A lighting panel system includes a lighting panel including a first string of solid state lighting devices configured to emit light at a first wavelength and a second string of solid state lighting devices configured to emit light at a second wavelength, different from the first wavelength, and a current supply circuit configured to supply a drive current to the first string in response to a control signal. A photosensor is arranged to receive light emitted by the panel, and a control system is configured to sample an output signal of the photosensor and adjust the control signal responsive thereto to thereby adjust an average current supplied to the first string by the current supply circuit. Methods of operating a lighting panel are also provided.