Online monitoring techniques can play a major role in understanding and controlling the mechanisms involved in the fouling of membrane filtration systems. The present study describes the application of an in situ optical method to monitor the growth of a fouling layer using a photointerrupt sensor array. To verify the validity of this method, the effects of the voltage applied, slurry concentration and membrane materials on the signal from the sensor array were examined. In addition, the application of this method in monitoring the growth of the fouling-layer thickness during the filtration of a high-turbidity influent stream under constant flux was studied. The results from three-dimensional (3D) fouling contour studies showed that the average fouling-layer thickness increased proportionally with time during the initial period in response to the rapid deposition of particles on the membrane surface, after which the growth rate of the fouling layer decreased. The 3D fouling contour analyses suggest that this online monitoring technique provides an effective in situ measurement of fouling-layer thickness and distribution. The photo-sensor array proved to be a promising online monitoring technique for scanning membrane surfaces on demand and can be applied to a pilot-scale water treatment unit for use in the real-time profiling of changes in fouling-layer thickness.