Chapter 2. Fundamentals of light and vision

10



Fundamentals of light and vision



absorption by the metal causes emission of a photon,

each emitted electron arising from the absorption of a

single photon. The energy of the photon is proportional to the frequency of the electromagnetic radiation, given by the following equation:

E = hν

(2)

Energy of a photon = Planck’s constant × frequency

The constant of proportionality in the above

equation is a universal constant, Planck’s constant,

with a value of 6.626 × 10–34 Joule seconds.



Optics

The study of the behaviour of light is termed optics.

It is customary to group the problems that confront us

in this study in three different classes, and to

formulate for each a different set of rules as to how

light behaves. The science of optics is thus divided

into three branches. Physical optics is the study of

light on the assumption that it behaves as waves. A

stone dropped into a pond of still water causes a train

of waves to spread out in all directions on the surface

of the water. Such waves are almost completely

confined to the surface of the water, the advancing

wavefront being circular in form. A point source of

light, however, is assumed to emit energy in the form

of waves which spread out in all directions, and

hence, with light, the wavefront forms a spherical

surface of ever-increasing size. This wavefront may

be deviated from its original direction by obstacles in

its path, the form of the deviation depending on the

shape and nature of the obstacle. Phenomena which

can be explained under the heading of physical optics

include diffraction, interference and polarization

which have particular relevance to the resolving

power of lenses, lens coatings and special types of

filters, respectively and are considered in Chapters 4,

5 and 6.

The path of any single point on the wavefront

referred to above is a straight line with direction

perpendicular to the wavefront. Hence we say that

light travels in straight lines. In geometrical optics we

postulate the existence of light rays represented by

such straight lines along which light energy flows. By

means of these lines, change of direction of travel of

a wavefront can be shown easily. The concept of light

rays is helpful in studying the formation of an image

by a lens. Phenomena which are explained by this

branch of optics include reflection and refraction,

which form the basis of imaging by lenses and are

fully described in Chapter 4.

Quantum optics assumes that light consists essentially of quanta of energy and is employed when

studying in detail the effects that take place when

light is absorbed or emitted by matter, e.g. a

photographic emulsion or other light-sensitive

material.



The electromagnetic spectrum

Of the other waves besides light travelling in space,

some have shorter wavelengths than that of light and

others have longer wavelengths. The complete series

of waves, arranged in order of wavelengths, is

referred to as the electromagnetic spectrum. This is

illustrated in Figure 2.2. There is no clear-cut line

between one wave and another, or between one type

of radiation and another – the series of waves is

continuous.

The various types forming the family of electromagnetic radiation differ widely in their effect. Waves

of very long wavelength such as radio waves, for

example, have no effect on the body, i.e. they cannot

be seen or felt, although they can readily be detected

by radio receivers. Moving along the spectrum to

shorter wavelengths, we reach infrared radiation,

which we feel as heat, and then come to waves that

the eye sees as light; these form the visible spectrum.

Even shorter wavelengths provide radiation such as

ultraviolet, which causes sunburn, X-radiation, which

can penetrate the human body, and gamma-radiation,

which can penetrate several inches of steel. Both

X-radiation and gamma-radiation, unless properly

controlled, are harmful to human beings.

The energy values in Figure 2.2 were obtained

from a combination of the previous two equations for



Figure 2.2 Electromagnetic spectrum and the relationship

between wavelength, frequency and energy



Fundamentals of light and vision



wavelength (1) and for the energy of photons (2).

Thus rearranging equation (1) gives: ν = c/λ, which

on substituting in to equation (2) gives:

E = hc/λ



(3)



Since h and c are constants, equation (3) allows us to

determine the energy associated with each wavelength

(λ). Putting the known values for h an c in to equation

(3) gives the following equation from which it is easy

to determine the energy for any wavelength and

provides the basis for the values given in Figure 2.2:

E = 1.99 × 10–25/λ Joules



(4)



This equation is valid provided that the λ is expressed

in metres.

Energies also are quoted in electron volts, particularly for electronic transitions in imaging sensors (see

Figure 12.1). The conversion of Joules to electron

volts is given by multiplying by 6.24 × 1018.

The visible spectrum occupies only a minute part

of the total range of electromagnetic radiation,

comprising wavelengths within the limits of approximately 400 and 700 nanometres (1 nanometre (nm) =

10–9 metre (m)). Within these limits, the human eye

sees change of wavelength as a change of hue. The

change from one hue to another is not a sharp one, but

the spectrum may be divided up roughly as shown in

Figure 2.3. (See also Chapter 16.)

The eye has a slight sensitivity beyond this region,

to 390 nm at the short-wave end and about 760 nm at

the long-wave end, but for most photographic pur-



Figure 2.3



The visible spectrum expanded



11



poses this can be ignored. Shorter wavelengths than

390 nm, invisible to the eye, are referred to as

ultraviolet (UV), and longer wavelengths than

760 nm, also invisible to the eye, are referred to as

infrared (IR). Figure 2.3 shows that the visible

spectrum contains the hues of the rainbow in their

familiar order, from violet at the short-wavelength

end to red at the long-wavelength end. For many

photographic purposes we can usefully consider the

visible spectrum to consist of three bands only: blue–

violet from 400 to 500 nm, green from 500 to 600 nm

and red from 600 to 700 nm. This division is only an

approximation, but it is sufficiently accurate to be of

help in solving many practical problems, and is

readily memorized.



The eye and vision

The eye bears some superficial similarities to a

simple camera, as can be seen in Figure 2.4. It is

basically a light-tight box contained within the white

scelera, having a lens system consisting of the cornea

and the eyelens which focuses the incoming light rays

on the retina at the back of the eyeball to form an

inverted image. The iris controls the amount of light

entering the eye which, when fully open has a

diameter of approximately 8 mm in low light levels

and around 1.5 mm in bright conditions. It has

effective apertures from f/11 to f/2 and a focal length

of around 16 mm. The retina comprises a thin layer of

cells containing the light-sensitive photoreceptors.

The electrical signals from light sensitive receptors

are transmitted to the brain via the optic nerve. These

light-sensitive receptors consist of two types – rods

and cones – which are not distributed uniformly

throughout the retina, as shown in Figure 2.5, and are

responsive at low light levels (scotopic or night



Figure 2.4 Cross-section through the human eyeball

(adapted from Colour Physics for Industry, R. McDonald,

ed.)



12



Fundamentals of light and vision



Figure 2.5



The distribution of rods and cones



vision) and high light levels (photopic or day vision),

respectively. Also, the cones are responsible for

colour vision, which is explained in Chapter 16.

From Figure 2.5 it can be seen that there is a very

high density of cones at the fovea but no rods, and the

gap or blind spot where there are no rods or cones is

where the optic nerve is located. At the centre of the

retina is the fovea which is the most sensitive area of

around 1.5 mm in diameter into which are packed the

highest number of cones, more than 100 000.

The mechanisms of vision which involve the

organization of the receptors, the complex ways in

which the signals are generated, organized, processed

and transmitted to the brain are beyond the scope of

this book. However, they give rise to a number of

visual phenomena which have been extensively

studied and have a number of consequences in our

understanding and evaluation of imaging systems. A

few examples of important aspects of vision are

outlined below, although it must be emphasized that

these should not be considered in isolation. Colour

has not been included here, partly for simplicity and

because those aspects of colour of particular relevance to imaging are considered in later chapters.



Dark and light adaptation

When one moves from a brightly lit environment to a

dark or dimly lit room, it immediately appears to be

completely dark, but after about 30 minutes the visual

system adapts as there is a gradual switching from the

cones to the rods and objects become discernible.

Light adaptation is the reverse process with the same

mechanism but takes place more rapidly, within about

5 minutes.



Luminance discrimination

Discrimination of luminance (changes in luminosity

– lightness of an object or brightness of a light

source) is governed by the level. As luminance

increases, we need larger changes in luminance to

perceive a just noticeable difference, as shown in

Figure 2.6.

This is known as the Weber–Fechner Law and

over a fairly large luminance range the ratio of the

change in luminance (ΔL) to the luminance (L) is a



Fundamentals of light and vision



Figure 2.6



13



Response and intensity

Figure 2.8 Spatial contrast sensitivity function for the

human visual system



constant of around 0.01 under optimum viewing

conditions:

ΔL/L = constant

The ratio of light intensities is the significant

feature of our perception. Figure 2.7 gives a visual

indication of reflected light intensities in which each

step increases by an equal ratio of 1, 2, 4, 8, i.e., a

logarithmic scale.



Spatial aspects

The human visual system’s (HVS) ability to discriminate fine detail has been determined in terms of its

contrast sensitivity function (CSF). The CSF is

defined as the threshold response to contrast where



contrast (or modulation) is the difference between the

minimum (Lmin ) and maximum (Lmax ) luminances of

the stimulus divided by their sum:

Contrast = (Lmax – Lmin )/(Lmax + Lmin )

A typical CSF for luminance shown by the HVS is

shown in Figure 2.8.

Because of the distribution of rods and cones in the

retina the CSF for the colour channels are different

from those shown in Figure 2.8, with lower peaks and

cut-off frequencies. For luminance the HVS has a

peak spatial contrast sensitivity at around 5 cycles per

degree and tends to zero at around 50 cycles per

degree.



White light and colour mixtures

More than three hundred years ago Newton discovered that sunlight could be made to yield a variety

of hues by allowing it to pass through a triangular

glass prism. A narrow beam of sunlight was dispersed



Figure 2.7 Equal steps in lightness, each step differing by

an equal ratio



Figure 2.9



Dispersion of white light by a prism



14



Fundamentals of light and vision



Table 2.1 Additive mixing of blue, green and red light

Colours of light mixed



Visual appearance



Blue + green

Blue + red

Green + red

Blue + green + red



Blue/green, or cyan

Red/purple, or magenta

Yellow

White



Table 2.2 Subtractive mixing of cyan, magenta and yellow

colorants

Colours mixed



into a band showing the hues of the rainbow. These

represent the visible spectrum, and the experiment is

shown diagrammatically in Figure 2.9. It was later

found that recombination of the dispersed light by

means of a second prism gave white light once

more.

Later experiments showed that by masking off

parts of the spectrum before recombination a range

of colours could be produced. Young showed that if

small parts of the spectrum were selected in the

blue, green and red regions, a mixture of appropriate

amounts of blue, green and red light appeared white.

Fifty years after Young’s original experiments (in

1802), Helmholtz was successful in quantifying

these phenomena. Variation of the blue, green and

red contents of the mixture resulted in a wide range

of colours. Almost any colour could be produced,

including magenta, or purple, which did not appear

in the visible spectrum. The results of mixing blue,

green and red light are listed in Table 2.1 and

illustrated in Figure 2.10.

The results of mixing blue, green and red light

suggested that the human eye might possess three

types of colour sensitivity, to blue, green and red

light respectively. This triple-sensitivity theory is



Figure 2.10

colorants



Visual appearance



Cyan (white-red)

Magenta (white-green)

Yellow (white-blue)

Cyan + magenta

Cyan + yellow

Magenta + yellow

Cyan + magenta + yellow



Blue + green

Blue + red

Yellow

Blue

Green

Red

Black



called the Young–Helmholtz theory of colour

vision. It provides a fairly simple explanation for

the production of any colour from appropriate

proportions of these primaries. This type of colour

mixing is applied in cathode ray tube displays

which have red, green and blue light-emitting

phosphors in their faceplates, whereas subtractive

colour mixtures are applied in most colour photographic materials, colour hard copy output devices

and liquid crystal displays. Subtractive colour mixing, which involves the overlaying of cyan,

magenta and yellow colorants, is shown in Table

2.2.



Bibliography

Bruce, V., Green, P.R. and Georgeson, M.A. (1996)

Visual Perception: Physiology, Psychology and

Ecology. Psychology Press, Hove.



Additive mixing blue, green and red light and subtractive colour synthesis using yellow, magenta and cyan



Fundamentals of light and vision



Falk, D. and Stork, D. (1986) Seeing the Light: Optics

in Nature, Photography, Color, Vision and Holography. Harper and Row, New York.

Jackson, R., MacDonald, L. and Freeman, K. (1994)

Computer Generated Colour. Wiley, Chichester.



15



McDonald, R. (ed.) (1987) Colour Physics for

Industry. Society of Dyers and Colorists,

Bradford.

Ray, S. (1994) Applied Photographic Optics, 2nd edn.

Focal Press, London.



3



Photographic light sources



Photographs are taken by the agency of light

travelling from the subject to the photoplane in the

camera. This light usually originates at a source

outside the picture area and is reflected by the subject.

Light comes from both natural and artificial sources.

Natural sources include the sun, clear sky and clouds.

Artificial sources are classified by the method used to

produce the light (see Table 3.1).



Characteristics of light sources

Light sources differ in many ways, and the selection

of suitable sources for photographic purposes is based

on the order of importance of a number of significant

characteristics. A summary of properties of photographic sources is given in Table 3.2. The more

important characteristics are discussed in detail

below.



Spectral quality

The radiation from most sources is a mixture of light

of various wavelengths. The hue of the light from a

source, or its spectral quality, may vary depending on

the distribution of energy at each wavelength in its

spectrum. Most of the sources used for photography

emit what is usually termed white light. This is a

vague term, describing light that is not visibly

deficient in any particular band of wavelengths, but

not implying any very definite colour quality. Most



white-light sources vary considerably among themselves and from daylight. Because of the perceptual

phenomenon of colour constancy these differences

matter little in everyday life, but they can be very

important in photography, especially when using

colour materials or where there is ‘mixed’ lighting. It

is essential that light quality is described in precise

terms. Light is a specific region of the electromagnetic spectrum and is a form of radiant energy.

Colour quality may be defined in terms of the

spectral power distribution (SPD) throughout the

spectrum. There are several ways this can be

expressed, with varying degrees of precision. Each

method has its own advantages, but not all methods

are applicable to every light source.



Spectral power distribution curve

Using a spectroradiometer, the spectral power distribution of light energy can be measured and

displayed as the SPD curve. Curves of this type are

shown in Figure 3.1 for the sun and in Figure 3.2 for

some other sources. Such data show clearly small

differences between various forms of light. For

example, the light sources in Figure 3.1 seen

separately would, owing to colour constancy effects,

probably be described as white, yet the curves are

different. Light from a clear blue sky has a high blue

content, while light from a tungsten lamp has a high

red content. Although not obvious to the eye such



Table 3.1 Methods of producing light

Method



Source of light



Examples



Burning



Flame from flammable materials



Candles, oil lamps, matches, magnesium ribbon, flash powder

and flash-bulbs



Heating



Carbon or tungsten filament



Incandescent electric lamps, e.g. domestic lamps, studio

lamps, tungsten-halogen lamps



Electric spark or arc



Crater or flame of arc



Carbon arcs, spark gaps



Electrical discharge



Gas or metallic vapour



Electronic flash, fluorescent lighting, metal halide lamps



Luminescence



Phosphors



Sodium and mercury vapour lamps



16



Table 3.2 The properties of some light sources used in photography

Source



Type of

spectrum:

C. continuous

L. line

B. line plus

continuum



Colour

temperature

(kelvins)



Daylight

Tungsten filament lamps:

General service

Photographic

Photoflood

Projector



C



2760–2960

3200

3400

3200



Tungsten–halogen lamps



C



2700–3400



Mercury vapour discharge lamps:

Low pressure

High pressure



L

L



Fluorescent lamps



B



Sodium vapour discharge lamps:

Low pressure

High pressure



L

L



Metal halide lamps



B



Pulsed xenon lamps



B



Flash bulbs



C



3800 or 5500



Electronic flash



B



*6000



Average

lamp life

(hours)



2000–20 000



C

C

C

C



Efficacy

(lumens

per watt)



*correlated value

†

typical value



Light output:

H. high

M. moderate

L. low



Constancy

of output:

P. poor

G. good

E. excellent

V. variable



Size of unit:

S. small

M. medium

L. large



Costs:

L. low

M. moderate

H. high

Initial



Running



H–L

†



Ease of

operation:

D. difficult

M. moderate

S. simple



P



L



L



S



13

20

†

40

†

20



1000

100

3–10

25–100



L

L

M

M



P

G

P

G



L

M

L

M



L

M

M

M



M

M

S

S



S

S

S

S



15–35



25–200



M



E



M



H



S,M



S



6

35–55



1000–2000



M

H



G

G



H

H



L

L



M

M



S

S



62



7000–8000



M



P



M



L



L



M



170

100



10–16 000



H

H



E

E



M

H



L

L



M

M



M

M



5600–6000



85–100



200–1000



H



E,V



H



H



M



M



5600



25–50



300–1000



H



G



H



L



M,S



M



*



3000–6500



†



†



used once only

40



H



E



L



H



S



S



M



G



H,M,L



L



L,M,S



M



18



Photographic light sources



Figure 3.1 Spectral power distribution curves of sunlight,

light from a blue sky and light from a tungsten lamp



differences can be clearly shown by colour reversal

film. Colour film has to be balanced for a particular

form of lighting.

Analysis of SPD curves show that there are three

main types of spectrum emitted by light sources. The

sources in Figure 3.1 have continuous spectra, with

energy present at all wavelengths in the region

measured. Many sources, including all incandescentfilament lamps, have this type of spectrum. Other

sources have the energy confined to a few narrow

spectral regions. At these wavelengths the energy is

high, but elsewhere it is almost nil. This is called a

discontinuous or line spectrum, and is emitted

typically by low-pressure discharge lamps such as

sodium- and mercury-vapour lamps.

A third type of spectrum has broad bands of energy

with a continuous background spectrum or continuum

of varying magnitude, and is given by discharge

sources by increasing the internal pressure of the

discharge tube, e.g. a high-pressure mercury-vapour

lamp. Alternatively, the inside of the discharge tube

may be coated with ‘phosphors’ that fluoresce, i.e.

emit light at longer wavelengths than the spectral

lines which stimulate them. Another method is to

include gases in the tubes such as xenon or argon, and

metal halide vapour.



source is by means of its colour temperature. This is

defined in terms of what is called a Planckian

radiator, a full radiator or simply a black body. This

is a source emitting radiation whose SPD depends

only on its temperature and not on the material or

nature of the source.

The colour temperature of a light source is the

temperature of a full radiator that would emit

radiation of substantially the same spectral distribution in the visible region as the radiation from the

light source. Colour temperatures are measured on the

thermodynamic or Kelvin scale, which has a unit of

temperature interval identical to that of the Celsius

scale, but with its zero at –273.15 °C.

The idea of colour temperature can be appreciated

by considering the progressive change in colour of

the light emitted by a piece of metal as its temperature

is raised, going from dull black through red and

orange to white heat. The quality of the light emitted

depends on the temperature of the metal. Luminous

sources of low colour temperature have an SPD

relatively rich in red radiation. With progression up

the colour scale the emission of energy is more

balanced and the light becomes ‘whiter’. At high

values the SPD is rich in blue radiation. It is

unfortunate that reddish light has been traditionally

known as ‘warm’ and bluish light as ‘cold’, as the

actual temperatures associated with these colours are

the other way round.

The idea of colour temperature is strictly applicable only to sources that are full radiators, but in

practice it is extended to those that have an SPD

approximating to that of a full radiator or quasiPlanckian source, such as a tungsten-filament lamp.

The term is, however, often applied incorrectly to

fluorescent lamps, whose spectra and photographic

effects can be very different from those of full

radiators. The preferred term describing such sources

is correlated colour temperature, which indicates a

visual similarity to a value on the colour temperature

scale (but with an unpredictable photographic effect,

particularly with colour reversal film).

The approximate colour temperatures of light

sources used in photography are given in Table 3.3.

In black-and-white photography, the colour quality

of light is of limited practical importance. In colour

photography, however, it is vitally important, because

colour materials and focal plane arrays (FPA) are

balanced to give correct rendering with an illuminant

of a particular colour temperature. Consequently, the

measurement and control of colour temperature must

be considered for such work, or the response of the

sensor adjusted, usually termed ‘white balance’.



Colour temperature



Colour rendering



For photographic purposes a preferred method of

quantifying the light quality of an incandescent



With fluorescent lamps, which vary greatly in

spectral energy distribution, covering a wide range of



Photographic light sources



Figure 3.2



19



Spectral power distribution curves typical of some of the artificial light sources used in photography



Table 3.3 Colour temperatures of some common light sources

Light source

Standard candle

Dawn sunlight

Vacuum tungsten lamp

Acetylene lamp (used in early sensitometric work)

Gas-filled tungsten lamp (general service)

Warm-white fluorescent lamp

Photographic lamp

Photoflood lamp

Clear flashbulb

Daylight fluorescent lamp

Mean noon sunlight

Photographic daylight

Blue flashbulb

Electronic flashtube

Average daylight (sunlight and skylight combined)

Colour-matching fluorescent lamp

Blue sky



Approximate colour temperature



Mired value



1930 K

2000 K

2400 K

2415 K

2760–2960 K

3000 K

3200 K

3400 K

3800 K

4500 K

5400 K

5500 K

6000 K

6000 K

6500 K

6500 K

12 000–18000 K



518

500

417

414

362–338

333

312

294

263

222

185

182

167

167

154

154

83–56



20



Photographic light sources



correlated colour temperatures, the results given by

two lamps of nominally the same properties may be

quite different if used for visual colour matching or

for colour photography. Various objective methods

have been devised to give a numerical value to the

colour rendering given by such sources as compared

with a corresponding full radiator, or with visual

perception. A colour rendering index (CRI) or value

is defined based on the measurement of luminance in

some six or eight spectral bands and compared with

the total luminance, coupled with weighting factors, a

value of 100 indicating ideal performance. Typical

values vary from 50 for a ‘warm-white’ type to

greater than 90 for a ‘colour-matching’ version.



Percentage content of primary hues

For many photographic purposes, the visible spectrum can be considered as consisting of three main

bands: blue, green and red. The quality of light from

a source with a continuous spectrum can be approximately expressed in terms of the percentages in

which light of these three hues is present. The method

is imprecise; but it is the basis of some colour

temperature meters, where the ratios of blue-green

and red-green content are compared. (The same

principles are used to specify the colour rendering

given by a lens, as absorption of light at the blue end

of the spectrum is common in optical glass.)



Measurement and control of colour

temperature

In colour photography, the colour temperature (CT)

of the light emitted by all the separate sources

illuminating a subject must agree in balance with that

for which the process is being used. The tolerance

permissible depends on the process and to some

extent on the subject. A departure of 100 K from the

specified value by all the sources (which may arise

from a 10 per cent variation in supply voltage) is

probably the maximum tolerable for colour reversal

material balanced for a colour temperature of around

3200 K. Colour negative material (depending on how

it is CT balanced) may allow a greater departure than

this, because a certain amount of colour correction is

possible at the printing stage.

A particular problem is that of mixed lighting,

where part of the subject may be unavoidably

illuminated by a light source of markedly different

colour quality from the others. A localized colour cast

may then appear in the photograph. Another example

is the use of tungsten lamps fitted with blue filters to

match daylight for fill-in purposes, where some



mismatch can occur. Note that both blue flashbulbs

and electronic flash may be used successfully as fillin sources when daylight is the main illuminant. A

visual comparison of the colour quality of two light

sources is possible by viewing the independently

illuminated halves of a folded sheet of white paper

with its apex pointing towards the observer. Any

visually observable difference in colour would be

recorded in a photograph, so must be corrected (see

later).

Instrumental methods such as colour temperature

meters are more convenient. Most incorporate filtered

photocells which sample specific regions of the

spectrum, such as red, green and blue. A direct

readout of colour temperature is given, usually

together with recommendations as to the type of

light-balancing or colour-correction filters needed for

a particular type of film.

A matrix array of several hundred CCD photocells

filtered to blue, green and red light, together with

scene classification data can also be used in-camera

to measure the colour temperature of a scene.

The CT balance of colour films to illuminants is

specified by their manufacturers. The colour temperature of a lamp may be affected by the reflector

and optics used; it also changes with variations in

the power supply and with the age of the lamp. To

obtain light of the correct quality, various precautions are necessary. The lamps must be operated at

the specified voltage, and any reflectors, diffusers

and lenses must be as near to neutral in colour as

possible. Voltage control can be by a constantvoltage transformer. The life of filament lamps can

be extended by switching on at reduced voltage and

arranging the subject lighting, then using the correct

full voltage only for the actual exposure. Variable

resistances or solid-state dimmer devices can be

used with individual lamps for trim control. To raise

or lower the colour temperature by small amounts,

light-balancing filters may be used over the lamps.

Pale blue filters raise the colour temperature while

pale yellow or amber ones lower it.

As conventional tungsten-filament lamps age, the

inner side of the envelope darkens from a deposit of

tungsten evaporated from the filament. Both light

output and colour temperature decrease as a result.

Bulb replacement is the only remedy; in general, all

bulbs of a particular set should be replaced at the

same time. Tungsten–halogen lamps maintain a more

constant output throughout an extended life, as

compared with ordinary filament lamps.

To compensate for the wide variations encountered

in daylight conditions for colour photography, camera

filtration may be necessary by means of lightbalancing filters of known mired shift value as

defined below. To use colour film in lighting

conditions for which it is not balanced, colour

conversion (CC) filters with large mired shift values

are available (see Chapter 11).