| mean value of illuminance on the outer curved surface of a very small (real or imaginary) sphere at a point in space
Note 1 to entry: The spherical illuminance can be expressed by
where Ω is solid angle and Lv is luminance.
Note 2 to entry: The spherical illuminance is the quotient of the luminous flux of all the light incident on the outer surface of an infinitely small sphere centred at the given point and the area of the diametrical cross-section of that sphere.
Note 3 to entry: The analogous quantities "spherical irradiance", Ee,o, and "photon spherical irradiance" (also termed "photon fluence rate"), Ep,o, are defined in a similar way, replacing luminance, Lv, by radiance, Le, and photon radiance, Lp, respectively.
Note 4 to entry: The spherical illuminance is expressed in lux (lx = lm·m−2).