A camera-based sensor device (200) and method for use in a controllable lighting system (100). The camera-based sensor device (200) comprises: a communications interface (203); a camera (201) for capturing images of a scene, each image comprising an array of pixels; and a processor (202). The processor (202) is configured to determine at least one light performance indicator, LPI, from an image captured by the camera (201). The LPI is a combined metric derived from a plurality of pixels of the array of pixels in the image. The processor (202) is configured to transmit, via the communications interface (203), the determined at least one LPI to the controllable lighting system (100) for use by the controllable lighting system (100) to make a control decision based on the LPI. The processor (202) does not transmit any of the images of the scene.