Abstract: A method of testing large area panes of a transparent material for optical quality, especially automotive windshield, uses a light box having a uniformly illuminated wall along which a planar grating is provided. The grating has an area at least equal to that of the windshield which is spaced from that gravity. An objective in line with the windshield and the grating forms an image of the grating through the windshield on a plane having a reference grating of much smaller size to form a Moire-pattern superposition which is picked up by a video camera and subjected to phase shift analysis.