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Figure 1Given the same output of power at each wavelength, the visual system will sense the yellow-green region as the brightest and the red or blue regions as the dimmest. This is why, among equally efficient light sources, a light source that has most of its power in the yellow-green area will have the highest visual efficacy, i.e., the highest lumens per watt. However, without a reasonable proportion of red or blue in its output, a light source will not be able to render colors satisfactorily.
How we see color depends on the wavelengths emitted by the light source, the wavelengths reflected by the object, the surroundings in which we see the object, and the characteristics of the visual system. Our conception of the color of an object is a constantly changing, highly dynamic process. It depends on what colors surround the object, how long we have been exposed to the scene, what we were looking at before, what we expect to see, and perhaps what we would like to see. Added to that is the fact that about 8% of the male population and about 0.4% of the female population has a color deficiency or is "color blind" to some degree.
Trying to describe exactly what we see is, of course, extremely difficult since it depends on language to describe what is really in our mind. It is difficult to visualize the color effect in space - and then to specify the necessary object and light sources to make it happen.
Primary Colors
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Figure 2Light and pigments mix differently to form colors. Since the human visual system has three color sensors, it is possible to pick three suitable colors and generate the other colors by mixing these. By convention, the primary colors of light light are chosen to be red, green and blue (RGB). Because they produce white light when they are added together, color mixing of light is called "additive." Any two primaries of light can be combined to form a secondary color - magenta (red plus blue), cyan (green plus blue), and yellow (red plus green). When a secondary is mixed in proper proportions with its opposite primary, the resulting light will be white. Therefore, any primary color is considered to be complementary with the secondary color produced by mixing the other two primaries. Yellow and blue are complementary colors of light - as are cyan and red, and magenta and green (Fig.2).
In pigments, however, a primary color is defined as one that subtracts or absorbs a primary color of light while transmitting or reflecting the other two. For example, the pigment that absorbs red light and reflects blue and green would be called cyan. So, the primary pigment colors (sometimes called subtractive primaries) are cyan, magenta, and yellow - the secondary colors of light.
The color appearance of an object or surface clearly depends on the light used to illuminate it. Often daylight is considered a "standard" but it is obvious that the color of "daylight" changes with the position of the sun in the sky, how cloudy or overcast it is and also which direction of the sky we are sampling, e.g. northern sky or southern sky. In specifying lamp color, the first thing that must be decided is how" warm" or how "cool" a lamp is to be selected. This is generally a subjective decision entirely. The color temperature of the lamp in Kelvins specifies the degree of coolness or warmth of the light source. Another color rating of lamps is the Color Rendering Index (CRI). Lamps with high CRI tend to have a "natural" look no matter what the color temperature. These two terms are explained in more detail later.