IIT JEE , AIEEE Forum - Physics , Optics - optics: can you explain luminouus intensity,luminous flux,illuminance,luminance

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These are photometric quantities, defined as follows

Photometric Quantities:

Radiant flux is usually measured in watts.
Luminous flux - the fundamental quantity measuring the rate of flow of radiant energy modified for its effectiveness in creating the sensation of seeing i.e. Luminous flux = radiant flux x relevant spectral sensitivity of visual system. In SI units, luminous flux is measured in
- lumens (lm). Luminous flux is useful for describing the total light output of light sources. However, to describe the distribution of light from a source, luminous intensity is used.
Luminous intensity - the luminous flux emitted per unit solid angle in a specified direction. The measure is the
- candela (cd) which is equivalent to the lumens per steradian (lumens steradian -1). Formally the candela is defined as "the luminous intensity in a given direction of a source emitting monochromatic radiation at 555 nm of which the radiant intensity is 1/683 watts steradian-1." Both these have AREA measures associated with them.
Illuminance - the luminous flux per unit projectedarea of a surface in a given direction (luminous flux/unit solid angle/unit area) measured in
- candela metre2 (cd m2) (1 footlambert = 3.43 candela m-2)