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This site is organized systematically in the sense of reflecting Biyee’s approach to some aspects of color vision, but it is by no means a comprehensive information center for color vision or any aspect of color vision.
The initial goal was to develop quick color vision tests based on a standard CRT. Using a standard CRT imposes limitations on the tests. The word "standard" means complying with the CIE 1931 color matching functions that are used by CRT manufacturers worldwide. Color CRT's have three types of phosphors so it can generate only the colors that can be produced by combining the three phosphors. In other words, the colors that can be produced by a CRT falls into a triangle area in the CIE color diagram with the three points corresponding to the colors of the three phosphors. However, even with all the limitations imposed by using standard color CRT, the author believes that these tests can achieve the goal of quick color vision tests just like Ishihara color test. That is to find out typical dichromatic color deficiencies.
The CIE 1931 color matching functions are used simply because color monitors are manufactured according to them (ITU-R BT.1361). They are not necessarily the best color matching functions as some scholars argues (Stockman and Sharpe, 1999). Let's take a look at what would be an ideal set of color matching functions, what prevent us from getting them and how close can we get to them.
They
are functions (Fi) of the spectral radiance of an object defining quantatively
its color
Where
i = 0, 1, n; L(l) is the radiance in W/sr×m2×nm; k is constant called
maximum photopic luminous efficacy in lm/W (lm is a psychophysical unit of luminance,
cd/m2) . The set of dimensionless numbers (F1, F2, … Fn) defines the color corresponding
to L(l). The CIE 1931color matching functions are x(l), y(l), z(l)
(i.e. F1, F2, F3). Since L(l) and fi(l) are continuous functions (in reality
they are functions of discrete wavelengths over the visible range (approximately
380 – 780 nm)), for certain set of Fi, there are infinite number of L(l) that
satisfy the above equation. All of them, although different physically,
have the same color to the observer using the CMF; in other words, their colors
match. For example, using the CIE 1931 CMF, for each set of X, Y, Z, we have
infinite number of spectral radiances that produce them. The spectral
radiance of yellow color on your monitor is dramatically different from that
of the yellow light of the spectrum generated by a prism, but they look almost
the same to us.
The ideal set of CMF is one that can be used under any lighting condition, for an object of any size at any distance and for any "normal" observer, to accurately calculate color coordinates in a color space. Unfortunately this ideal set of CMF never exists although people often use CMF's as ideal ones. This will be explained in the following section.
Let’s describe a hypothetical visual system that allows us to have an ideal set of color matching functions. Suppose this visual system has three cones – short wavelength photoreceptor, middle wavelength photoreceptor and long wavelength photoreceptor (or S-cone, M-cone and L-cone). Each of them has a spectral sensitivity determined by the spectral absorptance of its photopigments. The spectral absorptance is essentially the spectral probability of photon absorption. Each absorbed photon generates the same amount of psychophysical response and the response is additive. In other words, the response is proportional to the number of absorbed photons. The hue of color is determined by the ratio between the numbers of absorbed photons of the three cones. This system will give an ideal set of CMF’s that are the spectral sensitivities of the three cones or any set that they are linearly transformed to.
Now let’s look at the reality that defeats the conditions for ideal CMF’s.
Light has to pass cornea, aqueous humor, lens, vitreous humor and multiple layers of cells (pre-receptor cells of macula) before reaching the photopigments on the surfaces of cones to generate visual response. Although the visual functions of these components are not for modifying spectral characteristics of incident light (cornea and lens are the two major refractive components for focusing objects’ images on the retina), they have their own spectral absorptance that are multiplcation factors in calculating the effective cone spectral sensitivity. The two most important factors are lens and macular pigment spectral absorptance. They vary among people and the macular pigment spectral absorptance declines with eccentricity.
A human visual system has hundreds of thousands of cones and their distribution is uneven. Therefore the cone spectral sensitivities vary in different areas of retina. This results in the variation of the spectral sensitivities for objects of different sizes or at different view angles.
Even among normal trichromatic visual systems, there are slight variations of cone spectral sensitivities in terms of maximum absorptance wavelengths. For example, the spectral sensitivities of red, or green cones of one visual system may be different from that of another visual system.
The photopigment densities of cones depend on lighting conditions. Since the cone spectral sensitivities are determined partially by the photopigment densities, they can be selectively suppressed by manipulating lighting conditions. This affects color matching.
The electrical signals generated by cones in response to the absorption of photons are not always linear. They are proportional to the number of the absorbed photons only in a certain range of intensity to a certain degree.
The most complicated factor is the post-cone processing. Color matching functions are psychophysical parameters that represent the responses in the visual cortex. There are multiple types of cells involved in the signal processing and transportation between cones and visual cortex – horizontal, bipolar, amacrine, ganglion cells in retina, lateral geniculate nucleus (LGN) cells in thalamus or superior colliculus (SCN) cells in midbrain. All of these processing (including the final processing in visual cortex) and transportation do not always obey linear rules.
This site reflects two trends in information technology that Biyee strongly believes:
1. Web based applications will gradually become the norm of software application. With this belief, Biyee is building this site into one that interacts with visitors and offers a variety of functions that used to be available only in the traditional software applications such as VPMODEL.
2. Software componentization. Applications will be made of components that can be replaced or reused in other applications. This site has numerous components in such form as Java applets. Visitors can plug these components into their own applications or documents such as MS Office documents, Web pages, etc.