In order to understand color, it's first necessary to understand light. Light is a form of electromagnetic radiation that consists of millions of small particles, called photons, which travel at extremely high speeds and vibrate at different frequencies. Although it consists of particles, light is analyzed as a wave form. A light wave is a series of photons vibrating at the same (or nearly the same) frequency.
There are three characteristics of light which are often referred to as its physical characteristics. These characteristics are wavelength, intensity, and purity. The wavelength is the distance between two consecutive peaks or troughs. The intensity is the amplitude (or height) of the waves. The purity corresponds to the number of variations in wavelength in a beam of light—an impure wave contains numerous variations in wavelength while a pure wave contains few variations.
Not all light waves are visible. In fact, only a small percentage can be observed with the human eye. These waves are called the visible spectrum. The wavelength of waves in the visible spectrum varies from 380 to 770 nanometers. (A nanometer is one billionth of a meter.) The visible spectrum can be divided into three groups based on wavelength. The first group, with the shortest wavelength, is perceived as the color blue; the second group is perceived as the color green; and, the third group, with the longest wavelength, is perceived as the color red. These colors are referred to as the primary colors or primaries.
Although light possesses the three physical characteristics of wavelength, intensity, and purity, we use different terms to classify our perception of color. This is a result of the unique way in which our visual system processes the physical properties of one or more lightwaves. When light strikes the lens of our eye, it is refracted or bent. The refracted light strikes a sensitive part of the eye called the retina. The retina contains thousands of cells which are called cones and rods. The cones are sensitive to normal light and the rods are sensitive to dim light. As the light is refracted, the shorter wavelengths strike points towards the front of the eye, the medium wavelengths strike points towards the middle, and the long wavelengths strike points in the back of the eye. The cones and rods process the light waves, sending signals along the optic nerve to the brain. These signals, however, do not necessarily correspond to the physical characteristics of the light. For example, a change in the wavelength will not always result in the impression of a different color. For this reason, scientists coined three terms which describe our perception of light: hue, lightness, and saturation. Each of these terms loosely corresponds to one of the physical characteristics of light: hue corresponds to wavelength, saturation corresponds to purity, and lightness corresponds to intensity. The HLS model is supported in the ChooseColor common dialog which allows users to specify an HLS triplet that identifies a requested color. This dialog box is shown in the following illustration:
The ChooseColor Common Dialog Box