Abstract: Apparatus for detecting and measuring imperfections such as wedge, combing and lack of planarity in transparent sheets such as glass comprises an extended luminous source, focusing means to form a convergent beam containing an image of the source, a differential photoelectric cell positioned to receive that image after passage of that beam through, or reflection thereof at, the sheet, and circuit means connected with the cell for producing a changed output upon shift of the image on the cell due to changed refraction or reflection of the beam at the sheet produced by a defect. The beam of focused light is limited or stopped so that a cross-sectional dimension thereof is of the same order of magnitude as the width of typical defects in the sheets.