Illumination device and method for calibrating an illumination device over changes in temperature, drive current, and time
An illumination device and method is provided herein for calibrating individual LEDs in the illumination device, so as to obtain a desired luminous flux and a desired chromaticity of the device over changes in drive current, temperature, and over time as the LEDs age. The calibration method may include subjecting the illumination device to a first ambient temperature, successively applying at least three different drive currents to a first LED to produce illumination at three or more different levels of brightness, obtaining a plurality of optical measurements from the illumination produced by the first LED at each of the at least three different drive currents, obtaining a plurality of electrical measurements from the photodetector and storing results of the obtaining steps within the illumination device to calibrate the first LED at the first ambient temperature. The plurality of optical measurements may generally include luminous flux and chromaticity, the plurality of electrical measurements may generally include induced photocurrents and forward voltages, and the calibration method steps may be repeated for each LED included within the illumination device and upon subjecting the illumination device to a second ambient temperature.
1. A method for calibrating an illumination device comprising at least a first light emitting diode (LED) and a photodetector, the method comprising:
subjecting the illumination device to a first ambient temperature;
successively applying at least three different drive currents to the first LED to produce illumination at three or more different levels of brightness;
obtaining a plurality of optical measurements from the illumination produced by the first LED at each of the at least three different drive currents, wherein the plurality of optical measurements comprise luminous flux and chromaticity;
obtaining a plurality of electrical measurements from the photodetector, wherein the plurality of electrical measurements comprise a plurality of photocurrents, which are induced on the photodetector by the illumination produced by the first LED at each of the at least three different drive currents, and forward voltages that are measured across the photodetector before, during or after each induced photocurrent is measured; and
storing results of the obtaining steps within the illumination device to calibrate the first LED at the first ambient temperature.
2. The method as recited in claim 1 , wherein each of the forward voltages is measured across the photodetector by applying a non-operative drive current to the photodetector.
3. The method as recited in claim 2 , wherein the non-operative drive current applied to the photodetector ranges between approximately 100 μA and approximately 1 mA.
4. The method as recited in claim 1 , wherein before, during or after each induced photocurrent is measured, the method further comprises applying a non-operative drive current to the first LED and measuring a forward voltage that develops across the anode and cathode of the first LED.
5. The method as recited in claim 4 , wherein the non-operative drive current applied to the first LED ranges between approximately 0.1 mA and approximately 10 mA.
6. The method as recited in claim 1 , further comprising:
subjecting the first LED to a second ambient temperature, which is different from the first ambient temperature; and
repeating the steps of successively applying at least three different drive currents to the first LED, obtaining a plurality of optical measurements from the illumination produced by the first LED at each of the at least three different drive currents, obtaining a plurality of electrical measurements from the photodetector, and storing results of the obtaining steps to calibrate the first LED at the second ambient temperature.
7. The method as recited in claim 6 , wherein the illumination device comprises a plurality of LEDs including the first LED, and wherein the method is performed for each of the plurality of LEDs.
8. The method as recited in claim 1 , wherein the at least three different drive currents comprise a first operative drive current that is substantially equal to a maximum drive current associated with the first LED, a second operative drive current that is substantially less than the first operative drive current, and a third operative drive current that is substantially less than the second operative drive current.
9. The method as recited in claim 1 , wherein the illumination device comprises an additional photodetector, wherein the method further comprises obtaining a plurality of electrical measurements from the additional photodetector, wherein the plurality of electrical measurements comprise a plurality of photocurrents, which are induced on the additional photodetector by the illumination produced by the first LED at each of the at least three different drive currents, and a plurality of forward voltages that are measured across the additional photodetector before, during or after each induced photocurrent is measured.
10. The method as recited in claim 9 , wherein the first LED is a phosphor converted LED, wherein the plurality of photocurrents induced on the photodetector by the illumination produced by the phosphor converted LED correspond to a first portion of a spectrum of the phosphor converted LED, wherein the plurality of photocurrents induced on the additional photodetector by the illumination produced by the phosphor converted LED correspond to a second portion of the spectrum of the phosphor converted LED, and wherein the first portion and second portion comprise substantially different peak wavelengths.