Color sensation

Subjective Description of Color in Terms of Equivalent Stimuli. The observer, unable directly to measure or describe a color sensation in absolute terms, is able to evaluate it in terms of certain stimuli which produce an equivalent sensation. Subjectively the comparison is accomplished experimentally with a colorimeter, so designed that the color of the sample is seen in one half of a photometric field and the mixture of color produced by independently controllable components is seen in the other half. By proper adjustment of the components, a unique setting will be found which produces a match in the photometric field and the color of the sample can be specified in terms of the amounts of the chosen components. 

During the last eighty years, research has been conducted within the color community to test the response of the human eye to color stimuli. Scientists knew it was possible to match the color sensation by designing an experiment based on the mixing of three colored lights. W.D. Wright in 1928 and J. Guild in 1931 conducted the most important of these experiments. Their two experiments were inde-... 

To see if the environment has an impact on the perceived color, Land and McCann also moved the rectangular patches around on the Mondrian. They report that the color sensation does not change significantly if the rectangle is moved to a new neighborhood where it is surrounded by different colored rectangles. 

Macbeth 5000 K fluorescent, a Philips Ultralume fluorescent, and a Sylvania Cool White fluorescent tube. Some color constancy algorithms try to find out what type of illuminant produced the particular color sensation, which was measured by the sensor. If we measure the entire power spectrum for a particular patch of our object and also know the type of illuminant used, then it is easy to compute the BRDF. Let L(X) be the measured power spectrum and let E A.) be the power spectrum of the illuminant. Assuming a Lambertian surface, where the BRDF is independent of the normal vector of the patch Nobj, and also independent of the normal vector that points to the direction of the light source Ni, we... 

In order to reproduce the color sensation of an observer, we could reproduce exactly the same spectral characteristic for every point of the image. This sensation would be indistinguishable from the original sensation. However, we do not usually have a spectrometer to measure the entire spectrum for every image point, let alone a method to create a color spectrum given such measurements. However, quite a large number of colors can be reproduced using only three color primaries. 

In Chapter 2 we have seen that the human eye contains three different types of cones. Color sensations can be produced by varying the amount of light in the sections of the visible spectrum to which the cones respond. Let. S, with i e r, g, bj be the response curves of the red, green, and blue cones. Then the response <2 of each cone i is given by... 

The three primaries span a three-dimensional space of colors. By varying the parameters X, Y, and Z we can create different color sensations. 

Suppose that we want to create the color sensation created by monochromatic light at wavelength k = 500 nm. The response of the three cones are given by S, < X) with i e r, g, b). In order to create the same color sensation using the monochromatic primaries Lj with j e 1,2, 3, we have to solve the following set of equations ... 

In other words, the response of the three receptors at the given wavelength k have to be equivalent to the response to a linear combination of the three monochromatic primary colors for some X, Y, and Z. If any one of the parameters X, Y. or Z should become negative, then we cannot create the particular color sensation using only these three primaries. 

Figure 4.4 Weights used by a standard observer to create any given color sensation. Note that all of the weights are positive (data from International Commission on Illumination 1988).

This theory proposed by E. Hering and E. Mach believes the structure of the nervous system may fully explain at least some perceptual constancies as well as depth perception. E. Hering also proposed that there may be visual receptor cells organized into certain functional patterns that provide color sensation. And in fact these functionally patterned receptor cells have been found. 

Biochemical research has shown the importance of zinc metabolism in the retina (9). Zinc is found in high concentrations in the choroid, the retina, and especially the ganglion cells. Retinol dehydrogenase, a zinc-containing enzyme, interferes with the transformation of retinol (vitamin Ai), which is essential for color sensation and conal vision. Furthermore, zinc is involved in the biosynthesis of the specific transport of retinol from the liver to the effector cells. Ethambutol is a chelating... 

Equal Euclidean distances in the CIE-Yxy system are not necessarily perceived by a human observer as equal color distances. To match Euclidean and perceived distances, a wide variety of so-called uniform color spaces has been developed (Hunter and Harold, 1987). One of the first of these uniform systems was developed in the early 1900 s by the artist A.H. Munsell. Starting with the observation that brighter hues like yellow and red create a much stronger color sensation at color saturation than darker hues like blue or green, he mounted color chips on a rotating Maxwell disk to... 

Since the temperature on the test surface is determined from the reflection color on the liquid crystal surface, the color sensation for the reflected light from a liquid crystal surface becomes the essential element that affects the uncertainty in temperature or heat transfer measurements. Several parameters affect the color sensation the spectral characteristics of the incident light that illuminates the liquid crystal surface, the helical structure of the liquid crystal, the incidence and reflection angle of light, and the color capturing device [73]. 

Figure 1 A postulated representation of the interconnections between receptors and the subsequent signals on which color sensations depend. (Reproduced with permission from Hunt RWG (1998) Measuring Colour, 3rd edn., p. 24. Kingston upon Thames Fountain Press.)...

The color sensation associated with a Hght stimulus can be described in terms of three characteristics ... 

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