Using moire patterns to determine the distortion of graphic displays and graphic input devices

Before high-accuracy graphic displays can be successfully designed or implemented from existing displays, the display distortion need be known to a high degree of accuracy-typically of the order of 0.1 percent of the display area height or better. Present day methods of measuring distortion are clumsy and imprecise. Methods of measurement using moire fringes have been suggested before, but the treatment was intuitive and the interpretation quailtative. In this paper are derived quantitative relationships from which display distortion to the 0.1 percent accuracy mentioned can be obtained using moire fringes. Two distinct methods are described. Full experimental methods, limitations, accuracy of method, etc., are given. Actual case examples illustrate the method. Although the methods developed here were specifically for TV-mode displays, the methods are equally valid for any graphic display device such as cathode-ray tubes (storage or refresh), electromechanical X-Y plotters, flat-screen displays, etc., the only requirement being the ability to display or generate a parallel line pattern. The distortion of graphic input devices such as TV cameras, electronic X-Y input pads, etc., can also be obtained using these methods.