A system having a camera (18) for capturing video images of a scene in successive image frames, and a computer system (14 or 20) for receiving such video images. The computer system periodically generates a background image of the scene from multiple successive image frames (23) and extracts features in the background image (26), and extracts features for each new image frame received from the camera (28). For each new image frames, the new image frame and the last periodically generated background image are correlated at common locations (parts or regions) associated with the features extracted from the last periodically generated background image and features of the new image frame (30), to determine non-correlated features in the new image frame with respect to the last periodically generated background image (31). If the number and/or percentage of non-correlated features are sufficiently high, and/or the spatial distribution of non-correlated features is sufficiently low, the image frame is determined to have an invalid background (32, 33). When multiple successive frames are determined as having invalid backgrounds, the camera (20) represents an invalid camera.