the beam at the transforming lens(es) was examined to

9

ensure that the beam size was not large enough to introduce significant aberration or vignetting

effects.

3.3.9 Leica Z16 Zoom

wo Leica zoom microscope systems were assessed, the Z6 and Z16 models. The Z16 was

.3.10 Final 12-bit System Design

igure 8 shows the final components used in the 12-bit system for the measurement of angular

T

found to have a greater focal range and would allow the measurement of a greater range of LED

types. The Z16 is an apochromatic zoom system with central beam path. A planapochromatic

0.5X objective was used and the zoom range was 0.57× – 9.2×. This high quality optic has a

similar field of view to the Monozoom 7 but has significantly lower aberrations. The inbuilt iris

can also be employed to attenuate light levels when used in conjunction with a scatter screen.

The lockable zoom setting allows calibration at a particular fixed zoom level.

3

F

subtense.

Secondary

optical benchPrimary optical

bench

Achromats Rotating

diffuserCCD cameraND Filter and

Filter holder

Planofocal

Zoom Lens

LED

Secondary


bench

Achromats Rotating


Filter holder

Planofocal

Zoom Lens

LED

Figure 8 Final optical arrangement of the 12-bit system for the measurement of apparent source size

10

4 MEASUREMENT PROCEDURE

4.1 PREPARATION FOR MEASUREMENT

The optical arrangement for the measurement of apparent source size is detailed in the

schematic diagram, Figure 8. Prior to measurement the components must be carefully aligned to

ensure that the LED beam is parallel to the optical axis of the primary and secondary bench.

The following points detail the steps required to align the components used with the optical axis

of the bench.

a) Establish a reference He-Ne beam parallel to the optical bench by the use of at least two

movable irises or apertures.

b) Align the centre of the Zoom lens with the HeNe beam and use the imaging software to

ensure that the suitably attenuated beam is in the centre of the CCD array field of view.

c) Introduce the transform achromats one at a time and centre them on the beam. Ensuring

that the emerging beam is still creating a centred image in the camera.

d) Place the viewing screen at the focal point of the zoom lens by utilizing a reference grid

that can be resolved by the imaging software and can be coincident with the frosted side

of the screen.

e) A microscope, focused on the optical axis of the system, is used to locate and record the

positions of the components along the optical bench. This facility is used to set

accurately the appropriate distances between the LED and the achromat(s).

4.2 CALIBRATION OF CCD ARRAY AND ASSOCIATED EQUIPMENT

a) A reference grid or graticule is inserted in place of the frosted screen with the reference

grid plane coincident with the plane of the frosting, as determined using a telescope. A

CCD frame of the reference grid is recorded and analysed by the software to derive the

calibration factor (pixels/mm) to be used to convert subsequent beam pixel

measurements into linear dimensions.

b) All equipment used to measure the electrical characteristics of the LED were calibrated

and traceable to national standards, as is essential for such a system.

4.3 LED BEAM WIDTH MEASUREMENT

Once the system is aligned and the calibration procedures performed, the following steps are

required to predict the position of the beam waist from the vertex of LED.

a) A combination of ND filters and the iris of the Zoom lens are used to attenuate the

beam irradiance so that the full dynamic range of the CCD system is used. This is done

by locating the position of maximum irradiance, then placing filters in the beam path so

that the signal is just about saturating the CCD pixels. As readings are taken either side

11

of the maximum, the iris of the zoom lens and the exposure time of the camera can be

adjusted to maintain the signal level at the full dynamic range of camera. The Zoom is

also set so that approximately a quarter of the CCD field of view appears to be filled by

the largest diameter that is to be measured;

b) The image acquisition software is used to capture at least 10 equidistant beam images

either side of the beam waist. Each image has an associated image of background

optical noise captured at the same time by blanking out the LED with a black felted

beam stop;

c) The background frame is subtracted from the beam image frame before the digital width

analysis process is performed;

d) The corrected image is processed using the convergent second moment (CSM) method

to limit the dimensions of the CCD window that is subsequently analysed and hence

reduce noise contribution to the second moment evaluation. The CSM values of the

beam in the laboratory (CCD array) vertical and horizontal axes are calculated. A cross-

moment of the beam distribution in the converged window is used to calculate the

azimuth of the principal axes of potentially non-circular distributions. This figure

enables the calculation of the dimensions of the beam along its principal axes. The ratio

of the principal dimensions (ellipticity), the azimuth angle of the principal axes relative

to the laboratory axes; and the calibrated linear magnitude of the principal dimensions

are recorded. The convergence of the 2nd moment calculations can be seen in Figure 9

The program then outputs the final 2nd moment measurements in the X and Y axes;

33.7133.7133.71

Figure 9 CSM software illustrating the calculation of the second moment values.

e) A least-squares (maximum probability) process is used to discover the best fitting

hyperbolic envelope to the propagating beam in each of its principal planes. The

coefficients of the hyperbolas are processed to reveal: the locations of the beam waists

relative to the vertex of the LED; the transverse dimensions of the waists; the values of

the Rayleigh Lengths of the beam along their principal planes; and the far-field

divergences in those planes;

12

f) If the beam is found to be astigmatic (i.e. the ellipticity of the beam is found to be

greater than 1.15 or less than 0.83) and there is a monotonic variation in the azimuth of

the principal planes of the propagating beam (twist) then the beam is deemed to suffer

from general astigmatism and no further investigation or relief of the thermal hazard

factor C6 can be justified without a more detailed analysis procedure;

g) If the beam is identified as stigmatic or simple astigmatic the determined values of the

beam waist widths and far-field divergences can be placed on the angular subtense

contour map (Figure 4) and the contour below the lowest uncertainty ellipse can be used

to identify the angular subtense to be used to determine the appropriate value of the

thermal hazard relaxation factor C6.;

h) If the Rayleigh Length in the least divergent principal plane is less than 50 mm then the

location of the apparent source can be regarded as the location of the beam waist in that

plane. If the Rayleigh Length is greater than 50 mm then the possible error in hazard

assessment can be greater than 5% and the location of the centre of curvature of the

wavefront arriving at the most hazardous viewing distance should be used to identify

location of the apparent source.

Figure 10 shows the required optical elements of the system to measure the angular subtense of

an LED.

Enclosure to reduce

scattered light

Cooled CCD

Camera

Zoom lens

ND filters

LED in

goniometric mount

Transform achromats

Rotating diffuser

Figure 10 Experimental apparatus

13

4.4 TRANSFORM VALIDATION EXPERIMENT

To validate the suitability of the proposed measurement method for determining the beam

propagation parameters a Transform Validation Experiment can be undertaken. This technique

uses the beam propagation parameters to predict the size of a beam waist produced when a

known lens is inserted into the beam. This prediction is then verified by using the measurement

technique to measure the true diameter of the new beam waist with the lens inserted. The aim is

to achieve 10% (1 sigma) agreement between the predicted value and the measured diameter of

the beam waist. The transform validation experiment is shown in Figure 10 and schematically in

Figure 11. The points below detail the steps required in the transform validation method.

Measure the transformed waist and estimate original waist

Use the estimated waist to predict new waist formed by inserted lens

Measure and estimate new waist for comparison with step 2. If the estimate and the

prediction agree sufficiently well the validation is complete

LEDCCD

LED

LEDCCD

Estimate

Predict

Compare

Step 1

Step 2

Step 3

Measure

Measure

The results of the Transform Validation Experiment are presented in Section 5.2.

Figure 11: The three steps of the primary ISO Transform Validation Experiment 6

14

5 RESULTS

5.1 INITIAL RESULTS

The ideal methodology to measure the LED beam would be through direct imaging. Some

difficulties were encountered due to vignetting of the beam by the zoom lens. This effect can be

seen in the asymmetry of the hyperbolic plot produced from the second moment analysis, Figure

12.

IR LED measurement showing Vignetting effect

y = 0.0003x2 - 0.0142x + 3.4009

R2 = 0.9827

y = 0.0003x2 - 0.0146x + 3.297

R2 = 0.9854

2.500

3.000

3.500

4.000

4.500

5.000

-60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0 100.0

Distance from Beam Waist

2n

d m

om

en

t B

eam

rad

ius

Horizontal width (mm)

Vertical width (mm)

Poly. (Vertical width (mm))

Poly. (Horizontal width (mm))

Figure 12 Skewed fit of second moment values obtained showing vignetting effect ofzoom aperture

Noise effects from the intereflections between the filters used to attenuate the light from the

LED were found to be a particular problem. The differences in measured second moment

diameter caused by different filter combinations can be seen on Figure 13. The stray light noise

levels on the camera were very high and a discriminator level of 50 was required to produce the

analysis.

For the final measurements the procedure was adapted to only utilise the minimum number of

filter elements by manually finding the camera position that resulted in the greatest local

irradiance. The integration time of the camera and/or the iris in the zoom lens were then reduced

as much as possible to reduce the signal output from the camera pixels to a point where a ND

filter would reduce the signal levels to just below saturation. This was to ensure that the greatest

dynamic range of measurement was employed.

15

IRED through 2 lens transform - X-axis (Aug 26 Disc 50)

8.0

10.0

12.0

14.0

16.0

18.0

20.0

280 285 290 295 300 305 310

Distance past vertex (mm)

Co

nverg

ed

2M

beam

wid

th (

mm

)

LSq Fit Hyperbola

4 f ilter set A

2 f ilter set B

1 f ilter C

1 f ilter D

4 f ilter set E

3 f ilter set E

2 f ilter set F

Figure 13 IRED Led measurements demonstrating filter effects

Initial measurement work concentrated upon the confirmation of earlier work using an Osram

IR LED 2. Details of this LED can be found in Appendix 3. Early evaluations were pursued with

a 50 mm focal length singlet lens to examine the field of view required for the experiment.

16

5.2 8-BIT TRANSFORM VALIDATION EXPERIMENT RESULTS

5.2.1 Direct Measurements of IR LED

The 8-bit system using the Cohu CCD camera and Leica monzoom 7 lens was used for the

initial work to confirm the method for measurement of angular subtense. Figure 14 details the

required apparatus to measure the beam waist of an LED directly. Measurements of the beam

width were made using the procedure in Section 4.3.

Secondary


bench

Rotating


Filter holder

Planofocal

Zoom Lens

LED

Secondary


bench

Rotating


Filter holder

Planofocal

Zoom Lens

LED

Figure 15 Plot of converged beam width for IRED LED

he results, shown in Figure 15 and Table 1, indicate that the IR LED produces a beam with a

waist external to the LED. The M2 value is very large but the beam still fits the hyperbolic

Figure 14 Optical set-up for direct apparent source size measurement

SFH 400 IRED (950 nm) @ 50 mA Raw Beam (5 Aug)

2.5

3.5

4.5

5.5

6.5

0 5 10 15 20 25


Co

nverg

ed

2M

beam

wid

th (

mm

)

T

17

envelope well. The existence of an external beam waist allows the effective measurement of the

direct propagation envelope.

Table 1 Calculated beam parameters for Osram IRED LED

.2.2 Two Lens transform of IR LED

he secon e beam waist

iameter. This requires the use of a transfor ment (see Figure 16).

Parameter Value Units

5

T

Waist position from LED vertex zo 5.61 mm

Waist diameter Wo 3.38 mm

Rayleigh distance Zr 9.76 mm

Divergence 347 mrad

M2 969

Figure 16 Optical set-up for apparent source size measurement using two achromat lenses

Secondary


bench

Achromats Rotating


Filter holder

Planofocal

Zoom Lens

LED

Secondary


bench

Achromats Rotating


Filter holder

Planofocal

Zoom Lens

LED

d step in the validation of the measurement method is to predict th

m lens in the optical arranged

Two achromats were used to produce a beam waist that would not overfill the field of view of

the CCD. The lenses also ensured that the Rayleigh Length was sufficiently long to provide an

appropriate number of measurement planes. Figure 17 shows the results of these measurements

with a hyperbolic fit to the data points.

18

IRED 2 Lens Transform X-axis

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

220 230 240 250 260 270


CS

M w

idth

(m

m)

Figure 17 Hyperbolic fit to data from IR LED through two achromat lenses

A summary of the beam characteristics for the Osram LED is presented in Table 2. The

complete summary of the Transform Validation Experiment is located in Tables 4 and 5 of

Section 5.3 as it was thought more appropriate to put them in the context of the 12-bit system

and hence allow comparison.

Table 2 Summary of beam characteristics for IRED LED using 8-bit camera system

SFH 400 IRED using 8-bit Camera

Transformed Beam

Beam property Goodness of fit

Waist position from LED vertex Zo 254.1 mm 0.01

Waist diameter Wo 3.12 mm 0.10

Rayleigh distance Zr 8.6 mm 0.27

Divergence 361 mrad 16

M2 950 67

Original Beam from Inverse Transform

Beam property Uncertainty





M2 950 129

Uncertainties Used

Focal length etc. 0.5 %

Datum positions 0.1 mm

Width measurements 0.025 mm

Measurement locations 0.15 mm

19

5.3 12-BIT TRANSFORM VALIDATION EXPERIMENT RESULTS

The 12-bit PCO Sensicam camera with the Leica Z16 zoom lens was used for further

measurements of apparent source size on the Osram IRED LED and a range of visible LEDs.

The details of all LEDs measured can be seen in Appendix 3.

5.3.1 Low Level Noise

It was found that the discriminator level used to eliminate low level noise caused a much greater

effect upon measured beam width than might be expected. The intensity of the imaged LED was

always set as close as possible to the saturation point of the CCD to produce the greatest

dynamic range possible. With the 12-bit system, the maximal number of bits of dynamic range

would be 4096. Figure 18 shows the effect upon the second moment beam size due to change of

discrimination level. Hereafter the discrimination setting was kept at 5 bits.

IRED raw beam width vs Discriminator Level

28

33

38

43

48

0 5 10 15 20

Discriminator Level

Seco

nd

mo

men

t (X

-axis

)

Pos 1

Pos 3

Pos 6

Pos 6

Pos 10

Pos 11

Pos 12

Pos 13

Pos 14

Pos 15

Figure 18 Variation in discriminator level with beam width

5.3.2 Transform Validation Experiment results from IRED LED

The measurements performed using the 8-bit system (described in section 5.2) were repeated to

demonstrate the differences between the two systems and to validate the procedure for

measurement of angular subtense. The beam width of the LED was measured directly using the

measurement arrangement shown in Figure 14.The resulting data is plotted in Figure 19.

The next step in the Transform Validation Experiment was to measure the beam of the LED via

two achromat lenses. Several measurements were made and a hyperbolic fit was made to the

resulting data. The fit is shown in Figure 20.

20

IRED Raw - X-axis (SEP 30)

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

5 10 15 20 25


Co

nv

erg

ed

2M

be

am

wid

th (

mm

) -

2%

un

ce

rta

inty

30

Figure 19 Plot of converged beam width for IR LED

IRED 2 Lens Transform X-axis

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

220 230 240 250 260 270


CS

M w

idth

(m

m)

Figure 20 Hyperbolic fit to data through two achromats from IR LED

21

Table 3 Summary of beam characteristics for IRED LED using 12-bit camera system

SFH 400 IRED

Transformed Beam






M2 588 43







M2 588 78

Uncertainties Used




Measurement locations. 0.15 mm

Beam propagation parameters calculated from the hyperbola equation. These parameters

were then back propagated by calculation through the lens using the known lens

parameters allowing calculation of the LED emitted beam properties. The beam

parameters are presented in Table 3.

These can then be compared with the previous measurements of the LED direct beam.

Tables 4 and 5 give a complete summary of the Transform Validation Experiment

results for both the 8-bit and 12-bit camera systems.

22

Table 4 Validation results for IRED LED - X axis

LED SFH 400 IRED (950 nm) X – Axis

12-bit camera + Z16 lens

Property Direct

beam

(DB)

UC

%

Trans.

beam

UC

%

Inverse

transformed

beam (ITB)

UC

%

Difference

(ITB-RB)

Agreement %

(Difference/RB

x 100)

Waist

location

(mm from

vertex)

4.28 0.04 252.8 0.03 4.7 9.7 -0.42 -9.8

Waist

diamter

(mm)

3.06 0.01 2.73 0.14 3 0.58 0.06 2.0

Rayleigh

length

(mm)

12.13 0.03 10.7 0.56 13 1.8 -0.87 7.2

Divergence

(mrad)253 1 255 19 232 55 21 8.3

8-bit camera system + monozoom 7 lens

Waist

location

(mm from

vertex)

5.6 254.1 4.9 0.7 12.5

Waist

diamter

(mm)

3.38 3.12 3.47 -0.09 2.7

Rayleigh

length

(mm)

9.76 8.6 10.7 -0.94 9.6

Divergence

(mrad)346 361 324 22 6.4

Note: UC = Uncertainty (%)

23

Table 5 Validation results for IRED LED - Y axis

LED SFH 400 IRED (950 nm) Y - AXIS

12-bit camera + Z16 lens

Property Direct

beam

(DB)

UC

%

Trans.

beam

UC

%

Inverse

transformed

beam (ITB)

UC

%

Difference

(ITB-RB)

Agreement %

(Difference/RB

x 100)

Waist

location

(mm from

vertex)

4 0.03 252.8 0.03 4.5 9.9 -0.5 -12.5

Waist

diamter

(mm)

3.19 0.007 2.77 0.16 3.05 0.61 0.14 4.4

Rayleigh

length

(mm)

11.93 0.03 10.3 0.58 12.5 1.8 -0.57 -4.8

Divergence

(mrad)267 1 270 21 245 60 22 8.2

8-bit camera system + monozoom 7 lens

Waist

location

(mm from

vertex)

5.66 253.9 4.45 1.21 21.4

Waist

diameter

(mm)

3.34 3.09 3.45 -0.11 3.3

Rayleigh

length

(mm)

10.01 8.36 10.43 -0.42 4.2

Divergence

(mrad)333 370 331 2 0.6

24

5.4 YELLOW LED - LIGITEK LUY 3833/A29

The measurements of the yellow LED were performed using an arrangement of two

achromats. Images of the beam were taken and the beam width calculated. The

measurement points and the resultant hyperbolic fit are plotted in Figure 21. The

hyperbolic fits the measured data well. It is always difficult to make an initial estimate

of the position of the waist from the LED vertex. Ideally iterative measurements would

allow the spread of data to be symmetric around the beam waist position.

Yellow LED 2 Lens Transform X-axis

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

220 230 240 250 260 270


CS

M w

idth

(m

m)

Figure 21 Hyperbolic it to data from a yellow LED through two achromats

25

Table 6 Summary of beam characteristics for Yellow LED using 12-bit camera system

Yellow LED - Ligitek LUY 3833/A29

Transformed Beam






M2 735 20



Waist position from LED vertex Zo -3.9 mm 2.8




M2 735 64

Uncertainties Used

Focal length etc. 0.50%




Table 6 presents a summary of the beam characteristics for the yellow LED. From the results

the waist position of the inverse transformed beam (the direct beam) can be seen to be inside the

LED chip. This provides an interesting contrast to the IR LED. It should also be noted that the

divergence of this LED is significantly less than the other "display" LEDs examined in this

study.

26

5.5 BLUE LED - NICHIA NSPB500 RANK WS

Measurements of the 2-lens transformed beam from the blue LED were made at positions either

side of the beam waist and the results are shown in Figure 22.

The departure of data from the smooth fitted curve, shown in Figure 22, was thought to be due

to filter changes creating intereflections and problems for the CSM beam width measurement.

Blue LED 2 Lens Transform X-axis

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

235 240 245 250 255 260


CS

M w

idth

(m

m)

Figure 22 Hyperbolic fit to data from Blue LED through two achromats

27

Table 7 Summary of beam characteristics for Blue LED using 12-bit camera system

Blue LED - Nichia NSPB500 Rank WS

Transformed Beam






M21171 112







M21171 170

Uncertainties Used

Focal length etc. 0.50%




With these results an externally located beam waist location outside the Blue LED package can

be seen from the positive waist position in the inverse transform section of Table 7. This is

similar to the Osram SFH 400 IRED, but the waist is not as conveniently far away from the

LED vertex which facilitated the direct beam measurement in section 5.2.

28

5.6 GREEN LED - NICHIA NSPG500 RANK GS

Measurements of the beam width were made at positions either side of the beam waist and the

results are shown in Figure 23.

Green LED 2 Lens Transform X-axis

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

235 240 245 250 255 260


CS

M w

idth

(m

m)

Figure 23 Hyperbolic fit to data from Green LED through two achromats

Figure 23 shows a smaller data divergence from the fitted curve. These deviations can be

disregarded because the rest of the data fits so well.

29

Table 8 Summary of beam characteristics for Green LED using 12-bit camera system

Green LED - Nichia NSPG500 Rank GS

Transformed Beam






M2 1129 93







M2 1129 148

Uncertainties Used





The results in Table 8 summarise the beam characteristics for the green LED. It shows an

external waist from the LED. The measurement procedure requires at least 10 measurements of

the beam width either side of the waist position, therefore the waist would not be positioned far

enough away from the LED vertex to be easily measured as a direct beam.

30

5.7 RED LED - KINGBRIGHT L-53SRC/E



Red LED 2 Lens Transform X-axis

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

235 240 245 250 255 260


CS

M w

idth

(m

m)

Figure 24 Hyperbolic fit to data from Red LED through two achromats

31

Table 9 Summary of beam characteristics for Red LED using 12-bit camera system

Red LED - Kingbright L-53SRC/E

Transformed Beam






M2 1052 75







M2 1052 126

Uncertainties Used





A summary of the beam characteristics for the red LED is presented in Table 9. As seen with

the yellow and blue LEDs in sections 5.4 and 5.5, a beam waist location inside the red LED

package can be seen from the negative waist position in the inverse transform section of Table

9.

32

5.8 WHITE LED - NICHIA NSPW500 RANK BS



White LED 2 Lens Transform X-axis

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

235 240 245 250 255 260


CS

M w

idth

(m

m)

Figure 25 Hyperbolic fit to data from White LED through two achromats

33

Table 10 Summary of beam characteristics for White LED using 12-bit camera system

White - Nichia NSPW500 Rank BS

Transformed Beam






M2 911 90







M2 911 131

Uncertainties Used




Measurement locations. 0.15 mm

A summary of the beam characteristics for the white LED is presented in Table 10.

34

5.9 ORANGE LED - TOSHIBA TLOH190P

Orange LED One Lens Transform. X-axis

6.0

7.0

8.0

9.0

10.

11.

12.

13.

180 200 220 240 260 280


CS

M w

idth

(mm

)

Figure 26 Orange LED one achromat transform

The orange LED only used one achromat to perform the beam transformation to give an

appropriate image to analyse on the 12-bit camera.

The orange LED data did not fit well to a hyperbola. This is clearly demonstrated in Figure 32.

Further study of the measurement data indicated that the propagating beam was astigmatic and

hence would not fit to the beam propagation model. Astigmatic beams could be treated using the

methodology described in ISO 11146-2 7 but this is beyond the scope of this study.

The astigmatic nature of the beam can be discovered from the steady change of azimuth angle as

the beam propagates, see Figure 33. The apparent sudden jump of the angle is due to the beam

widths in the X and Y direction reaching the same value at that point in the Z direction. This

indicates a nearly circular beam and makes the azimuth angle indeterminate. As described in the

ISO standard 11146-2 the insertion at this point of a cylindrical lens at the right azimuth angle

may remove the astigmatism.

35

Astigmatism in beam from Orange LED

-10.00

-5.00

0.00

5.00

10.00

15.00

20.00

170 190 210 230 250 270 290 310

Distance past Vertex (mm)

WoX

WoY

Elipticity x 10

Azimuth (degrees)

Figure 27 Plot demonstrating the astigmatism of the orange LED and hence the lack of fit to a hyperbola.

36

5.10 HIGH POWER BLUE LED - LUXEON STAR

Figure 28 Photo of Luxeon Star LED with Fraen 10° lens

The Luxeon Star LED, including the Fraen 10° lens associated with the LED, is shown in

Figure 29. Details for the Luxeon Star are given in Appendix 3.

Luxeon V-Star Batwing LED (Royal blue) + Fraen 10° Lens. (x-axis)

7.0

8.0

9.0

10.0

11.0

12.0

13.0

340 350 360 370 380 390 400 410


CS

M w

idth

(m

m)

Figure 29 Hyperbolic fit to data from high power royal blue LED through one achromat

37

The values for the beam width of the Luxeon Star LED are plotted in Figure 30. The angular

subtense of this device far exceeds max (100 mrad) and hence falls outside the region where the

coefficient C6 (IEC 60825-1) value depends upon angular subtense. It should be noted that this

LED carries a Class 2 warning label.

38

6 UNCERTAINTY ANALYSIS

With reference to the ISO Guide to Uncertainty in Measurement 18 the uncertainties are

separated into:

Type A, those uncertainties evaluated by statistical methods

and Type B, those evaluated by other methods.

The equations used to derive the beam propagation parameter are partially differentiated with

respect to all the measured quantities to produce contributions to the uncertainty budget.

A simplified summary of the Type B uncertainty budget is shown in the Table 11. The

uncertainties quoted are the reduced values (coverage factor k=1).

Table 11 List of Type B uncertainty values

Source of Uncertainty Value

Focal lengths 0.5 %




The second moment width measurements were fitted to a hyperbolic curve and the curve

coefficients were then used to derive the beam propagation parameters. The uncertainty of this

measurement was therefore derived by the partial differentiation of the equations defining the

propagation parameters. This was checked using a step-wise uncertainty analysis, which

produced close agreement with the original method (partial differentiation is a more rigorous

method).

Correlation has not been considered in this analysis but the uncertainties were combined using

sum of squares to give the most conservative estimate of uncertainty.

The uncertainty derivations had to consider three measurement configurations

a) No Transform lenses used (LED has an external waist)

b) One Transform lens used

c) Two Transform lenses used

The tables below give examples of each configuration. The Type B uncertainties listed in Table

11 are added in quadrature to provide the uncertainty values for component positions.

39

6.1.1 No Transform Lens Used

This example is for the 12-bit direct beam measurement of Osram IR LED. Table 12 details the

uncertainty components. The uncertainty references are at the end of the Section 6.

Table 12 Uncertainties for measurement of Osram IR LED

Waist position zo 4.28 mm +/- 0.04

Waist diameter Wo 3.06 mm +/- 0.0067

Rayleigh distance Zr 12.13 mm +/- 0.0287

Divergence 252 mrad 0.81

M2 639 +/- 3.17

The uncertainties for the raw beam were calculated by knowledge of the Type B uncertainty in

the measurement of distance modified by the local gradient of the hyperbola. The resulting

covariances were then added in quadrature to obtain estimates for the 1 standard deviation level.

6.1.2 One Lens datasheet

This example is for the 12-bit direct beam measurement of high power Luxeon Star LED Osram

IR LED (optical arrangement and dimensions are shown in Figure 31). Table 13 details the

uncertainty components for this measurement.

Figure 30 Diagrams illustrating the required dimensions for the Luxeon star LED

40

Table 13 Uncertainties for measurement of Luxeon Star LED

Symbol FormulaeDistance

(mm)1

Uncertainty

Uncertainty

calculation code

LED Vertex to lens datum L4 253.00 0.21 RMS

Datum of Lens 1 to 1st principle

planeL8 19.7 0.16 M/F

Separation of principle planes L9 5.3 0.11 M/F

Effective focal length of lens fe 76.2 0.38 M/F

Location of measured beam

waistZo1 370.08 0.04 Ref 1

Distance of measured waist from

focal plane X1 15.88 0.48 RMS

Rayleigh Length of measured

beamZr1 24.51 0.50 Ref 2

Waist width of measured beam Wo1 7.56 0.16 Ref 3

Far-field divergence of measured

beam 1 308.3 8.97 Ref 4

Transform Parameter G1 6.81 0.24 Ref 5

Distance of output waist from

focal plane Y1 108.1 5.0 Ref 6

Location of output beam waist

wrt vertex of LEDZo2 88.4 5.0 Ref 7

Waist width of LED output beam Wo2 19.7 0.5 Ref 8

Rayleigh Length of LED output

beamZr2 166.9 6.8 Ref 9

Far-field divergence of LED

output beam 2 118.2 18.3 Ref 10

C

BZ

20

eo fLLLZX 98411

2

4

11BCA

CZr

C

BAWo

4

2

310r

o

Z

W

)( 221

2

rZX

fG

11 XGY

eo fYLLZ 12 84

112 GWW oo

12 rr ZGZ

310r

o

Z

W

41

6.1.3 Uncertainty Budget For Beam Waist (two lens transform)

Figure 32 illustrates some of the critical measurements made for the Ligitek LUY 3833/A29

Yellow LED evaluation.

Figure 31 Critical measurements for two lens transformation

Table 14 Uncertainties for measurement of Yellow LED

Symbol FormulaeDistance

(mm)

1

Uncertainty

Uncertainty

calculation

code

LED Vertex to lens 2

datumL5 161.8 0.14 RMS

Datum of Lens 2 to

1st principle planeL10 13.23 0.15 M/F

Separation of

principle planes of

Lens 2

L11 4.9 0.10 M/F

Effective focal length

of Lens 2fe2 100 0.50 M/F

LED Vertex to lens 1

datumL4 81.60 0.14 RMS

Datum of Lens 1 to

1st principle planeL8 19.7 0.16 M/F

Separation of

principle planesL9 5.3 0.11 M/F

Effective focal length

of lensfe1 76.2 0.38 M/F

Location of measured

beam waist Zo1 242.43 0.02 Ref 1

Distance of measured

waist from input

cal plane of Lensfo 2

X1 -37.50 0.55 RMS

Rayleigh Length of

measured beamZr1 16.70 0.20 Ref 2

C

BZ

20

eo fLLLZX 98411

2

4

11BCA

CZ r

42

Waist width of

measured beamWo1 3.81 0.05 Ref 3

Far-field divergence

of measured beam

vert D

1

C

BAWo

4

2

228.2 3.85 Ref 4


Distance of waist of

intermediate beam

from focal plane of

Lens 2

Y1 -222.5 6.8 Ref 6

Location of

intermediate beam

waist wrt vertex of

LED

Zo2 297.5 6.8 Ref 7

Waist width of

intermediate beamWo2 9.3 0.2 Ref 8

Rayleigh Length of

intermediate beamZr2 99.1 2.9 Ref 9


of intermediate beam 2

93.7 0.6 Ref 10

Distance between

intermediate waist

and input focal plane

of Lens 1

X2 114.7 6.8 RMS


Distance of waist of

input beam from

focal plane of Lens 1

Y2 28.98 2.7 Ref 6

Location of input

beam waist wrt

ex of LE

Zo3 -3.88 2.8 Ref 7

Waist width of LED

output beamWo3 4.67 0.2 Ref 8

Rayleigh Length of

LED output beamZr3 25.03 0.73 Ref 9


of LED output beam 3 186.41 9.36 Ref 10

310r

o

Z

W

)( 2

1

2

1

2

21

rZX

fG

11 XGY

212 105 eo fYLLZ

112 GWW oo

12 rr ZGZ

310r

o

Z

W

198422 eo fLLLZX

)( 2

2

2

2

2

12

rZX

fG

222 XGY

128403 fYLLZ

223 GWW oo

323 rr ZGZ

3

3

3

3 10r

o

Z

W

43

6.1.4 Uncertainty References

The fitting used for the measurement data was :

22 )()()( zcCzbBaAW W

Example of one lens analysis as fitted to the Luxeon blue LED propagation data.

Where A = 13074 B = -70.35 C = 0.0950

and a = 0.972 b = 0.00518 c = 6.911E-06

Example of two lens analysis as fitted to the Yellow LED transformed output beam

Where A = 3075 B = -25.25 C = 0.0521 sl = 0.1and a = 0.143 b = 0.00117 c = 2.395E-06

These are the partially differentiated equations for the derived parameters and therefore provide

the measurement uncertainty of the named parameters.

2

2

2

1

C

Bcb

CZoReference 1:

222

2

222

1A

C

B

C

c

C

bBa

CZr

ZrReference 2:

2

22

2

422

1

C

cB

C

bBa

Wo

WoReference 3:

2

2310

r

oZW

r Z

W

Z roReference 4:

2

2

2

2

2

2

2

2

2

22

222 andwheref

ZG

Z

G

f

G

f

G ,

f

XG

X

G

Z

G

f

G

X

G

rii

ri

iiiii

i

i

ri

i

Z

if

i

i

XGriii

Reference 5:

22

iXiGY GXiiiReference 6:

222

12)1(2

orio fYilZReference 7:

2

2

2)1(

GiG

Woiio W

i

oiGiW

Reference 8:

44

22

)1(GiZ

riir zriGiZReference 9:

2

2310

ri

oiZW

ri

iZ

W

Z rioi

Reference 10:

45

7 CONCLUSIONS

The results from this investigation show that this method can be applied successfully to the

analysis of beam propagation parameters 17 and hence the apparent source size determination for

stigmatic and simple astigmatic beams from LEDs. It is also capable of identifying beams that

suffer from general astigmatism and which are not currently eligible for relaxation of

classification limits using this simplified form of analysis. The good agreement obtained for the

ISO Transform Validation Experiment for both the 8-bit and 12-bit systems shows that the

technique is highly robust. This level of agreement was unexpected, due to the increased noise

levels inherent in the 8-bit system as a result of the smaller dynamic range and the un-cooled

analogue camera.

Assuming a maximum permissible inaccuracy is 10% at a measuring distance of 100 mm, the

geometric approximation can only be used with beams whose Rayleigh Length is less than

approximately 50 mm. This limit effectively marks the boundary above which diffraction

effects become noticeable and classical ray-tracing optics cannot be used. The measured LED’s

Rayleigh Lengths were all less than 50 mm which allows us to consider the apparent source size

and its location, to be the same as the direct beam waist.

The detection of general astigmatism in the beam from the orange LED shows that the in built

checks of the techniques applicability work well.

Figure 33 shows the high level of agreement between the propagation parameters derived

through the 8-bit and 12-bit methods using the IR LED. It also includes the parameters derived

through the Transform Validation Experiment. Figure 34 shows the measurement results from

each of the LED’s plotted on the contour plot for angular subtense as a function of the measured

beam characteristics of LEDs. The size of the ellipse indicates the level of measurement

uncertainty for each LED.

The montage, presented in Figure 32, clearly depicts how the beam images, profiles and results

correspond. The beam image mapped onto the beam propagation envelope with the appropriate

2D spatial intensity profiles is a helpful visualization of the evolution of the beam as it travels

through space. The important aspects to note are that the point in the beam propagation where

the LED chip structure is in focus does not correspond to the position of the beam waist. This is

an important result because it has been the practice of some safety assessors to use the position

of sharp focus to estimate the apparent source size. In this situation this methodology would

result in an estimate of the apparent source size that was greater that the real value. This would

produce a lower value of the potential hazard of the LED than actuality.

46

Figure 32 Montage of the spatial beam profiles which make up the propagation envelope of the LED

47

Comparison of Beam Parameter Measurements - Y-axis

0

2

4

6

8

10

12

14

Zo Wo Zr Theta

mill

ime

ters

or

d/r

ad

12-bit raw beam

12-bit t ransformed

8-bit raw beam

8-bit transformed

Comparison of Beam Parameter Measurements - X-axis

0

2

4

6

8

10

12

14

16

Zo Wo Zr Theta

millim

ete

rs o

r d

/rad

12-bit raw beam

12-bit transformed

8-bit raw beam

8-bit transformed

Figure 33 Comparison of 8-bit and 12-bit camera results

48

Figure 34 Plot showing contours of angular subtense, including results for the measured LEDs

49

7.1 FUTURE DIRECTIONS

This project has allowed the identification of many areas where the technique can be improved

with further work. An improvement of the length measuring system would result in a reduction

of the uncertainty of the validation process and would produce greater accuracy in the

determination of beam waist, divergence and thus retinal hazard. An optically encoded servo

motor slide would reduce the distance measurement uncertainty by an order of magnitude. The

replacement of a manual vernier slide with an electrically driven version would remove the need

to manually read measurement position. This would allow better exclusion of background light

by the use of a local light tight enclosure coated with diffusing black paint. The lack of

extraneous light would improve the measurement dynamic range and reduce the probability of

problems caused by optical artefacts on the CCD images.

A custom produced achromat with a larger diameter and shorter focal length would serve to

reduce the cumulative uncertainty. The reduction in the number of optical surfaces through

which the light propagates would serve to reduce aberration of the beam wavefront and the

production of scattered light. A custom manufactured graticule would allow calibration of the

whole field of view at higher magnification zoom settings.

An ideal development of this project would be to determine the real spot size produced by a

given source by producing an “artificial” eye or eye analogue. Apparent source size was created

as an artifice to allow the comparative measurement of the effect of viewing sources larger than

a “point” source yet smaller than the 100 mrad subtense advocated in the IEC safety standard.

This would then inform the debate about the effect of problematic beam profiles on the retina

and thus would allow a thermal diffusion model to be produced with finite element analysis.

Figure 35: Diagram showing the lens of an eye transforming a LED beam. The size ofthe spot on the retina is not measurable with current techniques.

The correct treatment of generally astigmatic beams from LEDs and other intermediate sources

would require the use of the mathematical method outlined in “ISO 11146-2 7 Lasers and laser-

related equipment. Test methods for laser beam widths, divergence angle and beam propagation

ratio. Part 2: General astigmatic beams”. The existing software could be adapted and the

measurement technique amended to provide the required technique 8.

50

APPENDIX 1: SECOND MOMENT, AZIMUTH AND PRINCIPLEWIDTH DERIVATION

The reduced second order moments can be determined by a measurement of the energy density

distribution over a limited area or window:

2

2

2

2

2

2

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

),,(

),,()()(

),,(

),,()()(

yzyxI

zyxIyyz

xzyxI

zyxIxxz

y

y

x

x

y

y

x

x

y

y

y

x

x

y

y

x

x

x

where the summations are carried out over a rectangle parallel to the x- and y-axes and:

yy

xx

dyydyy

dxxdxx

2

3

2

3

2

3

2

3

21

21

and

The concept of second moment measurements is extended to include the “mixed moments” of

the spatial and divergence properties of the beam. For example, the spatial mixed moment is:

xyzyxI

zyxIyyxxz

y

y

x

x

y

y

x

xxy

2

1

2

1

2

1

2

1

),,(

),,())(()(

2

The three spatial moments describe the lateral extent of the power density distribution of the

beam in the reference plane. The directions of minimum and maximum extent are called

principal axes which are always orthogonal to each other. Any power density distribution is

characterized by the extents along its principal axes and the orientation of the principal axes.

The beam width along the direction of that principal axis, which is closer to the x-axis of the

laboratory system, is given by:

½½

22

2222 422 xyyxyxzd x

and the beam width along the direction of that principal axis, which is closer to the y-axis by:½

½2

22222 422 xyyxyxzd y

51

where22

22

22sgn

yx

yxyx

Finally, the azimuthal angle between the principal axis that is closer to the X-axis and the X-

axis is :

22

2arctan½

yx

xy

52

APPENDIX 2: DESIGN AND TECHNICAL SPECIFICATION FOR AFACILITY TO DETERMINE THE APPARENT SOURCE SIZE OF

LIGHT EMITTING DIODES

Introduction

The angular subtense of an apparent source of radiation in the 400 nm to 1400 nm wavelength

range is required by current laser safety standards to permit calculation of the relaxation factor

C6, for extended sources. It is the ratio of the angular subtense of the source in question to that

of a source that would form the realistic minimum spot size on the retina (1.5 mrad).

Classification or assessment of the thermal hazard from a source requires that both the angular

subtense and location of an extended source be known before there can be a relaxation of the

maximum permissible exposure (MPE) of the eye. The location of a source is required so that

the angular subtense can be calculated for viewing from the minimum conceivable eye

accommodation distance of 100 mm.

It is a simple matter to measure the physical size of the chip of a LED that has a Lambertian

radiation pattern but it is more difficult to know or measure the location or size of the apparent

source of collimated beams from an LED. Such beams can have a nearly plane wavefront which

would imply that the apparent source is located at infinity with an unknown angular extent.

However, recent advances in the characterization of optical beams, both coherent and

incoherent, enable prediction of their propagation envelopes. It is now possible to assess the

intrabeam viewing hazard by using known beam characteristics to estimate the angular subtense

of an extended source that would present the greatest hazard to a retina.

Measurement of the optical constants of the propagation envelope of a beam have been the

subject of considerable research over the last ten years. A consequence of this work is the

evolution of ISO standards1 for the measurement of the diameter and divergence of a beam. The

procedures and techniques that are proposed here for the determination of the diameter and

location of the apparent source of a beam2 are based on the principles underlying the ISO

standards for simple astigmatic beams. Should a beam display general astigmatism (twist) no

relaxation of the laser safety criteria should be given.

Beam measurements

There are a number of methods available for measurement of the diameter of a beam as well as

its far-field divergence. The basic principles for those methods have been established by the ISO

standard. They are applicable to laser beams with a relatively small beam propagation ratio, M2.

Recent research has demonstrated that adequate steps have to be taken to counter the effects of

noise and offset errors when measuring the transverse irradiance distribution of a beam. When

these steps are taken, the propagation behaviour of incoherent broadband beams as well as high-

1 ISO 11 146:1999. “Test methods for laser beam parameters: beam widths, divergence angle and

beam propagation factor”.

2The measurements proposed in this document are applicable to beams whose full divergence angle

is less that 30°.

53

quality laser beams can be predicted reproducibly with considerable precision. The methods

leading to estimates of the diameter of a beam use a procedure known as the Converging Second

Moment diameter or width measurement (CSM). Those methods are being defined in the

revision of ISO 11146 that is currently in preparation.

The preferred method for measuring all the propagation characteristics of a beam is to perform

CSM diameter measurements at a number of locations either side of the beam waist.

The Optical System

The beam measurement process consists of using a CCD sensor to image the irradiance profile

at about ten locations, ideally either side of the beam waist. The proposed optical system

contains variable magnifying optics that are designed to facilitate imaging the transverse

irradiance profiles so that they occupy approximately one quarter of the sensor screen height.

Other components are included in the system to attenuate the beam power and avoid sensor

saturation and to provide spatial calibration of the pixel array of the sensor.

A schematic diagram of the proposed system is displayed later in this section.

The main base bench contains two sub-systems, the LED Bench and the Imaging Bench.

The LED Bench

The LED bench is capable of axial movement so as to transport the emerging beam relative to

the imaging bench. The movement can be achieved with manual or automatic means. The

movement distance relative to a datum should be determined with an uncertainty less than 0.1

mm. The bench is to be 400 mm long and fitted with two or three component holders that are

capable of smooth and fine adjustment about a number of specified axes.

Unit 1 - The first carriage is a holder for a range of LEDs. The LED should be firmly held in a

cantilevered stand and restrained from any movement that might arise from movement of its

power supply leads. An arrangement consisting of a split clamping plate with a suitable range of

collets that will accommodate the diameters of the selected LEDs. This unit should be capable

of adjustment along the transverse x and y axes, the axial z axis and rotation about the Ox and

Oy axes. The LEDs will probably all have a cylindrical form with discrete diameters of 3 mm, 5

mm, 8 mm or 10 mm.

Unit 2 - The next carriage has a cantilevered lens holder that is capable of placing a lens with

its vertex touching the output face or vertex of the lens of the LED. The mount should hold the

lens normal to the optical axis of the system and be capable of smooth adjustment along the

transverse x and y axes and the axial z axis.

i) The lens should not need to have an aperture greater than f/2. This lens is

designed to transform beams whose waist is virtual and behind the LED into one with

an accessible waist in the measurement region of the system. The design of the lens

should be based on a multi-element composition aimed at minimising aberrations. It

should also be broadband A/R coated to minimise reflections at the wavelength range

that is to be studied.

ii) An examination of the capability of a +50 mm focal length transform lens on

the range of beam parameters under consideration is given in the Annex. The results

suggest that this lens will produce transformed beams whose parameters are all within

the measurement capability of the proposed system.

54

A second carriage, similar to Unit 2 should be provided so that a second transform lens can be

mounted on the LED bench for the “validation” trials.

The Imaging Bench

A static imaging bench is to be provided with three main carriages. These are for an attenuator

wheel, a calibration graticule and the CCD camera fitted with a “zoom” microscope lens system.

Unit 3 - This is the main beam attenuation device. It consists of an indexable wheel containing,

say, eight neutral density filters. The apertures in the filter holder should be sufficient to

accommodate at least 95% of the beam power. The ND filters should exhibit a high degree of

uniformity across 95% of their full aperture and, ideally, should be antireflection coated for the

wavelength range that is to be studied.

The filter wheel holder should be capable of adjustment along both transverse x and y axes. It

should also be capable of rotation about the Oy axis so that reflections can be diverted from the

beam path.

Unit 4 - This carriage contains a holder for a calibration graticule. It should be capable of fine

adjustment along all the x, y and z axes. The location of the graticule along the z axis should be

capable of placement with a separation D1 from the objective of the microscope between 30

mm and 300 mm. The graticule should be capable of being adjusted by rotation about an Oz axis

through the centre of its aperture. The design of the graticule should enable viewing with the

CCD array to present contrast and sharpness sufficient to enable use of the provided calibration

software with an adequately low level of uncertainty. Trials will be performed with a number of

graticules to permit analysis and select a suitable design.

The function of the graticule is to enable calibration of the transverse dimensions and linearity

of the CCD sensor array. The graticule is not required during actual beam measurements

although it could be advantageous to use the beam to illuminate the graticule at its own

wavelength.

The graticule has to be removed from the beam path during measurements but it should be

capable of being relocated in its original position in the object plane of the microscope without

requiring readjustment.

Units 5 and 6 - A sturdy carriage is required to hold firmly a zoom microscope and CCD

camera with a high degree of stability. The function of the zoom microscope is to enlarge the

image of the transverse irradiance distribution in the object plane and focus it on the CCD array

of Unit 6. The zoom function enables adjustment of the magnification so that, ideally, the image

fills approximately one quarter of the array.

i) The Microscope - The zoom microscope is required to have a field of view that

will be adjustable between 0.25 mm and 50 mm. A microscope that can satisfy this

requirement is the Leica MonoZoom77 Video Microscope System. When fitted with

either x0.25 or x2 objectives in combination no amplifier lens or a x3 amplifier, it may

just cover the required field size range when used in conjunction with an appropriately

sized CCD array sensor. In addition, an aperture diaphragm (P/N 007/023) can be

incorporated into the system to provide continuously variable attenuation of the

transmitted power. This is a very attractive facility since it will enable power

adjustment between the steps available with the ND filters.

55

A MonoZoom 7 instrument has been hired to enable assessment of its optical

performance and the magnitude of residual aberrations. Should the performance be

inadequate an alternative set of components could be assembled from the Leica Z16

APO system.

ii) The Camera - It is thought that a CCD sensor system with a 12-bit dynamic

sensitivity is required to provide sufficient resolution and noise control for beam

profile analysis. Furthermore, a pixel number in excess of 1 million is estimated to be

required to give sufficient resolution for spot diameter measurements. However, these

aspects are to be studied in one of the principle topics of this programme.

One further aspect influencing the selection of a camera is the physical size of the

CCD array. The height dimension of a 4:3 video array is the dimension that will

control the field of view seen through the microscope. The dimensions of an array are

not always given in a camera specification. It could be referred to as a b” screen where

it will have a height of 6.6 mm and width of 8.8 mm, giving an 11 mm diagonal.

Alternative array sizes can be quoted as ½” and a”. These dimensions must be

obtained before all the lens accessories for the microscope system can be selected.

Additional Components

Three more optical aspects of the system need to be considered and provided. These are a

“beam stop” and background illumination shields. The need for these components arises from

the requirement to eliminate background optical noise from the CSM width estimates. When the

beam passes through optical components, residual multiple internal reflections and other sources

of stray light will combine with the main beam and tend to enlarge estimates of beam widths.

The first precaution is to place the whole system in a light-tight enclosure so that light from the

environment does not reach the CCD array. The next stage is to attenuate any light scattered or

reflected out of the main beam by placing an absorbing shield with an aperture at some distance

down the beam path. The aperture should limit beam divergences above 30°.

The final precaution is to make a record of any residual light or optical imperfections by

recording a frame of the average background field and subtract it from the beam profile. This

can be done by inserting a physical beam stop into the path of the beam so that the only light

that passes is from the sources of extraneous noise and recording this as the background. The

exact nature and location of this beam stop must be the subject of further discussion.

56

Figure 36 Proposed optical apparatus for measurement of apparent source size

57

APPENDIX 3: LED TECHINICAL DATA SHEETS

VISIBLE LEDsRed: Kingbright L-53SRC/Ehttp://www.us.kingbright.com/data/spec/W1503SRC-D.pdf

Yellow: Ligitek LUY 3833/A29http://www.ligitek.com/2-2.htm

Green: Nichia NSPG500 Rank GShttp://www.nichia.co.jp/specification/led_lamp/NSPG500S.pdf

Blue: Nichia NSPB500 Rank WS http://www.nichia.co.jp/specification/led_lamp/NSPB500S.pdf

White: Nichia NSPW500 Rank BShttp://www.nichia.co.jp/specification/led_lamp/NSPL500S.pdf

IR LED IR: Osram SFH 400 http://www.osram.convergy.de/scripts/product_family.asp?CLSOID=10024&FAMILYOID=20412

HIGH BRIGHTNESS LEDSOrange: Toshiba TLOH190Phttp://www.semicon.toshiba.co.jp/td/ja/Opto/Visible_LED/20030620_TLOH190P(F)_datasheet.pdf

Blue: Luxeon Star http://www.lumileds.com/pdfs/DS23.pdf

Table 15 Summary of optical and electrical characteristics of LEDs

LED

model

Size

(mm)

Colour Peak

(nm)

Typical

Lumious

Intensity

(mcd)

Typ

current

(mA)

Max

Fwd

voltage

(V)

Bandwidth

(nm)

Divergence,

½

Kingbright

L-53SRC/E

5 Red 660 1500 20 2.5 20 15

Ligitek

LUY

3833/A29

5 Yellow 595 2700 20 2.8 - 12

Nichia

NSPG500

Rank GS

5 Green 520 11600 30 4.0 40 -

Nichia

NSPB500

Rank WS

5 Blue 470 3460 30 4.0 30 -

Nichia

NSPW500

Rank BS

5 White 595 6400 30 4.0 N/a -

Osram

SFH 400

5 Ired 950 - 300 5 55 6

Toshiba

TLOH190P

10 Orange 612 20000 50 4 10 6

Luxeon

Star

- Blue 470 100000 700 5 25 10

58

REFERENCES

1“ICNIRP statement on light-emitting diodes (LED’s) and laser diodes:

implications for Hazard assessment”, Health Physics June 2000, Volume 78,

Number 6 (http://www.icnirp.de/documents/led.pdf)

1.2

Ward B.A. “Measurement of Laser and LED Beams for prediction of

Angular Subtense ILSC 2003 conference Jacksonville, FL, USA

3BS EN 60825-1:1994, Incorporating Amendment 1,2 and 3, Safety of laser

products. Equipment classification, requirements and user’s guide

4ISO 11146:1999 Lasers and laser-related equipment -- Test methods for laser

beam parameters -- Beam widths, divergence angle and beam propagation factor

5BS EN ISO 11554:2003 Optics and optical instruments. Lasers and laser-related

equipment. Test methods for laser beam power, energy and temporal

characteristics

601/714513 DC ISO/CD 11146-1. Lasers and laser-related equipment. Test

methods for laser beam widths, divergence angle and beam propagation factor.

Part 1: Stigmatic and simple astigmatic beams

701/714514 DC ISO/CD 11146-2. Lasers and laser-related equipment. Test

methods for laser beam widths, divergence angle and beam propagation ratio.

Part 2: General astigmatic beams

8ISO/PDTR 11146-3. Lasers and laser-related equipment. Test methods for laser

beam widths, divergence angle and beam propagation ratio. Part 3: Alternative

test methods and geometrical laser beam classification and propagation (BSI

draft 01/714515 DC)

9ISO 13694:2000 Optics and optical instruments – Lasers and laser-related

equipment – Test methods for laser beam power (energy) density distribution

10IEC 60825-13. Ed.1. Safety of laser products. Part 13: Measurements for

classification of laser products (BSI draft 03/307798 DC)

11IEC TR 60825-14 ed. 1. Safety of laser products. Part 14. A user's guide (BSI

Draft 02/206661 DC)

12Henderson R and Schulmeister K “Laser Safety” Bristol, IOP,2004

13ISO 11145:2001, Optics and optical instruments . Lasers and laser-related

equipment . Vocabulary and symbols.

59

14ISO 13694, Optics and optical instruments . Lasers and laser-related equipment .

Test methods for laser beam power (energy) density distributions

15IEC 61040:1990, Power and energy measuring detectors. Instruments and

equipment for laser radiation.

16Amarande S, Giesen A Hügel H “Propagation analysis of self-convergent beam

width and characterization of hard-edge diffracted beams” APPLIED OPTICS

Vol. 39, No. 22, 1 August 2000

17 Siegman A.E “ Defining the Effective radius of Curvature for a Nonideal

Optical Beam” IEE Journal of Quantum Electronics Vol 27 No 5 May 1991

18ISO Guide to the expression of uncertainty in measurement 1995, ISBN 92-67-

10188-9

19Wood. RM “Laser-Induced Damage of Optical Materials” Bristol IOP 2003

60

GLOSSARY

Gamma

A numerical value, or the degree of contrast in a television picture, which is the

exponent of that power law which is used to approximate the value of the magnitude of

the output signal as a function of the input signal over the region of interest.

Interline Transfer

A technology of CCD design, where rows of pixels are output from the camera. The

sensor's active pixel area and storage register are both contained within the active image

area. This differs from "frame transfer" cameras that move all active pixels to a storage

register outside of the active area.

Vignetting

In an optical system, the gradual reduction of image illuminance as the off-axis angle

increases, resulting from limitations of the clear apertures of elements within the

system. This is called vignetting and is shown in Figure 37.

Figure 37 Illustration of vignetting effect

Stigmatism

Property of a beam having circular power density distributions in any plane under free

propagation and showing power density distributions after propagation through a

cylindrical lens all having the same or orthogonal orientation as that lens

Simple astigmatism

Property of a non-stigmatic beam whose azimuth shows a constant orientation under

free propagation, and retains its original orientation after passing through a cylindrical

optical element whose axis is parallel to one of the principal axes

NB: The principal axes of a power density distribution corresponding to a beam with

simple astigmatism are called the principal axes of that beam.

61

Generalised Rayleigh length (ZR,g)

Distance along the beam axis from the generalized beam waist where the generalized

beam diameter is a factor of 2 larger than the generalized beam waist diameter.

EFL (Effective Focal length)

The effective focal length (EFL) or equivalent focal length (denoted f in Figure 38) is

the distance from the focal points of the lens (F and F" in the Figure) to the respective

principal points (H or H"). The EFL determines magnification and hence the image size.

The term f appears frequently in the lens formulas and tables of standard lenses.

Unfortunately, the principal points are usually inside the lens, so that it is an

inconvenient measurement for precisely positioning a lens or determining mechanical

clearances. Consequently, most lenses specifications include measurements made from

the focal planes to the surfaces (verticies) of the optic (e.g., the front focal length ff, and

the back focal length fb).

Figure 38 Illustration of optical path through a lens

Back Focal Length

The Back focal length fb is the distance from the secondary vertex (A2) to the rear focal

point (F"), as illustrated in Figure 38.

Front Focal Length

The front focal length ff is the distance from the front focal point (F) to the primary

vertex (A1), as illustrated in Figure 38.

62

Printed and published by the Health and Safety ExecutiveC30 1/98

Printed and published by the Health and Safety ExecutiveC1.10 06/05

RR 345

£25.00 9 78071 7 661 084

ISBN 0-7176-6108-3

investigation of a measurement technique to determine led

Education

diodes led

measurement technique

initial results

validation experiment

led beam width measurement

led package

measurement procedure

optical system