An in situ optical method was developed to determine the thickness of the cake layer by using a low-cost photointerrupt sensor. The effects of slurry concentration, agitating speed (i.e. slurry transport velocity), background light and color of slurry on the accuracy of measurement were examined to verify the validity of this method, The accuracy of the thickness measurements was confirmed using uniform sized PMMA spheres. Experimental results show that the developed technique is useful for cake thickness ranging from 10 mum to 5 mm and yields a vertical resolution as high as 10 mum in the same order as that of gamma-ray absorption and NMR micro-imaging methods. This method has benefits of inexpensive, easy installation and a comparable accuracy compared with gamma-ray absorption, NMR micro-imaging and CATSCAN techniques. The developed method was then applied to measure the time course of cake layer thickness variation on the membrane surface during cross-flow micro-filtration of a PMMA suspension. Results of measurements show that the dynamic surface shape is not at all flat during a course of CFMF.