Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
1999-12-02
2001-07-03
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S224000, C315S149000, C315S155000, C315SDIG004, C250S205000
Reexamination Certificate
active
06255784
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to controllers for lamps used to illuminate liquid crystal displays (“backlights”) and the like and, in particular, to a backlight controller that accurately controls the apparent brightness of the backlight to the human eye.
Liquid Crystal Displays (LCD) provide a rugged and flexible display suitable for use in automotive applications. The LCD is backlit typically by a cold cathode fluorescent lamp (CCFL). Such fluorescent lamps are bright and relatively efficient and can be fabricated to provide even illumination over a large area.
Unfortunately, CCFL's are sensitive to temperature and vary in brightness as the passenger compartment and console warms up. One method of correcting for this variation is to make a temperature measurement at the CCFL and to vary its driving power to maintain constant illumination. The relationship between driving power, illumination and temperature is a complex function which may be implemented in lookup tables or algorithms incorporated into a microcontroller providing an output to the lamp. This approach is not entirely successful at low brightness levels and elevated temperatures where small changes in power, typically less than the quantization errors of the algorithms or tables, can result in greatly varying brightness.
A second approach to controlling the brightness of such lamps is to monitor their light output using an electronic light sensor and to use the measured output to control the power to the lamp by means of a feedback loop. A silicon photodiode may be used to measure the light from the lamp. Silicon photodiodes are inexpensive, rugged and available in a wide range of different package types including those suitable for surface mounting on a printed wiring board. Surface mount packages are smaller than packages that require sockets or holes in the printing wiring board to receive leads. Surface mounting lowers the cost of the photodiode.
Silicon photodiodes have sensitivity to a range of light frequencies that extend significantly into the infrared region invisible to the human observer. Unfortunately at cold temperatures, the CCFL's output a large amount of infrared radiation. For this reason, if an ordinary silicon photodiode is used to control a CCFL, the perceived brightness of the CCFL to a human observer will vary, being dimmer at cold temperatures, for example, when the output of infrared radiation is greater.
This problem may be solved by the use of an infrared filter, such as a special glass that absorbs the infrared portion of the light before it strikes the photodiode. Photodiodes with such filters, however, are relatively expensive and are not available in packages desired for use in the automotive field.
SUMMARY OF THE INVENTION
The present invention provides a photodetector that approximates the response of the human eye (the photopic curve) and which therefore may be used to control the perceived brightness to a human observer of a CCFL or other similar backlight. Instead of using a relatively expensive “infrared-filtered” silicon photodiode, the present inventor has recognized that common “daylight-filtered” silicon photodiodes (filtering out all but the infrared region) may be used in conjunction with an unfiltered silicon photodiode to produce the same result. The signal from the daylight-filtered photodiode is subtracted from the unfiltered photodiode to approximate the photopic curve.
In contrast to infrared-filtered photodiodes, daylight-filtered silicon photodiodes are widely available at low cost and a wide variety of packages for use in consumer electronics such as infrared remote controls where the daylight filtering prevents ambient light from interfering with the infrared control signals. For this reason, combining the two devices of a daylight-filtered and unfiltered silicon photodiode may produce a photopic detector of lower cost and smaller package size than a single infrared-filtered silicon photodiode.
Specifically then, the present invention provides an illumination control for a fluorescent backlight having a first light sensor producing a first electronic signal dependent on sensed light in a first and second frequency range of sensitivity where the first frequency range of sensitivity corresponds substantially to light frequencies that the human eye sees and the second range of sensitivity corresponds substantially to light frequencies that the human eye does not see. A second light sensor is also used producing a second electronic signal dependent on sensed light in a third frequency range of sensitivity substantially overlapping the second frequency range but not the first frequency range. The fluorescent backlight illuminates the first and second light sensors by an amount dependent on a drive signal from a fluorescent driver circuit, which varies the drive signal to the fluorescent bulb according to a feedback control signal input. A subtractor subtracts the second electronic signal from the first electronic signal to produce the feedback control signal.
Thus it is one object of the invention to provide a light sensor that effectively conforms to the photopic curve without the need for expensive filters or a limited selection of device packages. By independently sensing light that is invisible and visible to the human eye and subtracting one from the other, a feedback control signal related to the desired response curve may be obtained.
The first and second light sensors may be silicon photodiodes and the second light sensor may include a filter blocking light in the first range.
Thus it is another object of the invention to make use of commonly available silicon photodiodes having similar or identical electrical characteristics. A filter on one photodiode provides the necessary frequency discrimination. It is another object of the invention to make use of commercially available daylight-filtered photodiodes widely used in consumer electronics.
The fluorescent driver circuit may also receive a desired brightness signal and may also control the drive signal of the fluorescent bulb according to the desired brightness signal.
Thus it is another object of the invention to provide a control that may accept an independent brightness signal allowing the user to change the brightness of the fluorescent tube.
The fluorescent bulb may be mounted against a first face of a printed wiring board and the first and second detectors may be mounted on the first face of the printed wiring board between the fluorescent bulb and the printed wiring board to receive light directly from the back of the fluorescent bulb. Alternatively, the printed wiring board may include at least one aperture and the first and second detectors may be mounted on a second obverse face of the printed wiring board over the aperture to receive light from the fluorescent bulb through the aperture.
Thus it is another object of the invention to provide a rugged and low cost assembly for supporting and controlling the fluorescent bulb.
A scaling circuit may be used to scale at least one of the first and second electronic signals prior to receipt by the subtractor.
Thus it is another object of the invention to provide for corrections of errors caused by filter absorption, small differences between the electrical characteristics of the first and second light sensors, and other dependencies such as temperature dependency, through the use of a scaling circuit that may be adjusted for the particular devices.
REFERENCES:
patent: 4368406 (1983-01-01), Kruzich et al.
patent: 5051924 (1991-09-01), Berman et al.
patent: 5057744 (1991-10-01), Barbier et al.
patent: 5170035 (1992-12-01), Webster et al.
patent: 5194782 (1993-03-01), Richardson et al.
patent: 5210467 (1993-05-01), Nagashima
patent: 5340974 (1994-08-01), Zalewski
patent: 5504661 (1996-04-01), Szpak
Philogene Haissa
Quarles & Brady LLP
Visteon Global Technologies Inc.
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