SECTION 1

I THE PHYSICS OF LIGHT PRODUCTIONv

The American Standards Association and the Illuminating Engineering

Society define light asiradiant energy evaluated according to its capacity to

'produce visual sensationS Radiant energy of the proper wavelength makesvisible anything from which it is emitted or reflected in sufficient quantity

to activate the receptors in the eye.

( Several concepts of the nature of radiant energy have been advanced. 1

They are

:

A. The corpuscular theory advocated by Newton, based on these premises:

1

.

That luminous bodies emit radiant energy in particles.~~ 2. That these particles are intermittently ejected in straight lines.

— 3. That the particles act on the retina of the eye stimulating the optic

nerves to produce the sensation of light.

— B. The wave theory) based on these premises:~ 1. That light is the resultant of molecular vibration in the luminous— material.

2. That vibrations are transmitted through the ether as wavelike move-ments (comparable to ripples in water).

3. That the vibrations thus transmitted act on the retina of the eye

stimulating the optic nerves to produce visual sensation.

C. The electromagnetic theory,

2 based on these premises:

— 1. That luminous bodies emit light as a form of radiant energy.

--2. That this radiant energy is transmitted in the form of electromagnetic

-_ waves.

3. That the electromagnetic waves act upon the retina of the eye thus

stimulating the optic nerves to produce the sensation of light.

D. The quantum theory, a modern form of the corpuscular theory, based

on these premises:"~ 1. That energy is emitted and absorbed in discrete quanta.

2. That the magnitude of each quantum is hv,

where h — 6.547 X 10~27 erg sec (Planck's constant)

and v = frequency in cycles per second.

E. The theory of wave mechanics first proposed by Schrodinger in 1925

in an attempt to reach an harmonious compromise between the quantumand the wave theories.

1. It utilizes wave characteristics and quanta particles as the need arises

in the solution of problems.

2. The mathematics involved is too complicated for present application

to illuminating engineering problems.

Note: References are Listed at the end of each section.

1-1

1-2 I E S LIGHTING HANDBOOK

Until such time as new data or concepts are available the quantumand the electromagnetic wave theories will unquestionably be used as thebasis of continued research in light phenomena. The electromagneticwave theory provides a convenient explanation of those characteristicsof radiant energy most frequently of concern to the illuminating engineer.Radiant energy may be evaluated in a number of different ways; two

of these are:

1. Radiant flux—the time rate of the flow of any part of the radiantenergy spectrum measured in ergs per second.

2. Luminous flux—the time rate of the flow of the luminous parts of theradiant energy spectrum measured in lumens.

Light and the Energy Spectrum

The wave theory permits a convenient graphical representation of

radiant energy in an orderly arrangement according to its wavelength.This arrangement is called a spectrum (Fig. 1-1). It is useful in indicatingthe relationship between various radiant energy wavelength regions.

Such a graphical representation must not be construed to indicate thateach region of the spectrum is divided from the others in any physical waywhatsoever. Actually there is a gradual transition from one region toanother.

FREQUENCY IN CYCLES PER SECOND

\

COSM IC RAYS

GAMMA RAYSX-RAYS

HARD SOFT HERTZIAN WAVESVACUUM U.V.

ULTRAVIOLET •—** INFRAREDNEAR FAR

- DIRECTIONALRADIO (RADAR)

FM

TELEVISION

VIOLET BLUE GREEN YELLOW RED_ SHORT

WAVE

0.5 0.6 0.7

WAVELENGTH IN MICRONS

I X-UNn" I ANGSTROM

0.76 BROADCAST

I CM I METER I KILOMETER

POWERTRANS-MISSION

,0-12 10 -I0 io-8 I0"6 10"4 10"2 ] io2WAVELENGTH IN CENTIMETERS

I04 I06 I0 8 10'0

FIG. 1-1. The radiant energy (electromagnetic) spectrum.

The known limits of the radiant energy spectrum extend over a range

of wavelengths varying from a few micromicrons (10~10 cm) to one hundredthousand miles (1.6 X 10 10 cm). Radiant energy in the visible spectrumhas wavelengths betAveen 0.38 X 10~4 and 0.76 X 10~4 cm.The Angstrom unit (A), the micron (/x), and the millimicron (m/x) are

commonly used units of length in the visible spectrum band. The rela-

tionship of several units for measuring wavelength is given in Table 1-1.

PHYSICS OF LIGHT PRODUCTION 1-3

All forms of radiant energy are transmitted at the same rate of speed

in vacuum (186,300 miles per second). However, each form differs in

wavelength and thus in frequency. The wavelength and velocity may be

altered materially by the medium through which it passes, but frequency

is fixed independently of the medium. Thus, through the equation:

V = n\v

where V = velocity of waves (cm per sec)

n — (index of refraction)

X = wavelength (cm)

v — frequency (c per sec)

it is possible to determine the velocity of radiant energy and also to indicate

the relationship between frequency and wavelength.

Table 1-2 gives the velocity of light in different media for a frequency

corresponding to a wavelength of 0.589 micron in air.

Table 1-1. Conversion Table for Units of Length

Multiply Number C/3

of OSwH

(AwH

PiW

& O w w W Hp4 fc C/3

gW

To Obtain \ Hen

oPS H H S

Number of \

1 \O u H (J W kJ J g% § § g S S w

ANGSTROMS 1 104 2.540XIO 5

2.540

X1083.04SX109

1.609XIO"

10 7 108 lOU

MICRONS 10-4 1 2.540xio

2.540

X1043.048xio*

1.609

X10'103 104 109

MILS 3.937 3.937 1 103 1.2 6.336 3.937 3.937 3.937

X10-6 X10-2 X104 XIO 7 xio X102 X10 7

INCHES 3.937 3.937 10-3 1 12 6.336 3.937 3.937 3.937xio-» XlO-s XW X10-2 xio-' X104

FEET 3.281 3.281 8.333 8.333 1 5.280 3.281 3.281 3.281

X10-' 8 X10"s X10-5 X10-2 X10» X10-3 X10-J X103

MILES 6.214 6.214 1.578 1.57S 1.894 1 6.214 6.214 6.214XlO-n X 10-19 XIO"8 XlO-s X10-4 xio-' xio-« xio-'

MILLIMETERS lO-' 10-3 2.540 2.540 3.04S 1.609 1 10 106

X10-2 X10 X102 XIO8

CENTIMETERS 10-8 10-4 2.540XIO"'

2.540 3.048xio

1.609

X10*0.1 1 10»

KILOMETERS 10-" 10-9 2.540XIO"'

2.540xio-»

3.048

X10-41.609 io-« 10"s 1

Table 1-2. Velocity of Light for a Wavelength of 0.589 Micron

(Sodium D-lines)

MEDIUM VACUUM AIR (760 mm 0°C) CROWN GLASS WATER

VELOCITY(cm per see)

(2.99776 ± 0.00004) X 10" 2.99708 X lOio 1.98212 X 10" 2.24903X1010

1-4 I E S LIGHTING HANDBOOK

Luminosity of Radiant Energy

The apparent differences in character between radiant energy of various

wavelengths are in reality differences in ability of various receiving anddetecting devices. 3

The reception characteristics of the human eye have been subject to ex-

tensive investigations. The results may be summarized as follows:

1. The spectral response characteristic of the human eye varies betweenindividuals, with time, and with the age and the state of health of any indi-

vidual, to the extent that the selection of any individual to act as a standard

observer is not scientifically feasible.

2. However, from the wealth of data available, a luminosity curve

has been selected for engineering purposes which represents the average

human observer. This curve may be applied mathematically to the solu-

tion of photometric problems so as to eliminate the disadvantages related

to all measurements dependent on the accurate reporting of human sensa-

tions. (See also Section 2.)

Recognizing these facts, the Illuminating Engineering Society in 1923

and the International Commission on Illumination (I.C.I.) in 1924 adopted

the standard luminosity factors of Table 1-3 from which the luminosity

curve of Fig. 1-2 was plotted.

Table 1-3. Standard Luminosity Factors

(Relative to unity at 0.554 micron wavelength)*'

WAVELENGTH FACTOR WAVELENGTH FACTOR WAVELENGTH FACTOR(micron) (micron) (micron)

0.380 0.00004 0.510 0.503 0.640 0.175.390 .00012 .520 .710 .650 .107

.400 .0004 .530 .862 .660 .061

.410 .0012 .540 .954 .670 .032

.420 .0040 .550 .995 .680 .017

.430 .0116 .560 .995 .690 .0082

.440 .023 .570 .952 .700 .0041

.450 .038 .580 .870 .710 .0021

.460 .060 .590 .757 .720 .00105

.470 .091 .600 .631 .730 .00052

.480 .139 .610 .503 .740 .00025

.490 .208 .620 .381 .750 .00012

.500 .323 .630 .265 .760 .00006

1 Luminosity factor = 1.0002 for 0.555 micron is maximum.

The standard luminosity curve represents an average characteristic

from which the characteristic of any individual may be expected to vary.

Goodeve's data (Fig. 1-3) indicate that most human observers are capable

of experiencing a visual sensation upon exposure to radiation of infrared

wavelengths (longer than 0.76 micron). It also is known that observers

exhibit a slight response to ultraviolet wavelengths (shorter than 0.38

micron).

PHYSICS OF LIGHT PRODUCTION 1-5

VIOLET BLUE GREEN1.0

0.9

0.8

£0.55DJ

0.4

>

<0.3_lLU<r

0.2

0.1

0.38 0.42 0.46 0.50 0.54 0.58 0.62 0.66 0.70 0.74

WAVELENGTH OF RADIANT ENERGY IN MICRONS

1 micron =10,000 angstroms = 1/10,000 centimeter

FIG. 1-2. The standard (I.C.I.) luminositycurve shows the relative capacity of radiantenergy of various wavelengths to producevisual sensation.

10-2 I0"4

0.70 0.75 0.80 0.85 0.90WAVELENGTH IN MICRONS

FIG. 1-3. Goodeve's investi-

gations reveal that high flux con-centrations of wavelengths justoutside the "visible region" arecapable of producing visual sen-sations. 7

Photoelectric Effect

This phenomenon, which may be observed when a clean metal surface

is illuminated, is the liberation of electrons from the surface atoms. If

the surface is connected as a cathode in an electric field (Fig. 1-4) the lib-

erated electrons will flow to the anode creating a photoelectric current.

An arrangement of this sort may be used as an illumination meter and can

be calibrated in footcandles.

CATHODE(METAL PLATE)""-,

--X LIGHT QUANTUM(ENERGY = hV)

—ELECTRON

(ENERGY = Vz mV2 = hV-E )

ANODE

ENERGY TO-'RELEASE

ELECTRON =E

FIG. 1-4. By the photoelectric effect, electrons may be liber-

ated from illuminated metal surfaces. In an electric field thesewill flow to an anode and create an electric current which may bedetected by means of a galvanometer.