Electric lamp and discharge devices: systems – Current and/or voltage regulation – Automatic regulation
Reexamination Certificate
2001-10-22
2003-10-07
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
Automatic regulation
C315S308000, C362S800000
Reexamination Certificate
active
06630801
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to RGB based LED luminaries, and more particularly, to a method and apparatus for controlling an RGB based LED luminary, in which the LED luminary is adjusted according to measured differences in wavelengths between the actual wavelengths output by each LED and a desired wavelength of each LED so that the LED luminary generates a desired color and lighting level.
2. Background Information
As well known in the art, red, green and blue (RGB) light emitting diode (LED) based luminaries generate various colors of light which when properly combined produce white light. RGB LED based luminaries are widely used in applications such as, for example, LCD back lighting, commercial-freezer lighting and white light illumination. Illumination by LED based luminaries presents difficult issues because the optical characteristics of individual RGB LEDs vary with temperature, forward current, and aging. In addition, the characteristics of the individual LEDs vary significantly batch-to-batch for the same LED fabrication process and from manufacturer to manufacturer. Therefore, the quality of the light produced by RGB based LED luminaries can vary significantly and the desired color and the required lighting level of the white light cannot be obtained without a suitable feedback system.
One known system for controlling an RGB LED white luminary uses a lumen-feedback temperature-feed-forward control system which controls a white LED luminary so as to provide a constant color light with a fixed lumen output. The temperature-feed-forward control system provides compensation for variations in the color temperature and supplies the reference lumens. The lumen feedback control system regulates each RGB LED lumens to the reference lumens. This type of control system requires the characterization of each type of LED with changes in temperature, which requires a costly factory calibration. In addition, this control system also requires that the LEDs be briefly turned off for light measurements. The turning-off of the LED light sources introduces flicker to the light source. Therefore the power supplies must have a relatively fast response time. In addition, a PWM (pulse-width-modulation) driving method is required to overcome the LED variations with forward current. With the PWM control, the implementation becomes complex and, in addition, the LEDs are not utilized to their full capacity.
Another known prior art system compares the feedback tristimulus values (x,y,L) of the mixed output light of the RGB based LED luminary with tristimulus values representative of the desired light, and adjusts the forward currents of the LED luminary in such a way that the difference in tristimulas values is decreased to zero. The system control includes a feedback unit including photodiodes which generate the feedback tristimulus values of the LED-luminary, and a controller for acquiring a difference between the feedback tristimulus values and the desired reference tristimulus values. The system generates control voltages which adjust the forward currents of the LED luminary so that the difference in tristimulus values is decreased to zero.
The tristimulus values under comparison may be either under the CIE 1931 tristimulus system or under a new RGB calorimetric system. In either case, the control of the luminary tracks the reference tristimulus values. Thus, under a steady-state where the feedback tristimulus values follow the desired reference tristimulus values, the light produced by the LED luminary has the desired target color temperature and lumen output, which are regulated to the target values regardless of variations in junction temperature, forward current and aging of the LEDs.
The efficiency and accuracy of these prior art methods depend on their ability to sense both the CIE chromaticity coordinates as well as the luminous intensity L of the white color point. There exists a need in the art for a system and method of controlling RGB based LED luminaries which is not dependent upon sensing CIE chromaticity coordinates as well as the luminous intensity L of the white color point.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to overcome disadvantages of the prior art systems and methods of controlling an RGB based LED luminary.
In accordance with one form of the present invention, a method of controlling an LED luminary including red, green and blue (RGB) light emitting diodes (LEDs) driven by forward currents to produce a mixed color light includes the steps of:
measuring an output signal of a filtered photodiode for each of the red, green and blue LEDs of the LED luminary;
measuring an output signal of an unfiltered photodiode for each of the red, green and blue LEDs of the LED luminary;
calculating a photodiode output signal ratio by dividing the output signal for the filtered photodiode with the output signal of the unfiltered photodiode for each of the red, green and blue LEDs;
utilizing the photodiode output signal ratio to determine the chromaticity coordinates for each of the red, green and blue LEDs; and
adjusting the forward currents for each of the red, green and blue LEDs to produce a desired color light.
In accordance with another form of the present invention, a control system for an LED luminary including red, green and blue (RGB) light emitting diodes (LEDs) driven by forward currents to produce a mixed color-light includes:
a feedback unit which generates feedback values representative of the mixed color light produced by said LED luminary, said feedback values corresponding to output signals of a photodiode; and
a controller which acquires a difference between said feedback values and reference values representative of a desired mixed color light, said controller adjusting said forward currents in accordance with said difference.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments, which is to be read in connection with the accompanying drawings.
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Lümileds USA
Tran Thuy Vinh
Wong Don
LandOfFree
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