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Chapter 11:
Image Quality Control and Maintaining Performance
Abstract
An understanding of phosphor and cathode aging leads to the obvious assumption that things change over time. Maintaining, as-new performance cannot be isolated to peak luminance level criteria while the image quality, such as contrast modulation, deteriorates in real terms.Whether automatic compensation or scheduled service cycles using human intervention are used to maintain a display, the end result is the same. It is up to the end user to evaluate the facility’s needs and the cost of ownership associated with the different vendor solutions. Internal compensation can either address the total system response, including both cathode and phosphor aging, or the cathode only. A circuit that compares volts driven for cathode current produced and constantly adjusts the drive (video gain) can provide the latter. The circuit would have the as-new values as a reference point for calculating the required adjustment. This approach ignores the phosphor aging contribution. By using P45, which ages very slowly under the harshest of operating conditions, this element is minimized and can be ignored in the short term as long as routine calibration cycles are performed. A blended phosphor such as PC104 would cause an error to accumulate in a relatively short time. A solution including phosphor aging requires a luminance sensor. This can be achieved using internal photometers integrated into the display that would provide hands-free calibration checks. A number of approaches have been used in the past that place the sensor just under the bezel and detect black-level settings well, but not peak luminance. The phosphor under the sensor sees very little electron energy in normal operation and would not be aging equally with the screen center.
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CHAPTER 11
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