ANSI/SAE Z26.1 15.5

There are several different methods of measuring the optical distortion of glazing materials used in automotive and other transportation windscreens. ASTM C1036 uses a "zebra board" to quantify the amount of waviness in a piece of glass; ASTM F801 uses light transmission with a detector array to measure the angular deviation due to non-parallelism (or "wedginess") of glass.

Most metropolitan transportation agencies (such as bus and light rail) purchase glass conforming to ANSI/SAE Z26.1. Section 5.15 of this standard provides a fairly quick and simple way to measure angular deviation and also optial distortion arising from density differences.

The angular deviation test is performed by passing light through a annular aperture with a central circular aperture. When this set of apertures is viewed through a specimen of flat glass, a secondary image of both the central "dot" and surrounding "ring" is visible. These secondary images appear to move relative to one another when viewed thorugh locally non-parallel regions of the flat glass. The results of the test are acceptable if the secondary image of the dot remains inside the image of the ring as the glass is scanned.

The images below show the secondary image when viewed though 100 year-old residential glazing with natural ripples in it. The secondary image of the dot can be seen as an elongated mark to the right of the dot.

The second part of ANSI Z26.1 15.5 uses a variation of a shadowgraph technique to find density differenced in glazing materials. These areas of density variation would normally apprear as optical distortions when viewed through the glass. The setup for this test is failry simple. A high-power light source (in this case, a 750 W incandescent lamp) is projected through a long focal-length lens to produce nearly parallel rays of light at the projection screen. This test can also be done with a mirror to divert the rays of direct sunlight into a darkened room onto a screen.

The figure below shows the appearance of light and dark areas on the screen when the projected light passes thorugh a piece of flat glass with regions of different optical density. These regions would be otherwise difficult to see when viewed thorugh the glass.

At Principia, we use this equipment and others to test automotive glazing materials to applicable standards.