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Lecture 2

Spectral response of natural light

Three colour theory

Additive and subtractive mixing of colours Luminance Hue and Saturation

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Before going into details of encoding and

decoding the picture signal, it is essential to gain

a good understanding of the fundamentalproperties of light. It is also necessary to

understand mixing of colours to produce different

hues on the picture screen together with

limitations of the human eye to perceive them.

Furthermore a knowledge of the techniques

employed to determine different colours in a

scene and to generate corresponding signalvoltages by the colour television camera is

equally essential.

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NATURAL LIGHT When white light from the sun is examined it is

found that the radiation does not consist of asingle wavelength but it comprises of a band offrequencies.

The visible spectrum extends over only an octave

that centers around a frequency of the order of 5 1014Hz.

When radiation from the entire visible spectrumreaches the eye in suitable proportions we see

white light. If, however, part of the range is filtered out, and

only the remainder of the visible spectrum reachesthe eye, we see a colour.

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COLOUR PERCEPTION

All objects that we observe are focusedsharply by the lens system of the eye on itsretina.

The retina which is located at the back side ofthe eye has light sensitive organs whichmeasure the visual sensations.

The retina is connected with the optic nervewhich conducts the light stimuli as sensed bythe organs to the optical centre of the brain.

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The light sensitive organs are of two typesrodsand cones.

The rods provide brightness sensation and thus

perceive objects only in various shades of greyfrom black to white.

The cones that are sensitive to colour are broadlyclassified in three different groups.

One set of cones detects the presence of blue colour inthe object focused on the retina

the second set perceives red colour and

the third is sensitive to the green range.

Each set of cones, may be thought of as being tuned toonly a small band of frequencies and so absorb energyfrom a definite range of electromagnetic radiation toconvey the sensation of corresponding colour or rangeof colour.

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The combined relative luminosity curve

showing relative sensation of brightness

produced by individual spectral coloursradiated at a constant energy level is shown in

Fig below.

It will be seen from the plot that thesensitivity of the human eye is greatest for

green light, decreasing towards both the red

and blue ends of the spectrum.

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THREE COLOUR THEORY

All light sensations to the eye are divided (providedthere is an adequate brightness stimulus on theoperative cones) into three main groups.

The optic nerve system then integrates the different

colour impressions to perceive the actual colour of theobject being seen.

This is known as additive mixing and forms the basis ofany colour television system.

A white colour is then perceived by the additive mixingof the sensations from all the three sets of cones.

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Mixing of Colours

Mixing of colours can take place in twowayssubtractive mixing and additive mixing.

In subtractive mixing, reflecting properties of

pigments are used, which absorb all wavelengthsbut for their characteristic colour wavelengths.

Since the pigments are not quite saturated (purein colour) they reflect a fairly wide band of

wavelengths.

This type of mixing takes place in painting andcolour printing.

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In additive mixing which forms the basis of colour

television, light from two or more colours obtained

either from independent sources or through filterscan create a combined sensation of a different

colour.

Thus different colours are created by mixing pure

colours and not by subtracting parts from white.

The additive mixing of three primary coloursred,

green and blue in adjustable intensities can create

most of the colours encountered in everyday life. The impression of white light can also be created

by choosing suitable intensities of these colours.

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GrassmansLaw

The eye is not able to distinguish each of the

colours that mix to form a new colour butinstead perceives only the resultant colour.

Thus the eye behaves as though the output of

the three types of cones are additive. The property of the eye of producing a

response which depends on the algebraic sumof the red, green and blue inputs is known as

GrassmansLaw.

White has been seen to be reproduced byadding red, green and blue lights.

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Tristimulus Values of Spectral Colours

Based on the spectral response curve of Fig.and extensive tests with a large number of

observers, the primary spectral colours and

their intensities required to produce differentcolours by mixing have been standardized.

The component values (or fluxes) of the three

primary colours to produce various othercolours have also been standardized and are

called the tri-stimulus values of the different

spectral colours.

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The reference white for colour television has

been chosen to be a mixture of 30% red, 59%

green and 11% blue. These percentages for the light fluxes are

based on the sensitivity of the eye to different

colours. Thus one lumen (lm) of white light = 0.3 lm of

red + 0.59 lm of green + 0.11 lm of blue.

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b d h if h i f

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It may be noted that if the concentration of

luminous flux is reduced by a common factor

from all the constituent colours, the resultant

colour will still be white, though its level of

brightness will decrease.

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LUMINANCE, HUE AND SATURATION

Any colour has three characteristics to specify its visual

information. These are (i) luminance, (ii) hue or tint, and (iii) saturation. These are defined asfollows:

(i) Luminance or Brightness

This is the amount of light intensity as perceived by theeye regardless of the colour.

In black and white pictures, better lighted parts havemore luminance than the dark areas.

Thus on a monochrome TV screen, dark red colour will

appear as black, yellow as white and a light blue colouras grey.

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(ii) Hue

This is the predominant spectral colour of the

received light.

Thus the colour of any object is distinguished by its

hue or tint.

The green leaves have green hue and red tomatoeshave red hue.

Different hues result from different wavelengths of

spectral radiation and are perceived as such by the

sets of cones in the retina.

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(iii) Saturation This is the spectral purity of the colour light.

Since single hue colours occur rarely alone, thisindicates the amounts of other colours present.

Thus saturation may be taken as an indication of howlittle the colour is diluted by white.

A fully saturated colour has no white.

As an example. vivid green is fully saturated andwhen diluted by white it becomes light green.

The hue and saturation of a colour put together isknown as chrominance. Note that it does not contain

the brightness information. Chrominance is also called chroma.

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This diagram is called the colour wheel and any particular spot on the wheel from

0 to 360deg is referred to as hue which specifies the specific tone of the

colour. Hue differs slightly from colour, because a colour can have saturation

and brightness as well as hue.

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The fig above shows difference between saturation and

brightness. Picked up a blue hue from the colour wheel,

and decreasing its saturation will make it blue gray, zerosaturation it turns gray.

Picking up a blue shade and increasing or decreasing its

brightness.

Increasing brightness would make the colour light blue

but decreasing saturation would turn it into gray shades.

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SUMMARIZE

Three colour theory-cones and rods

Additive and subtractive mixing of colours

One lumen (lm) of white light = 0.3 lm of red +

0.59 lm of green + 0.11 lm of blue

Luminance, Hue and Saturation.