OBJECTIVE This study aims to classify annular bed flow patterns in the bottom spray fluid bed coating process, study their influence on coat uniformity and investigate the feasibility of developing real-time annular bed flow pattern detection as a PAT tool. METHODS High-speed imaging and particle image velocimetry were used to visualize annular bed flow. Color coating and subsequent tristimulus colorimetry were employed to determine influence of annular bed flow pattern on coat uniformity. Feasibility of monitoring annular bed flow pattern through an observation window was tested using miniaturized particle velocity field and time series particle velocity orientation information. RESULTS Three types of annular bed flow patterns were identified. Plug flow gave the best coat uniformity followed by global and localized fluidization. Plug flow may be advantageous for high spray-rate conditions, large-scale coating and prevention of particle segregation. Plug flow could be differentiated from the other flow patterns through a simulated observation window. CONCLUSIONS Annular bed flow patterns were classified and found to influence particle coat uniformity noticeably. Availability of annular bed flow information for large-scale coaters would enable adjustments for process optimization. This study highlights the potential of monitoring annular bed flow pattern as a PAT tool.