Optics: measuring and testing – Material strain analysis – By light interference detector
Reexamination Certificate
2000-05-09
2002-07-09
Le, N. (Department: 2858)
Optics: measuring and testing
Material strain analysis
By light interference detector
C356S512000, C356S513000, C324S244100
Reexamination Certificate
active
06417916
ABSTRACT:
Further, this application claims priority rights based on German Patent Application No. 198 59 725.8, filed Dec. 23, 1998, which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
The invention relates to an apparatus and a method for examining objects. In particular, the invention relates to an apparatus and a method for examining deformations occurring in objects with diffusely scattering surfaces.
BACKGROUND OF THE INVENTION
Nondestructive tests of objects are of practical value particularly wherever quality tests of workpieces or other kinds of work objects are to be performed. One practical example is examining tires for defects.
To that end, German Patent DE 42 31 578 C2 describes illuminating the surface of a test object with laser diodes. Observation of the test object is done using an interferometer, which generates an interference pattern on a pattern sensor system. A mirror of the interferometer is movably disposed. The interference patterns generated are 2&pgr;-modulated; this means that a phase rotation exceeding 2&pgr; can not readily be distinguished from a corresponding phase rotation that is 2&pgr; less. As a result, a pattern of dots or lines can be generated that characterizes the deformation of the surface.
A plurality of laser diodes are used to light the surface of the object, and each laser diode produces one light spot. The light spots border on one another but overlap, if at all, only in peripheral zones. The lighted portions of the surface taken together make up the total surface. The projection quality of the system is limited.
With this as the point of departure, it is the object of the invention to create an apparatus and a method for observing the surfaces of objects with improved picture quality.
SUMMARY OF THE INVENTION
The apparatus of the invention has a measurement head, which generates an interference pattern from light backscattered from the surface of an object. An electronic pattern sensor, which can be integrated with the measurement head, detects the interference pattern and converts it into corresponding electrical signals that can then be further analyzed.
The image field detected by the measurement head is illuminated by a lighting unit, which has a plurality of laser diodes. The laser diodes are disposed such that they form a common light spot. This spot is preferably uniformly lighted, so that within the light spot only slight differences in brightness may be encountered. This is a result of a relatively major overlap of the beams emerging from the individual laser diodes. The common light spot is not subdivided into individual light spots. A plurality of laser diodes illuminate the entire area of the light spot in such a way that radiation backscattered from each point is incoherent. The laser diodes are arranged such that the backscattered total radiation of each portion of the total area observed when the laser diodes are all in operation is greater than when only one laser diode is in operation.
The overlap zones of the regions illuminated by the laser diodes are preferably larger than the nonoverlapping zones. This makes it possible to achieve a uniform distribution of light. Furthermore, it is preferable not to permit any zone that is lighted by only one laser diode. To ensure that no portion of the area is lighted by only one light source (laser diode), laser diodes can also be combined into groups and aimed in groups of two or more laser diodes at a selected portion of the area.
By means of the uniform illumination of the surface of the object to be examined, enhanced picture quality is achieved despite the lack of coherence of the individual components of the light striking the surface. In the interference pattern generated, the desired deformations are readily apparent both in the middle of the image and at the edges.
The beams of the laser diodes can overlap so markedly that more than half the area of the light spot receives light from two laser diodes. There may exist extended regions that are lighted with approximately the same intensity from the light of a plurality of laser diodes. The light spots of individual laser diodes can, as a result, occupy virtually the entire field to be illuminated. In this way, narrow edge zones in which the light spots adjoin one another, and where uneven light distribution could occur, are avoided.
There can also be a plurality of regions that are struck by the light from more than two laser diodes. The orientation is expediently arranged in most cases to provide a uniform distribution of brightness. It is also possible to have virtually the entire area of the light spot illuminated by more than two diodes or by other numbers of diodes.
From independent light sources, not coherent with one another but oriented virtually in the same direction, a plurality of coincidence speckle fields can be generated and projected simultaneously onto a pattern sensor. The resultant superimposed speckle field is detected and used to calculate the deformation of the object.
It is possible to have the laser diodes of the entire group shine simultaneously, preferably in continuous operation. In a modified embodiment, the laser diodes can be operated in pulsating fashion. This makes a higher light yield possible at the moment a picture is taken; as a result, either the object field under observation can be enlarged, or the lighting power or the exposure time can be reduced.
It is advantageous if the individual light sources are not coherent with one another, but have light wavelengths differing only slightly from one another. Furthermore, the angles of incidence (the angles at which the beams of light strike the object) of the individual light sources should not differ excessively.
It is also possible to use somewhat more-different light wavelengths, say, &lgr;
1
and &lgr;
2
. If two groups of light sources are used, the resulting sensitivity can then be calculated as
&lgr;
res
=(&lgr;
1
*&lgr;
2
)/(&lgr;
1
+&lgr;
2
).
Each wavelength component should be present with, as much as possible, the same intensity.
Alternatively, it is possible to trigger the laser diodes or to provide them with a shutter device, for instance, in such a way that the light beams of the laser diodes strike the surface of the object in a chronologically staggered fashion. The resulting individual interferograms can be superimposed at the pattern sensor and added together (integrated), or depending on the hardware, they can be detected individually and then combined with one another in a computer. As in the case of continuous lighting, the overall result is a uniformly illuminated field. The laser diodes can be disposed such that they are either stationary or in motion.
The pattern sensor is preferably connected to an image analysis device, which on the basis of a plurality of detected interference patterns determines a deformation of the surface of the object. This is expedient especially in cases in which the structure or form of the undeformed surface of the object is of no interest. Such measurements are expedient, for example, in workpiece testing or materials testing. For instance, they can be employed to detect defects in tires. The surface of the tire to be examined is detected at two ambient pressures different from one another. The resultant deformations are rendered visible.
The interferometer can function without a direct interference beam. This is possible if the beam backscattered from the object is split into two fractional beams, one of which is subjected to a phase displacement. The phase displacement can preferably be controlled or monitored.
To that end, it is advantageous to employ a device for phase displacement, preferably, a mirror that is adjusted by a piezoelectric actuator. It is possible to use a Michelson interferometer as the interferometer; however, an especially advantageous embodiment uses an arrangement in which the object beam, received at the measurement head, is split into two fractional beams that reach the pattern sensor over different paths and reunite only t
Dengler Stefan
Mähner Bernward
Deb Anjan
Gluck Jeffrey W.
Kelemen Gabor J.
Le N.
Venable
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