Radiance and LuminancePublished on 24/2/2012 and last updated on Saturday 30th of June 2018 at 06:56:53 PM
Radiance is denoted by Le,λ and it is equal to the double derivative of radiant flux with respect to projected surface area As and Solid angle ωs. where, ÆŸ is the angle between normal to the elemental and the given direction. dAs is the elemental area and dωs is the elemental solid angle containing the given direction. The unit of radiance is W/sr-m2.
In case of photometric quantity, the radiance is called as Luminance. We can use the conversion equation to obtain luminance from radiance. Where, Km is the constant which is called maximum spectral luminous efficacy and its value is 683 lm/W.
So Luminance is the Luminous flux radiated from a point light source per unit solid angle and per unit projected area perpendicular to the specified direction. Luminance is denoted by The unit of Luminance is Lm/sr-m2 or Cd/m2. If we take an analysis on the conservation of radiance and Luminance then we see that the radiance luminance or luminance from a source and the radiance and the luminance from the detector is the same i.e.
It is because, if we consider that the radiation is neither gained or lost in the medium in which propagation of energy takes place between the source and the detector, then it must be that Φs = ΦD. The luminance is a quantity that is conserved in the system. Luminance is the same from the source and at the detector. Luminance is neither source quantity nor detector quantity. Luminance is purely geometric quantity of the beam connecting the source and the detector. The conservation of luminance is also true for presence of lenses or other optics.
The basic relationship between the luminance and the luminous flux is given below, Φ = LG, G is the geometric angle in steradian.