Photometry is the science of the measurement of light, particularly as it affects illumination engineering. Because the brightness experienced when light strikes the human eye depends not only on the power conveyed by the radiation but also on the wavelength of the light (the visual sensation for a given power reaching a maximum at 555 nanometers), a special arbitrary set of units is used in photometric calculations. In SI 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 luminous flux (the photometric equivalent of the power radiating) from a point source is measured in lumens where 1 lumen (lm) is the flux radiating from a 1 cd source through a solid angle of a steradian. The illuminance falling on a surface (formerly known as its illumination) is measured in luxes where 1 lux (lx) is the level of illuminance occurring when a luminous flux of 1 lm falls on each square meter of the surface. See also apostilb.
Photometry in astronomy
Photometry is widely used in astronomy to make precise measurements of the amount of electromagnetic energy that is received from celestial objects. It can be applied, for example, to the detection and characterization of exoplanets, since planets in transit cause tiny periodic variations in the apparent light output of their host stars. From the period and depth of the transits, the orbit and size of the planetary companions can be calculated: the smaller the planet, the smaller the photometric effect. Transits by Earth-class planets, for example, are expected to produce only a tiny drop in stellar brightness of between 0.005 and 0.04 percent, and lasting for 4 to 16 hours.