<title>Lamp-life predictive model for avionics backlights</title>

Abstract

Active Matrix Liquid Crystal Displays (AMLCDs) used in avionics applications require high luminance, high efficacy, and long-life backlights. Currently, fluorescent lamps are the favored light sources for these high performance avionics backlights. Their spectral characteristics and high electrical efficiency are well suited to illuminating AMLCDs used in avionics applications. Fluorescent lamps, however, suffer gradual reduction in luminance output caused by various degradation mechanisms. Korry Electronics Co. recently developed a mathematical model for predicting fluorescent lamp life. The model's basis is the well characterized exponential decay of the phosphor output. The primary luminance degradation mechanism of a fluorescent lamp is related to the arc discharge. Consequently, phosphor depreciation is proportional to the discharge arc power divided by the phosphor surface area. This 'wall loading' is a parameter in the computer model developed to extrapolate long-term luminance performance. Our model predicts a rapidly increasing decay rate of the lamp output as the input power is increased to sustain constant luminance. Eventually, a run-away condition occurs -- lamp arc power must be increased by unrealistically large factors (greater than 5x) to maintain the required luminance output. This condition represents the end of the useful lamp life. The lamp life model requires the definition of several key parameters in order to accurately predict the useful lamp life of an avionics backlight. These important factors include the construction of the lamp, lamp arc power, a decay constant based on the phosphor loading, and the operational profile. Based on the above-mentioned factors, our model approximates the useful lamp life of an avionics backlight using fluorescent lamp technology. Comparisons between calculated and experimental lamp depreciation are presented.