Light and Color

Light is electromagnetic (EM) radiation, the fluctuations of electric and magnetic fields in nature. More simply, light is energy and the phenomenon of color is a product of the interaction of energy and matter. As a reasonable starting place for discussing color, we need to take a brief look at the physics of light and the particular nature of light sources.
Light has the properties of both particles and waves. Light particles, called photons, radiate from their source in a wave pattern at a constant speed of 186,000 miles per second. Like waves in the ocean, light waves have a crest and a trough. They are measured by wavelength, the distance between two crests (in meters or, sometimes, in angstroms which are 1/100,000,000th of a meter), and by amplitude, the vertical distance between the crest and the trough.

Other ways of measuring EM radiation are by frequency (measured in hertz or cycles per second) and energy (measured in electron volts). Shorter wavelengths are higher frequency and higher energy; longer wavelengths have lower frequencies and lower energy.
There are different types of EM radiation including gamma rays, x-rays, radio waves, ultraviolet, and infrared. The whole array of these is known as the electromagnetic spectrum, which runs in order of wavelength from longest (radio waves that range from 1 millimeter to several kilometers) to shortest (gamma rays at less than 0.1 nanometers, or 1/10,000,000,000th of a meter).
The human eye is only sensitive to EM radiation at wavelengths that range roughly between 780 nanometers and 380 nanometers. This small segment is called the visible spectrum or visible light. This is usually what we mean when we speak of "light" (though, properly speaking, all EM radiation is light). Infrared lies just below red light; ultraviolet exists just above violet light. Both are invisible to humans and other creatures (though some reptiles can see infrared and some insects can see ultraviolet).

The visible spectrum contains numerous colors that are distinguished by wavelength and amplitude; wavelength determines color and amplitude determines brightness. Of these colors, the human eye can distinguish about 10,000. The visible spectrum, however, is often identified by the seven prominent colors we see in the rainbow. In 1666, Isaac Newton named these colors red, orange, yellow, green, blue, indigo, and violet, which are often referred to by the mnemonic acronym ROY G BIV.
More commonly, however, the spectrum is arranged in order of wavelength, shortest to longest, and divided into segments identified as violet (380-450nm), blue (450-490nm), green (490-560nm), yellow (560-590nm), orange (590-630), and red (630-780):

The combination of these light waves produces white light, which is what we see from the Sun and from most artificial light sources. A breakdown of the individual colors themselves is only visible under certain circumstances. This occurs naturally in a rainbow; it also occurs when white light is refracted through a prism. In fact, it was by experimenting with a prism in 1666 that Newton conclusively proved that what we see in these refractions are the constituent colors of white light; that is, that white light is not homogeneous (as had been previously supposed), but a composite of myriad-colored wavelengths.

Light Sources

Light comes from a variety of sources. Because color depends on the reflection of light from an object, the nature of the light source is of the utmost importance. The most obvious light source in our experience is the sun; other obvious sources include flame and various kinds of electric lamps. There are still others that might not be as obvious, such as the phosphors that make sea foam glow.
We have already characterized light as energy. In general, then, any process that emits, re-emits, or conducts energy in sufficient amounts produces light. The most common means are the following:
* Incandescence
Solids or liquids heated to 1000 K or greater emit light. The sun is a natural incandescent source (at about 5800 K on the surface), so is a candle flame. The most common man-made source is the tungsten filament light bulb at about 2854 K.
* Gas Discharge
Gases emit light when an electric current passes through them. The nature of the light depends on the gas used as the conductor. The gas is typically at very low density to facilitate conduction, though variations in the density of the gas changes the nature of the light produced. Common types of gas discharge sources are sodium, mercury, and xenon lamps.
* Photoluminescence
Phosphors are substances that absorb and then re-emit light. In doing so they change the nature of the absorbed light. When the re-emission takes place concurrent with the absorption, the source is called florescent; when the re-emission continues after the light is no longer being absorbed, it is called phosphorescent. The obvious example of a photo luminescent source is a florescent lighting tube (which is actually a mercury lamp coated inside with phosphors).

Other, more obscure, means of producing light come from chemical reactions (producing light but no heat) or other means of exciting solids, liquids, and phosphors such as electric conduction and bombardment with electrons. None of these, however, are commonly encountered as light sources.

Illuminants

It is important to note at this point that color scientists use theoretical sources to determine the chromaticity, or colorfulness, of light as well as real sources. These model sources are called blackbodies or Planckian radiators (after Max Planck, the German physicist who developed Planck's Law, a formula for determining the spectral power distribution of a light source based on its temperature). The term source is used in color theory to identify a physical source of light, such as a light bulb. For theoretical models, the term used is illuminant.
Light sources, whether actual sources or illuminants, are primarily characterized by their color temperature and spectral power distribution.

Color Temperature

Color temperature refers to the heat of a light source. As color temperatures vary, so does the makeup of the light in terms of the relative power of its constituent wavelengths. Color temperature is always measured in Kelvin, units of measurement on the Kelvin scale (noted as K). The system was developed in 1848 by Lord Kelvin (William Thomson) to measure absolute temperature. Each unit on the Kelvin scale is equivalent to one degree Celsius. Kelvin can be converted to degrees Celsius by subtracting 273.13 and to degrees Fahrenheit by subtracting 459.6.

Spectral Power Distribution

Spectral power distribution refers to the wavelengths that make up the light emitted from a source or illuminant at a particular color temperature. Those with cooler color temperatures emit the longer wavelengths (red to yellow) in stronger amounts than the shorter wavelengths (blue to violet). Hotter blackbodies emit all wavelengths in more equal distributions, though tending to be slightly stronger in the blue to violet wavelengths. The following graphs represent the spectral power distributions for a standard CIE source and illuminant:

Compare these to representations of the spectral power distributions of average daylight and a normal florescent light source:

Note how the florescent source is relatively low in terms of relative power as compared to CIE Source A (a tungsten-filament bulb) and average daylight and how it's relative power spikes sharply at certain wavelengths. These spikes are also typical of gas-discharge lamps.
Objects seen in "hotter" light will appear more vibrantly colored than objects seen in "cooler" light. Blues seen in cool light will appear darker, greens appear more yellow, and purples redder because of the lower intensity of the blue-violet wavelengths of the spectrum.

Photometry

Photometry is the measurement of the attributes of light, though it is more commonly used to refer to measuring its intensity or flux. Luminous intensity, or luminance, refers to the amount of energy in a light source and is measured in units called candelas. Luminous flux, or the amount of light radiating from a source, is measured in units called lumens. Both of these units are fairly complex. In simplified form:
One candela is the intensity of the light radiating from 1/50th of a square centimeter of the surface of a blackbody heated to 2046K.
One lumen is equal to the flow of light radiating from a source whose intensity is equal to one candela.