The uniformity of coating applied to large particles and tablets in rotating-drum coating devices is of significant interest to the Pharmaceutical Industry, especially when the coating contains active material or provides a sustained release barrier for drug transport in a functional coat. The purpose of this research was to quantify parameters that characterize the movement of tablets through the region where they receive maximum coating. The three parameters of greatest interest are (1) circulation time (tau(circ)), (2) surface exposure time (tau(surf)) and (3) surface area of the tablet projected toward the spray source (nozzle) during each pass through the spray zone (A(tab)). In order to measure these parameters, a digital imaging system was developed and implemented to analyze images of the surface of the tumbling tablet bed in near real-time. The drum rotation rate was varied between 6 and 12rpm, at two different drum fills of 1/8 and 1/4. Three tablet sizes, 6.3, 7.9 and 10.4 mm, were used. The circulation time was found to vary from 2.9 to 15.5s and the surface time from 0.06 to 0.2s. Decreasing trends for circulation and surface times were observed with increasing drum speeds, drum loadings, and tablet size. Atab and surface velocities were also estimated for the movement of tablets through the spray zone in the rotating drum. The axial dispersion coefficient was estimated from random walk theory. It was found to lie in the range of 0.2 x 10(-3)-4 x 10(-3) cm(2)/s for the conditions studied and was found to increase with increasing drum speed. (C) 2004 Elsevier Ltd. All rights reserved.