Probe for surface measurement

Optics: measuring and testing – By polarized light examination – With light attenuation

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Details

25022711, 2502012, G01B 1114

Patent

active

056402406

DESCRIPTION:

BRIEF SUMMARY
This invention relates to apparatus for carrying out measurements to establish the form or profile, or the texture, of a surface.
Traditionally, the measurement of surface form has been effected by means of mechanical sensors or probes which make contact with the surface being investigated and which, though yielding accurate results, are inherently slow in operation and limited in spatial resolution. There have therefore been various proposals and attempts to make use of optical sensors in the measurement of surface form, since these in general can have higher speeds of operation than mechanical probes and, moreover, they do not make mechanical contact with the surface and both the probe and the surface are thereby protected from possible damage or deformation. However, optical probes (including fiber-optic probes) are not widely used, principally because the performance of such probes of known kinds is strongly affected by variations in the reflectivity, scattering properties and surface texture characteristics of the surface under investigation. This severely limits the range of applicability of such probes: typically, problems are encountered in attempting to employ optical probes in the measurement of surfaces which are matt black or highly polished or are of such materials as perspex or nylon.
It is an object of the present invention to provide apparatus, which includes a fiber-optic or comparable optical probe, for carrying out measurements to establish the form, profile or texture of a surface and of which the performance is in large measure independent of the reflectivity of such surface.
According to the invention there is provided surface-measuring apparatus comprising a probe and means for moving the probe towards and away from a surface under examination and for monitoring such movement, the probe comprising a plurality of closely spaced light-collecting elements arranged In a light-collecting plane, lens means positioned to produce a sharply focussed image of said light collecting elements at an image plane movable relative to said surface by movement of the probe, illuminating means so arranged that the lens unit forms an illuminated spot image of it in the image plane in co-incidence with the image in the image plane of one of the light-collecting elements, and means for measuring the incidence of light on the said one of the light-collecting elements and on surrounding ones of those elements.
In one embodiment of such apparatus, the probe incorporates a bundle of, say, seven optical fibers which each have one end exposed at an end plane of the bundle and constituting a respective one of the light-collecting elements, the fibers being connected at their other ends to photodiodes or other photodetector means for measuring the incidence of light on the exposed ends of the fibers. The fiber whose one end constitutes the said one of the light-collecting elements may be coupled both to the respective photodetector means and to a source of illumination, which may be a laser diode, whereby the one end of the fiber also constitutes the illuminating means. Alternatively, between the lens means and the light-collecting plane in which the light-collecting elements are disposed there may be interposed a beam splitter which enables the illuminating means to be physically separate, constituted for example by an optical fiber having one of its ends coupled to the laser diode and its other end positioned so that its Image in the beam splitter is superposed on the said one of the light-collecting elements.
In another embodiment of apparatus according to the invention, the light-collecting elements are the individual detector cells comprised in the detector array of a solid state camera. The detector cells of such an array may be organized in groups each having a central cell and a plurality of surrounding cells, and the illuminating means may be a screen with appropriately located apertures illuminated from behind, so placed relative to a beam-splitter arranged between the detector array and the lens means t

REFERENCES:
patent: 3940608 (1976-02-01), Kissinger et al.
patent: 4629324 (1986-12-01), Stern
patent: 4640620 (1987-02-01), Schmidt
patent: 4674882 (1987-06-01), Dorman et al.
patent: 4965442 (1990-10-01), Girod et al.
Journal Of Lightwave Technology, vol. 9, No. 4, Apr. 1991, pp. 545-551, Gang He et al; .music-flat.A Light Intensity Function Suitable For Multimode Fiber-Optic Sensors.
Soviet Journal of Quantum Electronics, vol. 18, No. 10, Oct. 1988, pp.1295-1298, L.I. Zhmurova et al: "Coherent Optical System With a Synthetized Aperture For Determination Of Geometric Parameters Of an Object".

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