<title>Measurement of complex shapes: optical noncontact methods</title>

Abstract

ABSTRACT For the purposes of this paper, the measurement of shapes by optical non contact methods is divided intotwo categories: simple shapes, and ones. Simple shapes are considered to be those that arenear flat or spherical, and can be tested using conventional optical methods. Complex shapes are thosethat depart from flatness or sphericity by an appreciable amount, and currently need alternative testmethods. A number of ways of testing complex shapes are described, and their relative merits arediscussed. 1 . INTRODUCTIONThe production of flat and spherical wavefronts has been the subject of much work in the optics industryfor a considerable length of time. As a result the optical components required to produce these shapeshave been manufactured to extreme tolerances, and often in large quantities. Optical methods of testingshape are therefore at their best on these types of surfaces. Alternatively, a surface which is notspherical or flat, but which produces such a beam if used in a particular way, can also be testedsuccessfully by optical means. One such surface is the parabola, since a spherical beam of lightoriginating from the focus of the parabola will produce a flat or collimated wavefront on reflection fromthe surface. This paper discusses the strengths and limitations of one technique developed to analysesimple shapes, namely phase interferometry.Methods of dealing with more complex shapes are then highlighted. Full field and single pointinstruments are compared. The difficulties of using optical methods for this purpose are pointed out,and a comparison is made between each of the approaches.2. THE MEASUREMENT OF SIMPLE SHAPES