High Precision Optical Measurement Methods

I. INIRODllCTION Noncontact optical methods are appropriate for mic;ro.. and macrostructure measurements. Time of flight and phase measuring techniques with a resolution in the order of a few mm to one mm. respectively arc well introduced. Furthermore active and passive triangulation techniques arc very robust and appropriate for different applications in precision measurements. Some optical 3-D-measurement techniques are summarized together with their depth resolution in table I. Tiroe of flight and phase measuring techniques are well established. For the measurement of the topography the principle of triangulation as well as its extension to light sectioning and pmjected fringe methods can be applied succesfully (1-7). Triangulation based 3-D-sensor.; are appropriate tools for inspection and measurement in industrial environments. They fuIfiI very often the high requirements upon range, resolution and robustness. Synchronized single-spot-scanner.;, based on triangulation can be very versatile for industrial applications. Moire techniques or pmjected fringe techniques are useful tools for topography measurements. They can be applied to determine surface shape or deviation of the shape or vibration amplitudes. The grating like strUCtUres can be generated by projecting a roIing or grating upon a surface which is seen through a detecting system with a reference grating like structure. Alternatively a grating pattern can be generated by interference of two plane waves upon the surface. Furthermore image plane locating systems can be applied to analyse the surface topometry. Recently confocal principles were studied to be applied for the measurement of the surface geometry and microstructure (11). Today, laser interferometry is probably one of the most commonly used technique for very high resolution measurements in metrology. A stabilized He-Ne-Iaser is frequently used as light source with an absolute frequency stability of better than 10-7. The accuracy for length measurements is lintited, by the variation of the refractive index due to atmospheric conditions (humidity, temperature, pressure) as well as vibrations, rather than by the Iaser stability. Furthermore, single mode diode laser are used in metrology eventhough frequency stability is stiII a problem especially in interferometry. Interferometry and Moire techniques as well as image plane locating systems will be applied more frequently, when used with image processing. They are becoming useful tools for precision measurements in research and for industrial applications. Computer analysis is increasingly iroportant for fringe analysis. The use of solid-state detector arrays, image memory boards together with microprocessors and computers for the extraction of the iilf'ormation from the interferograms and highresolution graphic boards find iroportant application in optical metrology. Much more information can be extracted from the sensor data, learIing to higher sensitivities and accuracies (3). Automated quantitative evaluation of interferograms requires accurate interference phase measurements, independent of fringe position and intensity variations superposed onto the interferograms. In many interferometric arrangements, FottrierTransform, phase shifting -<lr heterodyne techniques have boeo introduced for automated fringe analysis. The phase shifting technique is very appropriate for digital processing and TV techniques. Two-bearninterferometry together with video electronic processing lead to a sensitivity of 1/100 of a fringe at any