A white light source (light bulb or sun) emits all wavelengths (l's) within the visible region (between 400 and 700 nanometers). When white light is incident on a surface some wavelength's are absorbed and others are reflected. This gives us the perception of the color of the object.The dominant wavelength is called the color or hue of the surface.
Our eyes respond to two other sensations in addition to the dominant wavelength. These are the luminance or brightness, which is a function of the intensity (energy) of light and purity or saturation. Saturation is how washed out or "pure" the color is, e.g., pastels are not very pure. The chromaticity of light refers to the purity and dominant frequency.
From Figure1: the brightness equals the area under curve or total energy. The purity is equal to Ed - Ew. For example light is 100% pure (Ed - Ew = 100) when Ew = 0 and 0% pure (Ed - Ew = 0) when Ew = Ed.
We can combine light from two or more sources with different intensities to produce a range of colors. If 2 color sources combine to form white light, then they are complementary. For example red and cyan, green and magenta, blue and yellow are all complementary. We usually use three colors, called the primary colors, to produce a range of colors called the color gamut. No unique set of three primary colors will describe all possible wavelength's within the visible spectrum. But a good choice of three colors provides a color gamut that covers most colors.
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