Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-04-27
2002-09-10
Bruce, David V. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S216000, C250S210000
Reexamination Certificate
active
06448550
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to the field of solid state illumination, and more particularly to solid state illumination systems employing closed loop control to maintain spectral characteristics.
2. Art Background
High brightness Light Emitting Diodes (LEDs) have sparked interest in their use for illumination. LEDs have no moving parts, operate at low temperatures, and exceed the reliability and life expectancy of common incandescent light bulbs by at least an order of magnitude. The main drawback in implementing LED based light sources for general illumination purposes is the lack of a convenient white-light source. Unlike incandescent light sources which are broadband black-body radiators, LEDs produce light of relatively narrow spectra, governed by the bandgap of the semiconductor material used to fabricate the device. One way of making a white light source using LEDs combines red, green, and blue LEDs to produce white, much in the same way white light is produced on the screen of a color television.
Combining light from blue, red, and green LEDs of appropriate brightness yields a “white” light. The brightness of each LED is controlled by varying the amount of current passing through it. Slight differences in the relative amounts of each color manifests itself as a color shift in the light, akin to a shift in the color temperature of an incandescent light source by changing the operating temperature. Use of LEDs to replace existing light sources requires that the color temperature of the light be controlled and constant over the lifetime of the unit.
Some applications require more careful control of spectral content than others, and differing color temperatures may be desired for different applications. For example, spectral control is of extreme interest in applications such as lighting of cosmetics counters, and food outlets, while spectral control may not be critical in industrial lighting applications where reliability is more important.
There are two effects which make careful control of spectral content difficult. First is that the luminous efficiency of a given LED will not exactly match that of another LED manufactured by a nominally identical process. The second is that the luminous efficiency of a given LED, and its spectral content, may shift over the lifetime of the device.
The first problem may be addressed by testing, grading, and matching devices during manufacture. This testing is expensive, and does not address changes occurring with device aging.
What is needed is a method of automatically measuring the spectral content of a LED light source, and controlling the spectral content based on that measurement.
SUMMARY OF THE INVENTION
Spectral content of a solid state illumination source composed of Light Emitting Diode (LED) sources of different colors is measured by photosensors mounted in close proximity to the sources. The results of these measurements are used to control the spectral content by varying the current to the different color LEDs.
REFERENCES:
patent: 4716285 (1987-12-01), Konishi
patent: 6122042 (2000-09-01), Wunderman et al.
Agilent Technologie,s Inc.
Bruce David V.
Hobden Pamela R.
Martin Robert T.
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