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Abstract

An illumination device and method is provided herein for controlling individual light emitting diodes (LEDs) in an LED illumination device, so that a desired luminous flux and a desired chromaticity of the device can be maintained over time as the LEDs age. According to one embodiment, the method comprises applying drive currents to a plurality of LED chains substantially continuously to produce illumination, measuring a photocurrent induced on the photodetector in response to the illumination produced by each LED chain, one LED chain at a time, and received by the photodetector, and measuring a forward voltage developed across the photodetector by applying a non-operative drive current to the photodetector. For each LED chain, the method may further comprise determining an expected photocurrent value corresponding to the forward voltage measured across the photodetector and the drive current currently applied to the LED chain by applying one or more interpolation techniques to a table of stored calibration values correlating forward voltage and photocurrent to drive current at a plurality of different temperatures, and adjusting the drive current currently applied to the LED chain if a difference exists between the expected photocurrent value and the measured photocurrent.

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First Claim