14.1 Tactile Profile Measurement
14.1.1 Basic assessment of the technology
Metrology comprises a pointwise sampling method and apparatus to determine the surface shape of an object. When applied to a plano, spherical, or aspherical surface of an optical element, suitable sensors must be used to avoid damaging.
Unlike surface-covering interferometric metrology, tactile profile measuring does not need any null systems or compensators. Coordinates of sample points are referenced against an intrinsic coordinate system, which is supplied by the machine after a calibration process.
â¢ Method of measurement: Measured variables are x, z-coordinates (line scanning) or x, y, z-coordinates (surface scanning) of arbitrary points on the surface under test.
â¢ In-line: Some diamond-turning machines used for the production of optical elements have built-in profile metrology.
â¢ Off-line: The usual case is the off-line operating apparatus in a metrology room providing the necessary environmental conditions (cleanness, stable temperature and humidity, no vibrations or air turbulences).
14.1.2 Intended purpose of the technology
High-precision surface metrology can be applied universally to optical surfaces. The aim is to measure the shape (figure) as well as the roughness of optical surfaces in different states of the production process. Thus, not only polished but also a ground or lapped optical element of different materials should be measured. No a priori information about the shape should be necessary. Unknown elements should be measured as long as they fit into the measuring range of the apparatus.