Photometry

Photometry, science of the measurement of light, particularly as it affects illumination engineering. The brightness experienced when light strikes the human eye depends not only on the power but also on the wave-length of the light. In SI (international system) units, the photometric base quantity is luminous intensity, which measures the intensity of light radiated from a small source. The base unit of luminous intensity is the candela (cd). The illuminance falling on a surface (formerly known as its illumination) is measured in luxes (lx). Up to the 1970s scientists were not in agreement about the concepts and terminology to be used in photometry, so many alternative units—apostilbs, blondels, foot-candles, and lamberts—are still encountered.