Tristimulus Values and Chromaticity Coordinates
Tristimulus ValuesFundamentally there are three colors. They are Red (R), Green (G) and Blue (B). Any color that stimulates human eyes is the mixture of R, G, and B in a certain proportion. Let us consider C is the color of an object as the test color. We have taken three sources of R, G, and B color to take an experiment.
The screen is taken to match the color of the test light and the source lights. The upper half of the full screen is taken as Screen 1 and next half is taken as Screen 2. Now the screen 2 is illuminated by the test source C. we have to match this test source color on the screen 1 by adjusting the R, G and B source colors intensities. The three source colors are so adjusted that we get the main screen without any different color in two halves, i.e. the screen will be with test light color only.
Now we can write as per their intensities that As per the figure below the arrangement must be followed by.
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Here r, g , b are the value of their intensities. This color matching experiment is taken to get the spectral Tristimulus values of an object color. As per the above experiment the object color is achieved by adjusting the source color intensity. In the trichromator, it symbolizes the availability of intensities of these three matching stimuli.
If now for arbitrary color that is so selected by adjusting the stimuli R, G and B, the amount of three matching stimuli can be expressed in a new way, that is Where the symbol ≡ is “read” as matches. Now the interesting thing is that the monochromatic test stimuli are used to obtain the object color. But practically red color getting mixed with green and blue does not provide exact test object color.
Rather if red is mixed with the test object color then it gives the color same to same as the mixed color of green and blue following perfect intensities. So the color mixture of given amounts of the green and blue matching stimuli will match the mixture of the test and red stimuli. Now the color stimuli equation can be written as This does not mean that red light is negative. The color matching is additive. 1 unit of power of light with wavelength λ1 [C(λ1)] is matched with the R, G, B primaries, then and 1 unit of power of light with wavelength λ2 [C(λ2)] is matched with the R,G,B primaries, then then the additive mixture of the two monochromatic lights C(λ1) + C(λ2) will be matched with the additive mixture of the two amounts of the primaries: The R, G, B Tristimulus values of a stimulus with P(λ) spectral power distribution are Or using integral, The graph of the inverted r(λ), inverted g(λ) and inverted b(λ) color matching functions of the CIE 1931 Standard colorimetric observer is given below.
Chromaticity CoordinatesMainly colors are three types.
- Source color
- Object color
- Derived color