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Nature of Color

Electromagnetic waves have many different wavelengths and frequencies that span a range known as the electromagnetic spectrum (Fig. 1.1). Light is a narrow range of electromagnetic waves that the eye can detect. Light of different wavelengths produces different perceptions of color. The longest wavelengths produce the perception of red, while the shortest ones produce the perception of violet. The visible, ultraviolet, and infrared spectral regions are classified in Table 1.1.

For many centuries, humans have been quite interested in color. However, the scientific study of color only goes back to Newton when he performed his classical experiment with a prism.

The sensation of color is produced by the physical stimulation of light detectors, called cones, in the human retina. The spectrum of colors produced by a prism is referred to as spectrally pure or monochromatic. They are related to the wavelength, as illustrated in Fig. 1.2. Different spectrally pure colors are said to have a different hue. A spectrally pure or monochromatic color can be produced by a single wavelength. For example, an orange color is associated with a wavelength of 600 nm. However, the same color can be produced with a combination of two light beams, one being red with a wavelength of 700 nm, and another being yellow with a wavelength of 580 nm, with no orange component. In this book, when we refer to a spectrally pure light beam it does not mean it is formed by a single wavelength beam, as in traditional physical or interferometry books. Instead, it means that it has the same color as the single wavelength light beam matching its color. Only with an instrument called a spectroscope can two or more components used to produce a color be identified by the eye. For this reason we say that the eye is a synthesizer device. In contrast, when the ear listens to an orchestra, the individual instruments producing the sound can be identified. Thus, we say that the ear is an analyzer.

Not all colors in nature are spectrally pure, since they can be mixed with white. In this manner, a mixture of red and white produces a pink color that goes from pure red (100% saturated) to white (0% saturated), depending on the relative amounts of red and white. All of these colors obtained by mixing a spectrally pure color with white are said to have the same hue but different saturation. The degree of saturation is called the chroma. Therefore, the relative amounts of a mixture of white and a spectrally pure color determine the color saturation, or chroma.

Again, combinations of spectrally pure colors and white cannot produce all possible colors in nature. Let us consider two identical samples of a spectrally pure red color. If one of them is strongly illuminated and the other is almost in darkness, the two colors look quite different. In another example, if a pure red color is mixed with black, its appearance is different. In these two examples, the difference between the two red samples is its lightness or luminance.

In conclusion, any color has to be specified by three parameters, hue, saturation (or chroma) and luminance, or any other three equivalent parameters, as will be described in more detail later.