Non-linear spectral unmixing in practice.

In practice we do not know the albedo either of the pure components or of the mixture. Under various assumptions one can derive an expression which relates

• the reflectance;
• the albedo;
• the incident angle of the light;
• the viewing angle.

The particular material is imaged under a variety of imaging geometries, i.e. a variety of incident and viewing angles, and in each case the observed reflectance is recorded. The value of the albedo then is treated as a parameter that has to be adjusted until the theoretically derived formula fits the data. This way the albedos of the pure components and the mixture can be determined. Then linear unmixing can be performed with respect to the albedos.