Evidence for the need to update the definition of the BRDF - spectral considerations

In this presentation, we discuss the implications of increasing requirements on angular resolution in measuring the BRDF, the Bidirectional Reflection Diffusion Function, in particular for the control of manufactured artefacts. To account for fast angular variations with respect to the direction of illumination or to the direction of observation, coherence requirements increase, and speckle appears. However, speckle should not be considered as part of the BRDF. Means to adjust the definition of the BRDF and to circumvent the effect of speckle include an analysis of spatial and temporal coherence in the characterization instruments. In particular, the effects of spectral width on the speckle features in scattering diagrams are discussed qualitatively.