Methods and apparatus are disclosed for providing optical emission feedback control for an illumination system comprising mixed light including light from a first light source (135) and a second light source (140). Each light source is driven by a drive current configured using a control and/or modification signal associated with that light source. The control signal in turn can be configured using a modification signal associated with the light source. An optical signal indicative of the mixed light is generated, for example using an optical sensor (150), and the optical signal is processed based on a reference signal to provide measurements indicative of light from each light source, which are used for feedback control of the illumination system. The reference signals can be generated locally or based on a corresponding control or modification signal. To provide measurements for a light source, processing (198) of the optical signal can comprise mixing (235) and compensation (255) operations based on control and/or modification signals associated with that light source.