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Automatic Legibility Control (ALC) of Electronic Displays

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Final rept. 19 Aug 1994-11 Feb 2003

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Pilot legibility requirements are investigated to establish a basis for implementing automatic control over electronic and conventional displays, in day through night aircraft cockpit viewing environments. Applicable legibility test results, published by different experimenters over much of the past century, are examined and analyzed, and comparisons are used to assess their validity as display control criteria. The mathematical model of the human visual system developed provides a basis for the design of display automatic legibility controls, suitable for use in aircraft cockpits, by predicting the pilots image difference luminance requirements, in the presence of luminance specularly and diffusely reflected from the display viewing surfaces, and with discrete and distributed glare sources incident on the pilots eyes. Although this general automatic legibility control ALC model and the automatic brightness control ABC model, used in the newer Boeing airliners, appear quite different, comparisons show that, when properly implemented, the two models give equivalent results. The advantage of the ALC model is its greater design flexibility, owing to its better defined physical relationship to vision. Various practical design choices that should be taken into consideration to achieve a successful application of automatic legibility controls are explored in the balance of the report. This includes, for example methods of achieving compatibility with light reflective, transflective, emissive and transmissive operating mode displays the relative merits of viable alternatives to the recommended constant legibility control characteristic strategy interactions between controls for legibility and sensor-video signal conditioning the type and need for pilot legibility trim controls and possible response time compensations for the light and dark adaptation of the pilots eyes during the following glare source exposures.

Subject Categories:

  • Flight Control and Instrumentation
  • Optics

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