Direct red light-emitting diodes based on InGaAlP comprise a strong temperature sensitivity regarding their output flux. In étendue-limited applications, like digital projectors, these LEDs are usually driven at current densities exceeding 3 A/mm2 in pulsed mode. The losses inside the semiconductor lead to a large amount of heat, which has to be removed most efficiently by a heatsink to keep the junction temperature as low as possible and therefore to obtain the maximum output flux. One important performance parameter is the thermal resistance Rth of the LED, which has been improved during the last few years, e.g. by the development of new high-power chips and packages. In our present approach, we investigated the influence of the driving frequency – which is closely related to the thermal impedance Zth – on the luminous and the radiant flux of red LEDs. A simulation model based on the electro-thermal analogies was implemented in SPICE and the optical and electrical characteristics of one LED type (OSRAM OSTAR Projection Power LE A P1W) were measured under application-related driving conditions while varying the parameters frequency, duty cycle, forward current, and heatsink temperature. The experimental results show clearly that the luminous and the radiant flux go up when the driving frequency is increased while the other parameters are maintained. Moreover, it can be noticed that the degree of this effect depends on the other parameters. The largest impact can be observed at the lowest tested duty cycle (30 %) and the highest tested current density (4 A/mm2) and heatsink temperature (80 °C). At this operating point, the luminous and the radiant flux increase by 20 % and 14 % respectively when raising the frequency from 240 Hz to 1920 Hz.