radiance

n. brilliance, glowing quality; warmth, joy, happiness; amount of radiation per unit solid angle (Optics)

Radiance and spectral radiance are radiometric measures that describe the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from diffuse sources and reflection from diffuse surfaces. The SI unit of radiance is watts per steradian per square metre (W·sr-1·m-2).
See more at Wikipedia.org...

Noun
1. the amount of electromagnetic radiation leaving or arriving at a point on a surface
(synonym) glow, glowing
(hypernym) light, visible light, visible radiation
(hyponym) aureole, corona
2. the quality of being bright and sending out rays of light
(synonym) radiancy, shine, effulgence, refulgence, refulgency
(hypernym) brightness
(hyponym) gleam, gleaming, glow, lambency
(derivation) radiate
3. an attractive combination of good health and happiness; "the radiance of her countenance"
(hypernym) happiness, felicity
(derivation) glow, beam, radiate, shine

(n.)
Alt. of Radiancy
  

Radiant power, in a given direction, per unit solid angle per unit of projected area of the source, as viewed from the given direction. Note: Radiance is usually expressed in watts per steradian per square meter. (188 )