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Color Vision

What is color constancy?

color constancy is the phenomenon that most color surfaces appear to retain their approximate daylight appearance even when viewed under light sources that differ markedly from daylight. color constancy is surprising since the spectral distribution of light entering the eye from a surface can vary markedly from one light source to another. The phenomenon of color constancy is only approximate, however, and surfaces do not retain their daylight colors when viewed under certain fluorescent light sources or when viewed under monochromatic radiation. Certain surfaces appear to change markedly from one light source to another and such surfaces are said to lack color constancy; this phenomenon must not be confused with metamerism which is a phenomenon associated with at least two samples.

What is metamerism?

Metamerism refers to the situation where two color samples appear to match under one condition but not under another; the match is said to be conditional. Metamersim is usually discussed in terms of two illuminants (illuminant metamerism) whereby two samples may match under one illuminant but not under another. Other types of metamerism include geometrical metamerism and observer metamerism. Two samples that conditionally match are said to be a metameric pair. If two samples have identical reflectance spectra then they cannot be metameric - they are an unconditional match.

How do I measure whiteness?

Whiteness is a complex perceptual phenomenon that depends not only on the luminance of a sample but also on the chromaticity. To promote uniformity of practice in the evaluation of whiteness the CIE has recommended that the formula for whiteness, W or W10, be used for comparisons of the whiteness of samples evaluated for CIE Standard Illuminant D65:

W = Y + 800(xn - x) + 1700(yn - y),
W10 = Y + 800(xn,10 - x10) + 1700(yn,10 - y10),

γ€€γ€€ where xn and yn refer to the chromaticities of the illuminant (D65), and the subscript 10 distinguishes the 10 degree observer data from the 2 degree data.

The higher the value of W or W10 the greater the whiteness: the formulas, however, are only valid for samples that would commercially be considered white and within certain other constraints.

If the measurement of whiteness is important, and if samples may be fluorescent, then it is very important that the light source in the spectrophotometer be a close approximation to the D65 illuminant.

How do I measure yellowness?

The preferential absorption of light in the short wavelength region (380-440nm) by a nominally white substance usually causes an appearance of yellowness. A number of yellowness scales have been developed over the years.

What can I do if my sample is not uniform?

It must be realized that a spectrophotometer (or a colorimeter) spatially averages the light reflected by a sample: thus it is possible that a uniform grey sample and a black-and-white checkerboard sample could give rise to identical measurements. The CIE system is strictly limited to the measurement of uniform color stimuli.

Digital color camera devices are beginning to be used for color measurement - especially for textured samples. They measure color at many thousands of spatial locations on the sample but currently provide relatively poor color resolution and precision.

What is device-independent color space?

There is an increasing need to be able to communicate color from one device (such as a VDU screen) to another (such as a color printer) with no loss in color fidelity. One way to achieve this is if all devices are calibrated in terms of a device-independent color space. The CIE color specification is being adopted by industry as a device-independent space.
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