Radiance

Radiance is the radiant flux per unit projected area per unit solid angle ().

Intuitively, radiant expresses how much power arrives at (or leaves from) a certain point on the surface, per unit solid angle, and per unit projected area.

Like other radiometry quantities, radiance is properly represented by SPD, but we often approximate it with RGB color.

Given position and direction vector , we can express radiance as

The cosine term of the above definition is because that the projected area is the perpendicular of the surface orthogonal to the direction we are interested in.

Advanced_Global_Illumination_Figure_2_1.png Radiance is the quantity that captures the “appearance” of objects in the scene.

Property of Radiance

  • Radiance is invariant along straight paths and does not attenuate with distance. This property is only valid in the absence of participating media.
  • Sensors, such as cameras and the human eye, are sensitive to radiance.