How do we see color?

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Volume 8 Issue 1	October 2004

Color rendering index (CRI) - A measure of the degree of color shift that objects undergo when illuminated by a lamp, compared with those same objects when illuminated by a reference source of comparable correlated color temperature (CCT). A CRI of 100 represents the maximum value. A lower CRI value indicates that some colors may appear unnatural when illuminated by the lamp. Incandescent lamps have a CRI above 95. The cool white fluorescent lamp has a CRI of 62; fluorescent lamps containing rare-earth phosphors are available with CRI values of 80 and above. Correlated color temperature (CCT) - A specification of the apparent color of a light source relative to the color appearance of an ideal incandescent source held at a particular temperature and measured on the Kelvin (K) scale. The CCT rating for a lamp is a general indication of the warmth or coolness of its appearance. As CCT increases, the appearance of the source shifts from reddish white toward bluish white; therefore, the higher the color temperature, the cooler the color appearance. Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt. Spectral power distribution (SPD) - A representation of the radiant power emitted by a light source as a function of wavelength. Blackbody radiator - A temperature radiator of uniform temperature whose radiant output in all parts of the spectrum is the maximum obtainable from any temperature radiator at the same temperature. Such a radiator is called a blackbody because it absorbs all the radiant energy that falls upon it. All other temperature radiators can be classed as non-blackbodies. Non-blackbodies radiate less in some or all wavelength intervals than a blackbody of the same size and the same temperature. Chromaticity - The dominant or complementary wavelength and purity aspects of the color taken together, or of the aspects specified by the chromaticity coordinates of the color taken together. It describes the properties of light related to hue and saturation, but not luminance (brightness). Color appearance - The resultant color perception that includes the effects of spectrum, background contrast, chromatic adaptation, color constancy, brightness, size and saturation. Color consistency - The measure of how close in color appearance random samples of a lamp or source tend to be. Color matching - The action of making a color appear the same as a given color. Often used as a method of evaluating the ability of a light source to render colors faithfully. Color stability - The ability of a lamp or light source to maintain its color rendering and color appearance properties over its life. The color properties of some discharge light sources may tend to shift over the life of the lamp. Full-spectrum index (FSI) - A mathematical measure of how much a light source's spectrum deviates from an equal energy spectrum, based on the slope of its cumulative spectrum. Full-spectrum color index (FSCI) - A mathematical transformation of full-spectrum index into a zero to 100 scale, where the resulting values are directly comparable to color rendering index. An equal energy spectrum is defined as having an FSCI value of 100, a “standard warm white” fluorescent lamp has an FSCI value of 50, and a monochromatic light source (e.g., low pressure sodium) has an FSCI value of 0. Gamut area - A measure of color rendering based upon volume in color space. It is the range of colors achievable on a given color reproduction medium (or present in an image on that medium) under a given set of viewing conditions. Hue - The attribute of a light source or illuminated object that determines whether it is red, yellow, green, blue, or the like. Isotemperature - A set of coordinates within which all points have the same temperature. In a color space diagram, isotemperature lines represent lights with identical correlated color temperatures. Metamers - Lights of the same color but of different spectral power distribution. Photopic - Vision mediated essentially or exclusively by the cones. It is generally associated with adaptation to a luminance of at least 3.4 cd/m². Primary - Any one of three lights in terms of which a color is specified by giving the amount of each required to match it by additive combination.

Electromagnetic radiation, varying in wavelength from gamma rays to microwaves, is constantly bombarding us from all directions. Our eyes are able to detect how much radiation is entering them, and from what direction, only if that radiation is within the visible spectrum, which is between approximately 380 and 780 nanometers (nm).

The spectral power distribution (SPD) of a light source is graphical or tabulated data representing the amount of radiation emitted by a light source at each wavelength in the visible spectrum only. For example, Figure 2 shows the SPD of an incandescent lamp. The SPD provides the basic physical data needed to calculate light source color.

Figure 2. Spectral power distribution of an incandescent lamp

When people speak of color, they are usually discussing the appearance of an object (red, pink, purple, brown, white) or a light source (e.g., warm-white, cool-white). Although color appearance seems to come from the physical characteristics of the electromagnetic radiation reaching the retina, it is actually the result of signal processing performed by the visual system. More specifically, color appearance is the result of calculations performed by three separate "color channels" in the visual system. Each channel takes the same radiant power falling on the retina and processes it slightly differently. Research has produced a general model of the visual system, which provides a basic explanation of color appearance (CIE 2004).

For light to stimulate the brain it must be absorbed by photoreceptors in the eye's retina. There are three types of cone photoreceptors responsible for color vision, each defined in large part by the photopigment contained within that photoreceptor. Figure 3 shows the spectral sensitivity of the three cone photoreceptors, L, M, and S. The letters symbolize the photoreceptors' peak spectral sensitivities to Long, Medium and Short wavelength radiation, respectively.

Figure 3. The spectral absorption curves of the three cone types

Source: IESNA Handbook 2000

The neural signals generated by these photoreceptors are combined in different ways by the three color channels. One channel, the achromatic or luminance channel, calculates the amount of light falling on the retina from the sum of the outputs of the L and M cones. This luminance channel is related to the perceived brightness of the object or the light source. The other two channels, known as the color opponent channels, calculate the hues. One opponent channel subtracts the response of the L cones from that of the M cones to produce a Red versus Green response. When the signal strength of the channel is, for example, greater than zero, the channel signals "Red" to the brain; when the response is less than zero, it signals "Green" to the brain. The other opponent channel subtracts the response of the S cone from the sum of the L and M cones to produce a Blue versus Yellow response.

All colors are seen by the brain as the following combinations: red or green; and blue or yellow. For example, orange is seen as reddish-yellow, while turquoise is seen as greenish-blue. Since both reds and greens are formed by only one color opponent channel, we can never see reddish-greens. Similarly, we can never see bluish-yellows. The color "white" is seen when both color opponent channels are balanced (i.e., the light is neither red or green, nor blue or yellow), so only the achromatic channel is signaling the brain.

Although this model forms the foundation of color appearance, the actual appearance of a given color is based upon a much more complicated set of subtle interactions in the visual system. For example, a light may appear white when briefly flashed, but yellow when steadily viewed. A more common example of these neural interactions is the change in appearance of the same light at different adaptation levels. A light that looks orange at high light levels may look amber or brown under low light levels. A vivid example of how color appearance can change as a result of other neural activities in the brain is illustrated in Figure 4.

The same physical stimuli (the colored squares) appear to be different depending upon the color of their surrounding.

Figure 4. A demonstration of the complexity of color appearance

The two squares on the left are physically identical to those on the right, but appear to be different because of their respective surrounding colors.