Surface characterisation Light interaction with the materials surfaces is a key issue for accurate simulations. By measuring light scattering , by building simulation models for light scattering, by constantly developing our measurement instruments, OPTIS has increased its understanding of light and matter interaction. Generally speaking, a surface can be characterised by its BRDF (Bidirectionnal Reflectance Distribution Function) that describes the way the light interacts with the surface material. Simpler models can be applied when the surface shows a specific behaviour. Many surfaces as mate materials can be considered as lambertian. Glossy paintings have a specular behaviour, rough surfaces can be modelled with a gaussian distribution of the reflected light around the normal direction etc… These models are used in SPEOS in the simple and the advanced scattering model. Sometimes, the BRDF measurement cannot be fitted in SPEOS model because the material shows an anisotropic behaviour on reflection (holograms, specific diffusers, textured surfaces…) The surface can also be characterised by the BTDF (Bidirectionnal Transmittance Distribution Function) that described the way the light is transmitted by the material. This property can be assigned to a transmissive surface without taking into account the thickness of the material. This assumption is true for thin materials. If the sample is thick, some light can be coupled into the material (specially for high incidences), the measurement has to be done differently. Real samples and simulated results.