Photometry is defined as a science that study about light which is perceived by human eyes. It is different with radiometry which measure all of radiant energy including visible light. Photometry focus radiant energy which is sensitive to human eyes, that is wavelength range from 380 nm to 750 nm.
IEC said light is energy radiation which can be evaluated visually. Gigahertz-Optic defined light as part of electromagnetic spectrum radiation which can be seen. The point of these definitions is same but in different words. There are several parameters in Photometry: Luminous flux, Luminous Intensity, illuminance and Luminance.
Luminous flux is the total power electromagnetic radiation emitted by source which the spectral is can be sensed by human eye. In other words, it is on in range of visible light (380 nm -750 nm). Unit of this parameter is Lumen.
Luminous Intensity is total luminous flux per unit solid angle that emitted in certain angle. Unit of this parameter is Lumen/steradian or Candela (cd).
Illumninance is the total luminous flux which is received by a certain surface area. Unit of this parameter is Lumen/m2 or Lux.
Luminance is total luminous intensity which is received by a certain surface area. Unit of this parameter is candela/m2.
These parameters are similar with radiometry. The difference is photometry measure only electromagnetic radiation than can be sensed by human eye.
Source of light emits both of visible light and non visible light. The ratio of power in visible light range to the total radiant power is luminous efficacy which has unit Lumen/Watt.
Luminous efficiency is also known to determine the amount of visible light power in unit watt which emitted by total radiant power (also) in unit watt. This can be represented in percent (%)
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