A light emitting system comprises an LED-based light emitting device and a controller for controlling operation of the device. The device comprises at least two LEDs that are operable to generate light of different colors that contribute to the emission product of the device. The controller is operable to control light emission from the LEDs in response to the measured intensity of the first and second color light contributions in the emission product. To measure the individual light contributions the controller is operable to interrupt, or at least change, light emission from one LED for a selected time period and during this time period to measure the intensity of the emission product of the device. The intensity of light of the first and second color can be determined by comparing the measured intensity with the measured intensity when the light emission from the other LED is interrupted or changed.