Definitions
Luminous Flux (Power) (Φv): The measure of the perceived total optical power output emitted in all directions (isotropically) by a light source. The SI (Standard Internationale) is the Lumen (lm). One lumen is defined as the Luminous Flux of a monochromatic light source emitting an optical power of 1/683 Watts at 555 nm.
Luminous Intensity (Iv): The measure of the perceived optical power emitted by a light source in a particular direction. It determined by measuring the total Luminous Flux (in Lumens) per unit of solid angle (lm/sr). The SI unit for Luminous Intensity is the Candela (cd). One candela is about the intensity of a normal wax candle.
Steradian (sr): A steradian is the solid angle in space whose projected spherical surface area is equal to the square of its radius. A sphere contains 4π sr.
Human Eye Luminous Efficacy (Vλ): The light sensitivity (response) of the human eye is a function of frequency (wavelength) and it is called the Luminous Efficacy of the eye. Luminous Efficacy is different for the light-adapted (photopic) versus the the dark-adapted (scotopic) human eye. For example, in a lighted environment the human eye's peak perception is at 555 nm while in a dark environment its peak perception will shift down to 507 nm.
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