Visible Light Spectrum, Color Models and Color Temperature

Visible Spectrum

Visible spectrum is the region of the electromagnetic spectrum that the human eye can perceive. Electromagnetic radiation in this range of wavelengths is called visible light, or simply light. There are no precise boundaries in the visible spectrum. A typical human eye responds to wavelengths from 400 to 700 nm, although some people may perceive wavelengths from 380 to 780 nm. The colors of the visible spectrum and their infinite possibilities of mixing can be represented mathematically.

CIE Chromaticity Triangle

The CIE chromaticity diagram (often shown as a triangle) arranges all colors using three chromaticity coordinates x, y, z, which satisfy x + y + z = 1. This means any color or mixture of colors can be defined from two coordinates. The triangle has a curved top that is the locus of monochromatic radiation and is closed at the bottom by a straight line called the purple line. The central area of the triangle is achromatic and contains the colors of all typical artificial light sources. The center of this area is a white point where the coordinates x, y, z are equal (0.333 each). The further a color lies from the center point, the more saturated it appears.

Subtractive Synthesis

Subtractive synthesis explains the theory of mixing paints, dyes, inks, and natural colorants to create colors that absorb some wavelengths and reflect others. The perceived color of an object depends on which parts of the electromagnetic spectrum are reflected by it, or, conversely, which parts are absorbed.

Additive Color System

An additive color system involves light that is emitted directly from a source of illumination. The additive reproduction process usually uses red, green, and blue to produce other colors. Combining one primary color in equal proportion with another produces the additive secondary colors:

• Primary colors: red, green, blue
• Secondary colors: cyan, magenta, yellow

Combining the three primary colors of light at equal intensities produces white. By varying the intensity of each primary light, the full spectrum of colors can be reproduced.

Color Temperature

The color temperature of a light source is defined by comparing its spectral color to the color that would be emitted by a black body at a given temperature. For this reason, color temperature is usually expressed in kelvin (K), although it does not directly measure a physical temperature.