Distance measuring device

Details Distance measuring device Distance measuring device

1988-12-13

1991-10-08

Rosenberger, Richard A.

Optics: measuring and testing

By particle light scattering

With photocell detection

356376, G01B 1114, G01B 1124

Patent

active

050549260

DESCRIPTION:

BRIEF SUMMARY
This invention relates to the field of measurement of distances and more particularly to rapid non-contact distance measurement. It is particularly well suited to measuring the profile of a moving surface; accurately measuring the distance to an object; measuring the thickness of an object by means of two measuring heads; and measuring the shape of simple objects.
Most commercial profilers work by dragging a stylus across the surface of the object to be profiled. These devices suffer from the problem that they may alter or damage the surface as a result of their passage. The invention described hereunder, being a non-contact device, does not suffer from this problem.
Many non-contact distance measuring devices, whether used for profilometry or distance measurement, rely on fringe counting. As a result, when step changes in the height or distance occur, they lose track of the absolute distance. The present device is able to keep track of the absolute distance within a broad range.
Another problem common to non-contact height measuring devices is that while their height resolution is good, their lateral resolution generally is not. This becomes important when the surface being measured is very rough or when the height cf a small element of surface is required. The current invention combines good lateral resolution with good height resolution.
The range of techniques for measuring surfaces is discussed in "Surface Metrology Instrumentation" by D. J. Whitehouse (J. Phys. E: Sci. Instrum. 20 (1987) pages 1145-1155). Various non-contact measurement arrangements are disclosed in "Optical Profilometer for Measuring Surface Contours of 1 to 150 Microns Depth" by H. P. Kleinknecht and H. Meier (Laboratories RCA, Ltd., Zurich, Switzerland); "Electro-Optic Surface Profiler by J. W. Buechele and N. B. Seebe" (IBM Technical Disclosure Bulletin Vol 26 No. 3B August 1983); "Surface Profile Measurement with a Dual-Beam Optical System" by David Y. Lou et al. (Applied Optics Vol. 23, No. 5 of Mar. 1, 1984, pages 746-751); "Optical Profilometer: A Practical Approximate Method of Analysis" by Marek Dobosz (Applied Optics, Vol. 22, No. 24, Dec. 15, 1983, page 3983).
For almost all height measuring devices and profilers, both contact and non-contact, the relative velocity between the instrument sensing point and the surface being measured must be relatively low in order to measure the surface profile with adequate accuracy. The present invention is able to operate with high relative velocities while measuring the surface position. This makes it ideal for surface profilometry or positioning in a manufacturing environment, where speed is important.
While the present invention, in one form, employs one aspect of the standard confocal microscope for its operation, it differs in an important aspect, it is confocal in only one dimension. In the standard confocal microscope, if the surface is moved in a plane perpendicular to the axis of the microscope, small surface height variations result in large signal intensity variations, in comparison with conventional microscopes, at the detector. For the detector signals to ge amenable to analysis these height variations must be limited to a small range. That is, the conventional confocal microscope gives sub wavelength height resolution within wavelength range. The operation of confocal microscopes is well understood as exemplified by the disclosures in "Experimental Observations of the Depth- Discrimination Properties of Scanning Microscopes" by D. K. Hamilton et al. (Vol. 6, No. 12 Optics Letters of December 1981) and "Surface Profile Measurement using the Confocal Microscope" by D. K. Hamilton and T. Wilson (J. Appl. Phy Vol. 53, No. 7, July 1982, page 5320 et seq.). In the present invention, the measuring range is extended to correspond to the length of the line of illumination. A paper entitled "Optisch-Beruhrunqslose Antastung mit Hilfe des Astiqmatismusuerfahrens" by R. Theska (Feingeratetechnik, Vol. No. 36 (1987), No. 6, pages 263-6) published after the earliest priority date of the

REFERENCES:
patent: 4548504 (1985-10-01), Morander
patent: 4626103 (1986-12-01), Feldman et al.
patent: 4650333 (1987-03-01), Crabb et al.
patent: 4657393 (1987-04-01), Stern

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