The candela is the basic unit in photometry. All other luminous quantities can principally be derived from it.
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 Hertz and that has a radiant intensity in that direction of 1/683 Watt per Steradian.
One steradian (sr) is the solid angle that, having its vertex in the center of a sphere, segments an area on the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.
The candela is abbreviated as cd and its symbol is Iv. The above definition was adopted by the 16th CGPM (International Committee of Weights and Measures in Paris) in 1979.
Intensity sources are used to calibrate photometers beyond the photometric limiting distance (the distance from which the light source can be considered as approximated point light source).
Visible light is only a small section of electromagnetic radiation which produces a sensation of brightness and color in the human eye.
Electromagnetic radiation is a form of energy. The spectrum of such radiation provides information on its energy composition. The entire spectrum of electromagnetic radiation ranges from X-ray radiation at the high-energy, short-wave end to radio waves at the low-energy, long-wave end.
Radiometry is the measurement of optical radiation, which is electromagnetic radiation within the frequency range between 3×1011 and 3×1016 Hz. This range corresponds to wavelengths between 0.01 and 1000 micrometers (mm), and includes the regions commonly called the ultraviolet (UV), the visible (VIS), and the infrared (IR). Two of the many typically encountered units are watts/m2 and photons/sec-steradian.
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