Reflection and Transmission

Radiance L:
The radiance L is the quotient from the (incident) radiant power Φ passing through an area A in a direction (ε) and the product from the trans-illuminated solid angle Ω and the projection of the area (dA⋅cos ε) to a plane vertical to the direction observed [DIN 5031/1].

Spectral radiance factor β(λ):
Ratio of the radiance of the material to the radiance of the perfectly reflecting material (which is irradiated in the same way) in the case of reflection [DIN 5036/1].

Spectral reflectance ρ(λ):
Ratio of the reflected radiant power to the incident radiant power [DIN 5036/1].

Spectral radiance coefficient q(λ):
Ratio of the radiance of the material to the irradiance on the material [DIN 5036/1].

Bidirectional Reflectance Distribution Function (BRDF) f(r):
Ratio of the reflected radiance of the surface element - defined for directional incoming radiation - in the direction stated and the irradiance on the medium.

Irradiance E:
The irradiance is the quotient from the radiant power Φ incident on an area A and this area.

Lambertian reflector:
For surfaces with directionally independent radiance L, the radiant intensity I depends in a specific direction only on the cosine of the angle of inclination ε. This is referred to as Lambert's cosine law. A Lambertian surface, therefore, appears to be identically "bright" under all directions.

Perfectly Reflecting Diffuser:
Perfectly reflecting medium during reflection. Theoretical concept which cannot be realized experimentally, whose reflectance and radiance factor have the value 1.

Reflection indicatrix (for a given incident radiation beam):
Representation of the radiance and luminance distribution of a surface element of a medium scattering light by reflection or transmission in polar coordinates (indicatrix = distortion ellipse).