According to the ASTM standard, distortion is defined as follows:
“Distortion—the rate of change of deviation resulting from an irregularity in a transparent part. Expressed as the angular bending of the light ray per unit of length of the part, for example, milliradians per centimeter. May also be expressed as the slope of the angle of localized grid line bending”
Non-uniform thickness of a transparency, excessive curvature of a transparency, heat-induced stress, structural stress generated during the installation of a transparency into the canopy frame and the installation of a canopy on the aircraft are the most common causes of transparency distortion. Objects viewed through a distorted transparency can be magnified, lengthened, or misshaped, their movement can be misjudged, etc.
How is distortion usually measured today in the aircraft transparency industry? Transparency distortion is usually measured directly by the manufacturer, in specially equipped rooms. A typical apparatus for measuring distortion consists of a grid board, a digital camera and a stand for fixing the windshield/canopy that is to be examined. In order to measure the distortion, it is necessary to put the camera in the position equivalent to that of pilot’s eye and make a picture trough the transparency.
By visually examining the captured image(s), the inspector locates the region with clearly apparent distortion. In that region, the angle of tangent slope in a portion of deformed grid lines has to be calculated (the slope for both horizontal and vertical lines should be calculated) and then compared to the adequate standard.
Distortion has been measured in this way virtually since the dawn of the aircraft transparency industry! In practice this method has been shown to be slow, inefficient and highly susceptible to subjective interpretation by all links in the quality assurance chain, from the transparency manufacturer to the pilot as the end user.