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Detection of Imperfections on a Reflective Surface by Schlieren Illumination

IP.com Disclosure Number: IPCOM000059909D
Original Publication Date: 1986-Feb-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 2 page(s) / 40K

Publishing Venue

IBM

Related People

Cooper, L: AUTHOR

Abstract

Imperfections and/or contaminants on a reflective surface are detected by a Schlieren optical illumination system. In Fig. 1, the rigid disk 1 is illuminated by light from a laser source 3, via a beam expander 5 and a first collimating lens 7. The incident light rays are reflected as shown schematically, and the reflected light is supplied to a vidicon detector 9, via a second collimating lens 11, an opaque optical stop element 13, located at the prime focus of lens 11, and an objective lens 15 focussed on the disk surface. With no imperfections on the surface of disk 1, no image is formed on the vidicon. With any imperfection, light is diffracted past the stop 13, and is imaged as a bright line or bright points on the surface of the vidicon, contrasted with a dark background field.

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Detection of Imperfections on a Reflective Surface by Schlieren Illumination

Imperfections and/or contaminants on a reflective surface are detected by a Schlieren optical illumination system. In Fig. 1, the rigid disk 1 is illuminated by light from a laser source 3, via a beam expander 5 and a first collimating lens 7. The incident light rays are reflected as shown schematically, and the reflected light is supplied to a vidicon detector 9, via a second collimating lens 11, an opaque optical stop element 13, located at the prime focus of lens 11, and an objective lens 15 focussed on the disk surface. With no imperfections on the surface of disk 1, no image is formed on the vidicon. With any imperfection, light is diffracted past the stop 13, and is imaged as a bright line or bright points on the surface of the vidicon, contrasted with a dark background field. Such a system is extremely sensitive to any heterogeneity of the reflective surface, and is advantageous in not requiring a scanning system, or moving parts in the optical system. A variation is shown in Fig. 2, using only one collimating lens, and a beam splitter 17, the remaining elements being identical with their counterparts in Fig. 1. As manifest from the drawing, illumination of disk 1 by laser source 3 is via the beam expander 5, through the beam splitter 17, and collimating lens 7. The reflected light travels through the lens 7, is reflected through 90 degrees by beam splitter 17, and hence through o...