Achieving color point stability in RGB multi-chip LED modules using various color control loops

The continuing research effort in high power LEDs will allow their use in high quality lighting systems in the (near) future. There are still a number of issues to tackle, for instance the LED's (strong) temperature dependence. This dependence will change the emitted flux and the spectral distribution of the LED. In addition, these parameters will also change as the LED ages. When creating white light by mixing red, green and blue LEDs, the temperature effects described above will already result in a visible color difference after a small rise in temperature. To overcome this issue, a number of LED color control loops have been developed. These loops can be based on: the heat sink temperature, flux measurements of each primary color, a combination of these last two and an integrated color point. For this purpose, an RGB test set up has been built, equipped with a temperature sensor and various photo-sensors. The appropriate color control loops have been implemented and tested in software. Some control loops use empirically determined LED parameters (dλ/dT or T0), the value of these parameters has been determined for a different set of LEDs. In addition, initial optical LED (and sensor) calibration has been performed at a single temperature only. The color stability of the various color control loops has been measured for a temperature increase of about 50 degrees Centigrade. In this range, we find that, on short term, all color control loops show a significant improvement in the color error, except for the color control loop based on flux measurements of each primary color, which performs nearly as mediocre as open loop. However, the color control loop based on the heat sink temperature cannot offer color stability when the LED ages, which is expected to be significant. The color control loop based on an integrated color point seems the most expensive one.