In dead-end ultrafiltration, in situ characterization of fouling is of great importance to be able to evaluate cake properties during filtration runs. Moreover, local information is necessary to analyze and model the basic mechanisms involved in deposit formation. Many studies have investigated cake formation on flat-sheet membranes but there is a lack of methods suitable for confined geometries such as inside-out hollow-fiber membranes. This study focuses on development and validation of an optical method using a laser sheet for in situ cake characterization in a narrow channel. The method enables the measurement of time-variations of cross-section cake thickness ranging from 10 mu m to hundreds of micrometers with a 3 mu m resolution and a 2.5 mu m standard deviation. The reproducibility of the results and the order of magnitude are discussed on the basis of experimental results for clay suspensions. Limitations of the method are investigated; in the range of 0-2 g/l for clay suspensions, suspension concentration has no effect. Finally, future applications of the method as a tool for deadend fouling characterization are considered. (C) 2007 American Institute of Chemical Engineers.