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.