Abstract: A method for measuring the size of dimple-type defects in an area of the surface of a flat piece of transparent material, such as an optical disc. A large beam of collimated light is provided and passed through the piece of material in a normal orientation with respect to the surface being inspected. A screen is placed in the path of the beam emerging from the disc and circular shadows generated by refraction are measured to determine the size of the defects which produces them.

Abstract: Apparatus for producing a shadowgraph image of a transparent part to be examined, such as part of a video disc. The apparatus includes a source of coherent light, and an objective lens system for focusing the coherent light to produce a divergent beam, a concave mirror disposed in the path of the divergent beam, a transparent plate for supporting the disc to be examined in the aperture of the concave mirror, and a projection screen to receive light reflected from the concave mirror. The apparatus may also include plane mirrors to permit the projection screen to be positioned in a convenient location with respect to the disc being examined.