A method of estimating the junction temperature of a light emitting diode comprises driving a forward bias current through the diode, the current comprising a square wave which toggles between high and low current values (Ihigh, llow), the high current value (lhigh) comprising an LED operation current, and the low current value (IIOW) comprising a non-zero measurement current. The forward bias voltage drop (Vf) is sampled and the forward bias voltage drop (Vflow) is determined at the measurement current (IIOW)—The temperature is derived from the determined forward bias voltage drop.