basic lighting concepts, terms, units, abbreviations ...pavement/images/cpee/do… · reference: as...

Road and Public Space Lighting Workshop

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BASIC LIGHTING CONCEPTS, TERMS, UNITS, ABBREVIATIONS & RELATIONSHIPS

This Session follows through Figure 1.1 above from the production of light in the source to it

reaching the eye of the road user, elaborating the basic concepts of photometry. A ready

understanding of these concepts is essential for the designer in order to appreciate the

performance requirements of road and public space lighting, in terms of light technical

parameters, as set out in AS/NZS 1158.0, AS/NZS 1158.1.1 Clause 2.5 and in 3.1 Clause 2.5

and the computation requirements, as set out in AS/NZS 1158.2.

The components of Figure 1.1 to be considered are:

The light source or lamp which produces light from the electrical input

The lamp used in road and public space lighting

The luminaire (light fitting) containing the lamp and which directs light to the surface

of interest, e.g. part of the roadway; onto a person

llluminance or light on a surface - see the performance requirements of Category P

lighting in AS/NZS 1158.3.1

Luminance or the brightness of the surface of interest as seen by an observer - see

the performance requirements of Category V lighting in AS/NZS 1158.1.1

Reflectance or reflecting properties of the surface, particularly that of the

carriageway.

Reference: AS 3665 (1989) Simplified lighting terms


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This is the spectral power distribution of a tri-phosphor fluorescent lamp together with the

spectral sensitivity of the eye.

The Basic Concepts in Detail

Light source

The total light flux (F) available from a light source is given in lumens (Im); note the radiant

energy has been evaluated with respect to the sensitivity of the light adapted eye to the spectral

composition of the radiation produced by the source using the CIE λ function.

Two major influences have resulted in colour of light being specifically referred to in the 1158

Standards:

(i) Recent interest in the practical effect of the changing sensitivity of the eye to light

radiation as the light level falls; a shift of peak sensitivity towards to a shorter

wavelength suggests that a lamp emitting light with a large yellow component will be

over-valued in effective lumens compared to one emitting 'white' light.

(ii) The increasing desire to use white light in lighting schemes, with the availability of

metal halide and improved fluorescent lamps and LED sources.


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Until any universal definitive statement by the CIE the initial lamp lumens, as conventionally

evaluated and stated by the lamp manufacturer (using the V λ function), shall be used except

as here stated. No re-rating lumen multipliers other than 1.0 shall be used except that if used

in sub-categories P4 and P5, the lamp lumens for HPS lamps shall be derated to 0.75 and

those of LPS lamps to 0.5 of the quoted value.


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Lamps

The lamps used in road and public space lighting are discharge lamps, although there are two

basic modes of operation of conventional lamps:

(i) Discharge - through an ionised gas, needs high voltage starting and a ballast to control the

lamp current. These lamps have high efficacy, good lumen maintenance, long life.

(ii) Incandescent - heating tungsten wire, current controlled by resistance. Although the light is

white lamp' efficacy is low (~ 20 Im/ W) and lumen maintenance and life are both relatively

poor.

Three basic types of discharge lamps are used in public lighting:

(i) Sodium

• Low pressure; harsh yellow, efficacy ~200 lm/W; not now favoured

• High pressure (HPS); soft yellow, efficacy ~ 130 lm/W; the economic lamp of choice for traffic routes

(ii) Mercury

• Low pressure (fluorescent); white, now with good colour rendering and efficacy ~ 85

lm/W; used on local roads, the older less efficient tubular fluorescent lamps (2x20W or

1x40W) have been replaced by MPM (BOW) or are being replaced by the newer

tubular (2x14W) and compact lamps (42W)

• High pressure (HPM); white, moderate colour rendering, efficacy ~ 60 lm/W; becoming

obsolescent for use on traffic routes but used on local roads

(iii) Metal Halide (MH)

• High pressure; white, good colour rendering , efficacy ~100 lm/W; used mainly in city

and urban centres.

(iv) Solid State Lighting (LEDs)

• White, good colour rendering a range of CCT, luminaire efficacy about 120 lm/W long

lifetime, will have widespread use in P Category schemes and increasing use in V

Category schemes.

Ballast losses

The actual watts consumed by the lamp and its control gear is greater than nominal lamp

rating, e.g. 250W HPS uses 273W; BOW HPM uses 96W.

Lamp characteristics - Colour of the light emitted by a lamp

There are two distinct aspects, the rendition of colour in the lit space and the colour

appearance of the light source itself.

The apparent colour appearance of the light is indicated by the correlated colour

temperature (CCT), given in degrees Kelvin.

The Colour Rendering Index (CRI)

The Lamp luminous efficacy

Colour temperature

The first aspect of the colour properties of a light source is the correlated colour

temperature (CCT) given in °K (degrees Kelvin) and indicates the apparent colour

appearance of the light source. The CCT value is that of the perfect source to which the

particular light source is correlated by its chromaticity.


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The value of CCT for electric light sources ranges from about 2000 for HPS, to 3000

(incandescent), through to 6500 ('daylight' fluorescent). MH and fluorescent lamps are made in

a range of CCT and may be given the designations 'warm white', 'cool white', 'daylight' etc.

as shown in Table 1.1. The difference in appearance between sources of different CCT will be

not be readily apparent in isolation, except that sources of very high CCT will tend to look

'clinical'. However where sources of different CCT become mixed in a lighting scheme the

differences in colour appearance will be marked.

Table 1.1 - Lamp Colour Appearance - Correlated Colour Temperature CCT

CCT Class CCT

Warm CCT ≤ 3 300K

Intermediate 3 300K < CCT ≤ 5 300K

Cool 5 300K < CCT

Colour rendering

The rendition of colour by a lamp is specified by the CIE colour rendering index (CRI or Ra)

and is a measure of the light source to faithfully render the appearance a gamut of colours

spanning the colour space. The index has a maximum value of 100 which means a light

source with this value is as effective in colour rendering is as the correlated perfect light

source. The scale was designed so that a, then, 'standard' single phosphor fluorescent lamp

had a CRI value of 50, a high pressure mercury lamp will have a CRI of about 60. Metal

halide lamps and multi-phosphor fluorescent lamps have CRI values in the 80 to 90+ range

and that of high pressure sodium will be about 30.


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The huge range in lamps that is now available has resulted in the problem of differences in

appearance of coloured surfaces when illuminated by different lamps. The broader more continuous

the SPD of the lamp the better is the colour rendering of the lamp.

It has become necessary to classify lamps according to their colour rendering properties so

that lamps with poorer colour rendering such as Sodium lamps are not used where colour

appearance is important.

The CIE defines the Colour Rendering Index of a source as a measure of the degree to which the

perceived colours of an agreed set of Munsell tiles illuminated by the source agrees with the

perceived colours when illuminated by a reference source. The measure of the degree of

difference between colours of a Munsell tile when illuminated by the reference source and by the

test source is taken as the difference in their chromaticities as represented in the 1964 UCS

colour space.

The set of Munsell tiles comprises eight tiles which form a hue circle. They are:

• 7.5R 6/4 Light greyish red

• 5R 6/4 Dark greyish red

• 5GR 6/8 Strong yellow green

• 2.5G 6/4 Moderate yellowish green

• 1OBG 6/4 Lightish bluish green

• 5PB 6/8 Light blue

• 2.5P 6/8 Light violet

• 1OP 6/8 Light reddish purple

The reference illuminant is taken to be the black body radiator which has the closest

chromaticity coordinates to the test source, provided the test source has a CCT of 4000K or

less.

If the test source has a higher CCT then one of the phases of daylight is taken as the

reference illuminant. Again, the phase of daylight with chromaticity coordinates closest to the

test source.

After calculation of the colour difference for each tile (DE) a Special Colour Rendering Index is

formed:


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Data for lamps may give the CIE CRI group rather than the numerical value of CRI as given in

the Table 1.2 below:

Table 1.2 - General Colour Rendering Index Ra or CRI and Associated Colour Rendering Group

Colour Rendering Group Ra 1A Ra ≥ 90 18 80 ≤ Ra < 90

2 60 ≤ Ra <80

3 40 ≤ Ra < 60 4 20 ≤ Ra < 40

The use of lamps with high CRI will make the environment more colourful, providing a range

of colours are present with fairly saturated hues. Sources of the same CRI but different CCT

will not show up colours the same - sources of low CCT will tend to enhance reds and of high

CCT will tend to enhance blues.

AS/NZS 1158.1.1 Clause 2.8 and 3.1 Clause 2.7 state - The choice of light source should

be based on an analysis of all the factors relevant to the particular lighting scheme -

aesthetics and environmental, together with lamp mortality and lumen depreciation, cost,

energy use, etc. Compatible with the operational and economic requirements of the

lighting scheme, in general, the type of light source used should have the highest colour

rendering index (CR/) possible.

Applying this recommendation in practice results in white light generally finding

application in major urban centres and locations where aesthetics are important (MH/LED)

and on local roads (HPM/ fluorescent), whereas the generality of traffic routes will be lit by

HPS.

Applying this recommendation in practice results in white light generally finding application in major urban centres and locations where aesthetics are important (MH) and on local roads

(HPM/ fluorescent), whereas the generality of traffic routes will be lit by HPS.

Lamp Efficacy

This defined as:

Electrical Power in

Lamp Efficacy Lm/W

Luminous Flux


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Summary Table of Lamps

Lamp Type Luminous Efficacy (lm/W)

Colour Temp (K)

Colour Rendering Index(Ra)

Quartz Halogen 19 3000 100

Fluorescent 36W/PA

96 4000 80

Low Pressure 200 1700 --------

High pressure

Sodium 112 2000 30

Metal Halide 85 4000 80

LED Luminaire 120-150 3500-6500 70-80

Luminaire

The luminaire directs light in a complex light distribution (RL file) towards the surface of interest with directional intensities (I) in candelas (cd). The RL file is normally expressed in cd per 1000 lamp lumens and in the C, y angular coordinate system as shown Figure 1.2.


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The light distribution for a road lighting luminaire is such that light is emitted at high 𝛶 angles as possible to direct light along the roadway to give long spacings whilst controlling glare to the road user; these principles are demonstrated by Figure 1.3.

This results in the optimum light distribution, for economic luminaire spacing plus reasonable glare control, as employed for the semi-cutoff luminaire which finds general application for Category V lighting.

The driver's eyes are shielded by the roof of the car from the direct light from a luminaire

emitted at angles less than 𝛶 max (taken as -70°), therefore glare control is applied to the light

intensity distribution at 𝛶 angles greater than 70°, the light distribution runback.


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Figure 1.3 - The General Principles Controlling the Light Intensity Distribution of a Luminaire for Category V Lighting

Figure 1.4 shows the actual light distribution of a semi-cutoff Category V luminaire with a 250W HPS lamp. The dashed line shows the distribution, in elevation, along the road; note

the 𝛶 -angle is at 72.5°, for an aeroscreen luminaire this angle will be somewhat less. The full line shows the distribution, in azimuth, across the road; note that the maximum intensity is at C = 10°. This is the tow-in of the luminaire and is advantageous, especially for the lighting of wide roads, to achieve uniformity.

The total light output ratio (LOR) of this luminaire is 0.83, i.e. 83% of the light from the lamp is emitted by the luminaire, with almost all being downward (DLOR = 0.82). The higher the DLOR plus an optimum distribution the more efficient the luminaire will be in lighting the roadway. Some luminaire, aeroscreen in particular, will have DLOR somewhat less than 0.7.

A traditional luminaire cannot have a LOR much in excess of 0.80 since light is absorbed within the luminaire at reflecting surfaces, including at the emitting face. The LOR is not applicable for LED luminaires as the light source is not photometered separately. The luminaire as a whole is photometered and the intensities expresses in absolute photometry (in cd).

The light intensity distribution in the vertical plane parallel to the road edge ( the C0

plane), showing the general shape of the light intensity distribution; note the elevation of maximum intensity and the runback.


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350 0 10 20 30 40

-- = Gamma angle 72.5° DLOR = 0.82 -·- = C angle 10° ULOR = 0.01

Figure 1.4 - The Light Distribution of a Semi-Cutoff Category V Luminaire

A luminaire should be used which has a high IP rating in order to maximise the length of the maintenance cycle; currently IP = 6x for most general service luminaires. In addition the luminaire must comply with AS/NZS 60598.2.3 and may comply with SA/SNZ TS 1158.6 to ensure mechanical and electrical integrity and long service life.

llluminance

The illumination, or illumination as measure or specified, of a surface at a point is the light flux per unit area (E), in lumens per square metre, abbreviated to lux (Ix).

The basic photometric relation is:

E = I cosθ2

where θ is the angle of incidence of the light at the surface; this equation holds for any orientation of the surface, eg. horizontal or vertical.

This equation can be expressed as:

E = I cosθ3/ H2

(horizontal) or E = I cosθ sin2

θ / H2 (vertical)

where H is the luminaire mounting height.

The inverse square and cosine laws of illuminance effectively limit luminaire spacing

since the illumination of a point on a surface falls off sharply with the distance of it from

the light source. These laws influence the light distribution of a road lighting luminaire,

with the peak intensity being at a high 𝛶 angle to project light as far down the road as is

effective to maximise spacing without creating glare; see Figure 1.3.

llluminance is used solely in the performance specification of Category P lighting;

illuminance may be specified on a vertical plane in some situations, as well as on the

horizontal one, where lighting is installed as a crime deterrent. llluminance is specified for

some Category V lighting situations, e.g. intersections.


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Luminance The lumininance, or brightness as measure or specified, of a surface at a point of a

surface, for given directions of incidence and view, is intensity per unit area (L), in

candelas per square metre (cd/m2).

The basic photometric relation is:

L = E ρ / π

where ρ is the reflectance at the point for given directions of incidence and view. That directions of incidence and view are referred to implies that, firstly, an observer at a given

position, that views the bright surface from an angle of view at is implicit in the notion of

luminance (this is not so with regard to horizontal illuminance) and, secondly, that the

reflecting property of the road surface is complex, as illustrated in Figure 1.5.

(a) Smooth textured surface,e.g. R3. R4 & NZN4 (b) Rough textured surface,e.g. R1. R2 & NZN2

Figure 1.5 - The Appearance of a Light Patch on the Carriageway from a Single Luminaire for Two Different Road Surfaces

The light patches formed from the reflected light from the luminaire are elongated

towards the observer, i.e. the oncoming motorist. The head of the patch is indicative

of reflection from a matt surface whereas the elongation is indicative of mirror-like

reflections from the polished facets of the aggregate material of the surface. It can be

seen that the elongation is greatest for the smoother surface; if the road is very wet

from rain the reflecting surface will be, effectively, a water one and all reflections will

appear as long narrow streaks.

Luminance is used in the performance specification of Category V lighting for the straight road elements and in order to calculate luminance the complex reflectances

of the carriageway surface must be known.


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Reflectance

The reflectance is the reflection from a surface as a fraction of the incident light for

given angles of incidence and view. The reflectances of a road surface for a relevant range of angles of incidence and view are given in terms of the angular coordinate

system C, 𝛶, β (RS file), shown in Figure 1.6.

Angle α, the angle of view of vehicle driver, is taken as constant since it is small and

varies only over a small range for the carriageway area involved in the luminance calculation procedure.

Figure 1.6 - The CIE C,𝛶,β Angular Coordinate System Defining the Reflectance at a Point P

There are a series standard CIE road surfaces representative of the various surface textures for which there are reflectance tables; see Table 1.3.

The Table gives the physical description of the make-up of each surface of the series, so that the correct reflectance table can be matched to the surface.

These reflectance tables are included in the computer calculation software supplied with AS 1158.2; the default design surface in Australia is R3, an asphaltic concrete surface, unless known to the contrary, e.g. cement concrete.


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Table 1.3 The CIE Standard Surfaces and Allied Physical Structure

* The reflectance tables for these standard surfaces are taken from CIE 66 **The reflectance tables for these standard surfaces are given in AS/NZS 1158.2

CLASS DESCRIPTION

R1*

(a) Asphaltic road surfaces with at least 15% of artificial brightener

or with at least 30% of very bright anorthosites (arclyte,

labradorite or similar)

(b) Surface dressings with chippings where over 80% of the road

surface is covered and where chippings exist for a great man y

artificial brighteners or for I00% of very bright anorthosites

(c) Concrete road surfaces

R2* & NZR2**

(a) Surface dressings with harsh texture and with normal aggregates

(b) Asphaltic surface with 10% to 15% of artificial brighteners in

the mixture

(c) Coarse and harsh asphaltic concrete rich in gravel (>60%) and

with gravel sizes up to or greater than 10 mm

(d) Mastic asphalt (Gussasphalt) after dressing in new condition

R3*

(a) Asphaltic concrete (cold. mastic asphalt) with gravel sizes

up to 10 mm but with harsh texture (sandpaper)

( b) Surface dressings with coarse texture but polished

R4* & NZN4**

(a) Mastic asphalt (Gussasphalt) after some months of use

(b) Road surfaces with rather smooth or polished texture


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Notes

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basic lighting concepts, terms, units, abbreviations ...pavement/images/cpee/do… · reference: as...

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