Abstract: The present invention relates to a method for checking flat transparent surfaces permitting the discrimination between the flaw in the matter and dirt which has been deposited on the surface but does not affect the flatness or the transparency of the sheet. The apparatus includes a photosensitive captor located above the sheet to be checked. The captor is arranged at the limits of a fuzzy real image which an optical system gives in the plane of this captor of a light source located below the sheet. In this way, a flaw causes the excitation of the captor at a level higher than the normal light threshold of the fuzzy image because of refraction caused by the flaw in the matter. In contrast, the presence of dirt will cause, through occultation, an excitation lower than the normal light threshold thereby enabling differentiation between the flaw and the dirt.