Color Vision

In the retina of the human eye there are three types of color  receptors (cone cells), each sensitive to only part of the visible spectrum. The receptors are identified by their color sensitivity: red, green, and blue. The brain processes the inputs from these three types of cones to produce our full-color vision. Red, green, and blue are the primary colors of light from which white and all other colors can be mixed. This process of mixing colors of light is known as additive mixing and can be used to produce the range of colors in video imaging systems and also in color printing, where the inks used are cyan (blue + green), magenta (red + blue), and yellow (red + green).

  When mixing the pigments in paints and dyes that absorb (subtract) and reflect, rather than producing it, the primary colors are red, yellow, and blue. These primaries can be mixed to create the full spectrum of colors through the process known as subtractive mixing.

  If the cone receptors in your eyes do not cover the correct parts of the spectrum, you will inhibit some form of color confusion, generally known as color blindness. Only if you have no functioning cones in your retina will you have monochrome (scotopic) vision. However, at very low light levels, cone cells do not function, so in very low light everybody has scotopic vision. This is the basis for the sometimes clumsy "day-for-night" scenes in video productions where blue light  is used to stand in for monochrome moonlight.

Color Temperature

White light is produced when a light source produces a mix of the main colors in the visible spectrum. All types of light source produce different distributions of the visible colors, so many types of white light have a different tint—some are actually reddish white, and others are bluish white. This difference in balance of the spectral colors is referred to as a light source's color temperature.

  Color temperature is the scientific measurement for expressing the distribution of the spectral colors radiating from a light source, expressed in Kelvin (K). As the Kelvin measure is a reference to the colors emitted from a theoretical hot object under defined conditions, the measurement is expressed in kelvin, never degrees. The higher the color temperature, the bluer the light. The lower the color temperature, the more orange the light. Table 5-1 lists the color temperatures of some common light sources.

  Color temperature is important to visual display. Here are two examples of how you will see color temperature applied:

• Electronic cameras are substantially more sensitive to changes in light's color temperature than the human eye. Setting the correct white balance is a critical step in producing good-quality images from any electronic imaging device.

• Most monitors and projectors have color-temperature selection and adjustment capabilities. These allow you to alter the white balance of an image for the lighting conditions in the environment or match projected images and monitor displays in a presentation.