Methods and apparatus are disclosed for providing combined time-varying light comprising light from one or more light sources (132, 134, 136), and determining aspects of light from one or more light sources based on measurements of the combined light. Light sources of one or more colors can be controlled to provide time-varying combined light outputs (310, 360) using different switching sequences for different light sources, for example according to PWM, PCM, or other modulation methods. By appropriately configuring the timing of the switching sequences, the combined light output can be made to exhibit a plurality of lighting combinations. A broadband optical sensor (148) can be configured to measure (145) some or all of the plurality of lighting combinations, and the measurements used to determine light output measurements of portions of the combined light, and optionally of ambient light, by appropriate processing (150) of the measurements.