Abstract

The surface bi-directional reflectance distribution func-tion (BRDF) can be used to distinguish different materials.The BRDFs of many real materials are near isotropic andcan be approximated well by a 2D function. When the cam-era principal axis is coincident with the surface normal ofthe material sample, the captured BRDF slice is nearly 1D,which suffers from significant information loss. Thus, im-provement in classification performance can be achievedby simply setting the camera at a slanted view to capturea larger portion of the BRDF domain. We further use ahandheld flashlight camera to capture a 1D BRDF slice for material classification. This 1D slice captures important reflectance properties such as specular reflection and retro-reflectance. We apply these results on ink classification, which can be used in forensics and analyzing historical manuscripts. For the first time, we show that most ofthe inks on the market can be well distinguished by theirreflectance properties.