The rapid improvement of the white light efficacy achievable with light-emitting diodes (LEDs) opens up new opportunities in the general illumination market. An LED light source made of red, green, and blue LEDs (RGB-LEDs) can provide the unique feature of color variability, allowing the user to select the desired color point of the lamp. The white light color accuracy required in the general illumination market is a challenge for LEDs. The variation in lumen output and wavelength for nominally identical LEDs and the change in these parameters with temperature and time result in an unacceptably high variability in the color point of white light from RGB-LEDs. In this paper, we show that these problems can be overcome with suitable feedback control schemes that can be implemented in a practical LED lamp. We present results of experiment and theoretical modeling that shows the performance that can be achieved with a number of different control schemes.

The recent improvements in high-power lightemitting diodes (LED) technology with 100+ lumens per LED chip and efficacy exceeding that of incandescent lamps, brings the solid-state lighting close to a reality. An LED light source made of Red, Green and Blue (RGB) LEDs can provide a compact light source with unique features such as instant color variability. However, the white light generation using many compact, discrete RGB light sources has the following issues: uniform spatial light mixing and distribution, white color point maintenance and thermal management. Specifically, the white color point maintenance is a stringent requirement in many applications. Meeting this requirement is a severe challenge due to the variation in the optical characteristics of the RGB-LEDs with temperature, time and forward current and the spread in the LED performance. This results in 1) an unacceptably high variability in white light color point and 2) difficulties in manufacturing reproducible LED lamps.In this paper, we highlight the issues that introduce the variability in the color point and present feedback control schemes to overcome these problems. We also show the results of experiments and theoretical modeling for practical control systems.

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