Electric lamp and discharge devices: systems – With radiant energy sensitive control means – Radiant energy responsive load device
Reexamination Certificate
1998-12-31
2001-06-26
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
With radiant energy sensitive control means
Radiant energy responsive load device
C315S151000, C315S112000, C315S224000
Reexamination Certificate
active
06252355
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates, generally, to a control system for maintaining optimum efficiency of a backlight and, more particularly in a preferred embodiment, to a closed loop temperature controller for adjusting the temperature within a fluorescent lamp to thereby optimize lamp arc drive for a given predetermined brightness set point.
2. Background Art and Technical Problems
Screen displays which employ fluorescent lamp backlights are used extensively in commercial, military, and consumer electronic applications. For example, such backlights are commonly used in desktop computers, laptop computers, screen displays for industrial equipment, and in connection with “heads up” or other screen displays in the cockpits in both commercial and military aircraft.
Conventional fluorescent lamps are commonly employed in backlit Liquid Crystal Display (LCD) applications. In a typical LCD, alphanumeric characters and other graphical images are produced on the viewing screen by selectively energizing or de-energizing preselected pixels in a two dimensional matrix to display the information. In a normally black screen display, predetermined pixels are illuminated to display the data or information as illuminated characters on a black (or other dark shade) background. In a normally white display, on the other hand, the desired data and/or information corresponds to the non-illuminated pixels, such that the information appears as black (or other dark color) images on a white (or other light color) background. In either case, a bright, consistent “background” light is necessary to achieve desirable contrast on the flat screen display. Indeed, in certain applications (e.g., military avionics), the high contrast provided by a bright backlight is essential to proper operation of the display.
It is also desirable to obtain a desired brightness while minimizing power consumption. This is particularly important in portable electronics, for example laptop computers and the like, where battery life is an important product feature.
Presently known systems for controlling the brightness of a fluorescent backlight lamp typically involve a control system for supplying lamp arc drive to the backlight, to thereby excite the gas atoms within the sealed lamp enclosure to create visible light. The amount of visible light emitted by the lamp is sensed, for example by a photodiode, and a feedback signal indicative of the brightness output of the lamp is fed back to a control circuit. This feedback signal (indicative of actual brightness) is compared to an input signal representative of a desired brightness level, and presently known control systems drive the difference between this actual signal and the desired signal to a minimum. Under this control regime, if the actual brightness is less than the desired brightness, the controller increases the lamp arc drive applied to the lamp until the actual brightness equals the desired brightness. If, on the other hand, the actual brightness is greater than the desired brightness, the controller circuit reduces the magnitude of the lamp arc drive applied to the lamp until the actual brightness emitted from the lamp again equals the desired brightness level for the lamp. Presently known prior art brightness control systems typically employ a “cold spot” at a predetermined point on the lamp which functions to keep a certain amount of the gas (typically mercury) within the lamp in a condensed state. Such “cold spot” systems employ the well known principle that maintaining the temperature of the cold spot in a specified range allows for very efficient operation of the lamp. Presently known systems, however, often require expensive components to maintain the cold spot, and do not adequately compensate for drifting or degradation over time of some of the parameters which influence the efficiency of the lamp.
A fluorescent lamp control system is thus needed which overcomes the shortcomings of the prior art.
SUMMARY OF THE INVENTION
The present invention provides improved methods and apparatus for optimizing the operating efficiency of a fluorescent lamp. In accordance with a preferred embodiment of the present invention, a primary control system controls the lamp arc drive to the lamp. The primary control system includes a desired brightness set-point as a first input, and a feedback signal corresponding to actual brightness detected at the lamp as a second input. The primary controller is configured to drive the difference between the aforementioned first and second inputs to a minimum: that is, to the extent the actual (detected) brightness of the lamp is greater than the desired “set-point” brightness, the primary controller reduces the lamp arc drive applied to the lamp until the actual brightness exhibited by the lamp is equal to the desired set-point brightness. Conversely, to the extent the actual brightness of the lamp is less than the desired set-point brightness, the primary controller increases the lamp arc drive signal until the actual brightness exhibited by the lamp equals the desired set-point level. In accordance with one aspect of the present invention, the primary controller operates in real time, essentially adjusting the lamp arc drive instantaneously as a function of the detected brightness feedback signal.
In accordance with a further aspect of the present invention, a secondary controller is employed to fine tune the control of the lamp by determining whether the desired brightness level may be achieved more efficiently. In accordance with a preferred embodiment, the secondary controller has a slower response time than the primary controller, and is configured to vary one or more parameters associated with the lamp to maintain the desired output level in an optimally efficient manner by adjusting one or more of the following: lamp arc drive, lamp temperature, lamp pressure, lamp volume, the quantity of gas within the lamp, or any other parameter which may effect the relative proportion of gas within the lamp in the vapor phase compared to the condensed phase or the efficiency with which the lamp produces a desired brightness output level.
In accordance with a preferred embodiment of the present invention, when the primary controller achieves an output brightness which is equal to the desired set-point brightness level, the primary controller outputs a constant lamp arc drive signal to the lamp. During this period in which a constant arc drive is applied to the lamp, the secondary controller varies a control parameter (e.g., temperature) associated with the gas within the lamp to determine whether a change in the control parameter (either upwardly or downwardly) causes a corresponding increase (or decrease) in lamp brightness. If lamp brightness decreases in response to varying the control parameter, the secondary controller may be configured to either reverse the change in the control parameter which caused the decrease in lamp brightness, or alternatively, the secondary controller may be configured to simply stop varying the control parameter in the direction which caused the decrease in lamp brightness.
If, on the other hand, the lamp brightness increases as a result of the secondary controller varying the control parameter, the secondary controller may be configured to continue to adjust the control parameter in the direction which caused an increase in the lamp output brightness until the lamp output brightness is maximized. Preferably, the parameter monitored and controlled by the secondary controller is the lamp temperature. This is achieved, for example, by monitoring and controllably varying the voltage and/or current through a resistive wire coupled to the lamp surface.
In accordance with the further aspect of the present invention, a desired lamp brightness output level may be achieved while driving the lamp arc drive required to obtain the desired lamp brightness to a minimum. Thus, the desired lamp brightness may be achieved while reducing the power required to achieve the desired brightness
Meldrum Richard Mitchel
Pitman Bruce Anthony
Honeywell International , Inc.
Lee Wilson
Vu David
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