In order to realize a light source for multiple colors, e.g. different color temperatures, in Solid State Lighting, LEDs emitting light of different colors, are used in one device. These LEDs define a sub area in the color space indicating the color gamut that can be realized via all possible linear combinations. Colored light of desired intensity within the color spectrum of the color space can hence be controlled by means of the signal input of the individual LEDs. It should be noted that white light is included in the term colored light.
Unlike traditional light sources colored LEDs will “color shift” due to the changes of the junction temperature, the current, aging effects and binning of the LEDs. As the light output starts to wane from the pre-defined color point due to temperature and aging, the color point of the light output of the light source will become distorted. Consequently, the color point of the light source has to be stabilized via a control loop.
For the purpose of monitoring the light output from a light source it is quite common to use light detectors, such as RGB-sensors or true-color-sensors having the same sensitivity as the standard observer or approximating these curves. Alternatively, flux sensors are used, possibly in combination with one or more temperature sensors. In order to be able to determine the contribution from individual LEDs or colors, which can be a group of LEDs as well, filtering techniques have been used. Thus, for example, three different ‘filtered’ photo detectors detecting primary colors (such as red, green, and blue light), respectively, have been employed in combination with one unfiltered photo detector. Based on the photo detector output signals, a measured color point of the light output of the light source has been determined and compared with a target color point in order to adjust the driving currents to the LEDs, if necessary.
However, when the light detectors detect the light output there may be interference from other light sources, for instance ambient sunlight and/or other color-LEDs or other interference sources creating electrical noise. The measured color point is therefore often defective and does not solely represent the light output from the LEDs, in the light source itself. Consequently the feedback signal to the control device of the control loop is incorrect.