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Solid angle (ω) is defined as the area (A) intercepted by the cone on a sphere of radius R centered at the apex of the cone, divided by the square of the radius (R^2). The SI unit of solid angle is the steradian (sr).

Luminous efficiency is the total luminous flux per unit radiant flux. For a monochromatic source, it is the relative luminosity multiplied by 685 lumen/watt. Overall luminous efficiency for a light source is the luminous flux emitted divided by the power input to the source.

Luminous intensity (I) in a given direction is the luminous flux (dF) of the radiation emitted by the source in a small cone of solid angle (dω) constructed around that direction. The SI unit is candela (cd) or lumen/steradian. For an ideal point source, I = F / (4π sr).

Illuminance is defined as the luminous flux of the radiation striking a surface per unit area.