led stabilization forward voltage patent
The method can be used for temperature control and compensation of known spectral temperature dependency as well, I think. As far as I see, LED forward voltage vs. temperature is typically unspecified by manufacturers. It's not guaranteed, that the sum of junction forward voltage (NTC) and bulk resistance voltage drop (typically PTC) results in an unequivocal characteristic at all operation currents, but I guess you get overall NTC behaviour with most types.