Introduction

Conventional shape measuring instruments use mechanical probes and give either point-by-point or line-scan information about shape (1). An optical method of measuring shape has the advantage of being non contacting and can also give a field view of the surface under investigation. Thus, there has been considerable effort directed towards the development of optical shape-measurement techniques.

Holographic methods of measuring surface shape are based on a two-wavelength technique first reported by Hildebrand and Haines (2). The two wavelengths can be produced by using two laser lines of different frequencies, or alternatively by altering the refractive index of the medium surrounding the object. The fringes represent the intersection of the object surface with a set of surfaces which in general are hyperboloids, but may be a set of equispaced planes in which case the fringes represent true depth contours. A new hologram must be made each time a new component is inspected.

ESPI can be used to compare the shape of test components with a master wavefront. The fringes obtained represent the difference in depth along the viewing direction between the master wavefront and the test component. The master wavefront may be produced by conventional optical components (i.e. flat, spherical or cylindrical) or may be generated holographically using a master component. The system enables components to be inspected in rapid succession.

When two beams of light which interfere to form a fringe pattern are projected onto the surface of an object, the form of fringes observed on the object surface depends on the shape of the surfaces (3).