Bidirectional reflectance distribution function

In radiometry, the bidirectional reflectance distribution function describes how energy reflecting of a surface is spread over the hemisphere. It is a function of five variables:

• Spectral Location (e.g. Wavelength)
• Incident Zenith
• Incident Azimuth
• Exitant Zenith
• Exitant Azimuth

A surface whose BRDF spreads incident energy evenly over the hemisphere is called lambertian or "diffuse". A surface that for a given incident vector reflects all or most energy in to the mirrored direction is called "specular".

Because the BRDF is a function of five variables, it is often characterized by making measurements at a small set of angles and wavelengths and then fitting a mathematical model to the data.