Optical profilometry and its application to mechanically inaccesible surfaces Part I: Principles of focus error detection

Optical profilometers can in principle be used to measure the roughness of surfaces that are not accessible for mechanical profilometers. Optical measurements can be performed through a transparent layer because of the noncontacting nature of the measuring principle. In this article the properties of a focus error detection system are given for the general case of scanning free surfaces and for the particular case of a glass plate lying on the scanned surface. Tests show that scanning through a glass plate is possible, but there are some limitations. The conditions under which proper measurements can be performed are described. Some examples of scanning through a transparent layer will be presented in part II of this paper.