Sheet flatness measurement system and method

A system for measuring the flatness of large reflective and semi-reflective surfaces including in combination:

• a base member having a support surface in a first plane on which a sheet, the flatness of the surface of which is to be measured, is placed;
• a frame;
• a structured illumination system mounted on said frame over said base member to simultaneously illuminate the entire surface to be measured, said structured illumination system located in a second plane parallel to said first plane and producing a periodic pattern of opaque and illuminated lines forming a periodic variation in light level along at least one direction parallel to the support surface of said base member;
• means for moving the structured illumination system in said second plane relative to the base member along the direction of illumination;
• means for detecting the light level at multiple locations reflected from the surface of a sheet on the support surface of said base member;
• and
• computing means coupled with said detecting means for determining the local slope of the surface of a sheet on said base member from the reflected structured illumination pattern produced by slope variations in the surface of such a sheet.