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Transcript of ch3-lect 2
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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.