# Bidirectional reflectance distribution function

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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.