Rigid barrier filters are one of the options for particulate removal from gas streams by means of surface filtration. Non-uniform gas flow through the filter, possibly caused by inhomogeneities from the manufacturing process, affects the filtration behavior of such media, is suspected of causing an inhomogeneous dust cake build-up, and may influence regeneration adversely, resulting in an instable operating cycle. So far Little is quantitatively known about the distribution of cake properties (e.g., porosity, thickness, etc.) across the filter surface, possible variations of this distribution over a number of filtration cycles, and how such variations may be connected with the distribution of residual cake patches after regeneration. This paper introduces an optical in situ technique to measure dust cake height distributions across a flat area of filter surface of 14 cm in diameter. Distributions are recorded by an image acquisition process with a lateral resolution of about 280 mu m (1 pixel) and a vertical resolution of about 50 mu m. Sample data of a partially regenerated ceramic filter medium are presented to illustrate the capabilities of the technique. The experiments were performed with quartz dust at ambient temperature and pressure on a filter test rig according to VDI Standard 3926 [VDI Richtlinie 3926, Prufung von Filtermedien fur Abreinigungsfilter, VDI Handbuch Reinhaltung der Luft, Band 6, 1991]. The data can be used to determine point-by-point growth rates of the filter cake during filtration and residual cake thickness after pressure pulse or reverse gas flow regeneration.