Light Sources
Light sources can be characterized according to their wavelength spectrum as either narrow- or broad-banded. For example, atomic resonance lamps, such as sodium or mercury, emit light only at certain wavelengths.
The spectral bandwidth of light at each atomic line is generally very narrow. Lasers are effectively monochromatic (single wavelength), while arc lamps, such as xenon, carbon arc, and incandescent lamps, emit over a very broad wavelength range with few spectral features.
One useful measurement of light sources is luminous intensity or candela (CD = LM/SR).
Candela is the foundation unit for the measurement of visible light. It is the luminous intensity in a given direction of a source that emits light that has a wavelength of approximately 555 nm (yellowish-green) and a radiant intensity in that direction of 1/683 Watt per steradian.
The candela value is independent of distance. One can think of it as the emission from a lamp without any interest in what happens to the photons that it has ejected. Candela is frequently used when dealing with focused light for LEDs, flashlights, or spots.