DE102012002161A1 - Method for three-dimensional optical surface measurement of objects by measurement device, involves projecting spectral narrow band optical patterns on object, and detecting patterns as location-differing image pattern of object surface - Google Patents

Abstract

The method involves projecting multiple different spectral narrow band optical patterns to be changed and having higher image rate on the three-dimensional object surface to be measured. The projected different patterns with their changes are detected by a detector as location-differing corresponding image pattern of the object surface to be measured and are evaluated for gaining three-dimensional information of the object (1). Light waves of the spectral narrow band optical pattern are changed during the exposure time of the detector by a phase variation. The spectral narrow band optical patterns with their light wave changes are projected on the object and are detected by the detector as location-differing corresponding image pattern of the object surface to be measured. An independent claim is included for a device for three-dimensional measurement of objects.

Description

The invention relates to a method for the fastest possible and highly accurate 3D measurement of objects, in which spectrally narrow-band patterns are projected onto the object to be measured, which are detected by corresponding different image views of the object as corresponding image patterns of the object, for example by cameras subjective Specklerauschen to increase the image-side signal-to-noise ratio and thus to further increase the evaluation accuracy is suppressed. From the comparison of these different image patterns, spatial information for the three-dimensional reconstruction of the object is obtained.

In many areas optically measuring 3D measuring systems are required. They are used, for example, for quality control in assembly line applications, biometric recordings as well as architectural and art historical inventory, archiving, reconstruction and replication. Especially in the last-mentioned field of application, the measurement objects are often in ambient light unfavorable for the 3D measurement. The use of projectors for structured illumination under such unfavorable conditions, however, is not possible because of the technical limitations (low light output) of currently available projectors, because the contrast between ambient light and projected pattern is too small to obtain evaluable data. Therefore, for such tasks (for example, outdoors in sunlight) often resort to measurement methods that do not provide structured illumination of the survey objects. In general, however, methods without structured illumination or sequential projection of patterns are characterized by lower measurement accuracy, since they depend solely on the texture immanent to the object. Forced is therefore often measured with reduced accuracy, although in principle more accurate measuring methods are known. In detail, therefore, the difficulty with using structured illumination is to ensure the best possible separation between the projected pattern and the ambient light. For such applications, a spectrally narrowband light source, for. As a laser source, are used, the light is on the one hand filterable, and on the other hand realizes high light output powers. Due to the high light output, such pattern projectors are also very interesting for the 3D measurement of weakly scattering objects indoors. However, due to the spectral narrowbandness of these sources, parasitic, disturbing subjective speckle patterns, which depend on the surface structure and the detector configuration, occur on the detector surface in addition to the projected pattern structure and degrade the 3D measurement in their accuracy. In this context, therefore, the development of a narrow projection projection system with high projection rate, which does not cause subjective speckle noise in the detectors, is required for highly accurate and robust measuring systems as well as many applications.

Methods are known for high-precision (relative measurement uncertainty <10 ^-4 ) and dense 3D measurement of objects using structured illumination. These include, for example, strip projection methods ( W. Schreiber and G. Notni: Theory and arrangements of self-calibrating whole-body three-dimensional measurement systems using the fringe projection technique, Optical Engineering 39, 2000, 159-169 ; J. Gühring, Dense 3-D surface acquisition by structured light using off-the-shelf components, video-metrics and optical methods for 3D shape measurement 4309, 2001, 220-231 ) or methods using statistical patterns ( DE 196 23 172 C1 ; A. Wiegmann, H. Wagner, R. Kowarschik: Human Face Measurement by Projecting Bandlimited Random Patterns, Optics Express 14, 2006, 7692-7698 ). Many of the methods mentioned typically use white light sources as projection illuminants, which distribute their entire optical power over a broad spectral range or have short coherence lengths and thus do not cause subjective speckle noise in the respective detectors. However, the light outputs that provide broadband light sources, limited and thus poorly reflective objects vermessbar not with short exposure times.

Furthermore, lasers have also been used as illuminants for processes using statistical patterns as illuminants ( M. Schaffer, M. Grosse, B. Harendt, R. Kowarschik: High-speed three-dimensional shape measurements of objects with laser speckles and acousto-optical deflection, Optics Letters 36, 2011, 3097-3099 ). In this method, coherent illumination by the laser is used to generate the pattern patterns rapidly (f> 200kHz).

In this context, it has already been proposed to separate the ambient light from the signal by means of spectral filtering in order to achieve better results, in particular in unfavorable light conditions (for example due to glaring solar radiation in external measurements). This projection method is referred to below as high-frequency speckle projection (HSP). Again, however, could be affected by the occurrence of subjective Specklerauschen the measurement accuracy, so that it would be desirable, this completely eliminate possible sources of interference and further increase the evaluation accuracy.

Furthermore, methods of laser scanning are known ( DE 102 00 606 0108 A1 . DE 000 01 980 6288 A1 ), which are used outdoors and thus also in scenarios that are potentially characterized by strong ambient light, but in comparison to methods with planar, structured illumination have a longer measurement time, lower point density and higher measurement uncertainty and thus for a very fast and highly accurate 3D reconstruction are not suitable. Since these methods sequentially capture measurement points, the measurement does not represent an exact time, and changing measurement objects / scenarios are detected incorrectly. In addition, these methods are often used for longer distances and have a uniqueness interval that adversely limits the spatial depth extent and does not allow for adaptation to near-field applications.

Another scanning method is laser line triangulation, which could be used outdoors and thus also in scenarios that are potentially characterized by strong ambient light. In this case, either the object is guided during recording under a laser light line or the laser light line is moved with a deflection device over the object ( DE 603 00 824 T2 . DE 196 08 632 A1 ). As in the case of the abovementioned laser scanning method, different object locations are measured at different times, with which changes in the object between these times lead to motion artifacts in the 3D reconstruction. In addition, large, complex objects due to the necessary deflection and positioning are measured only slowly or with increased mutual shading. Although the methods themselves are fast, they capture the measuring points only sequentially and thus have problems with the evaluation of complex and variable forms during the measuring time.

In addition, there are methods for 3D measurement with laser light, which could also be used outdoors with strong ambient light and project the area characteristics and thus not scanning. In DE 103 08 383 A1 a system was patented, which projects markers onto the object with the help of diffractive optical elements. Such designs, which operate on a pattern basis, typically result in low density reconstructions since only single points per mark are calculated.

Furthermore, there are numerous research projects and prototypes in the field of laser projection and laser display systems for displaying films or multimedia content. In particular, methods for reducing subjective speckle noise of the projected images are known from this field. However, these methods have the limits known from conventional projection technologies with respect to the pattern projection rate (f <100 Hz), so that these methods are also not suitable for fast, dense and accurate 3D surveying.

In summary, it should be noted that so far no method is known, the area, dense, accurate (relative accuracy <1.0 × 10-4 of the 3D points) realizes 3D measurements using narrow-band pattern projection while suppressing disturbing subjective Specklerauschen, without the precision and speed significantly affect the 3D recordings.

The invention is thus based on the object, the object with little effort, as quickly as possible to measure three-dimensional and thereby despite spectrally narrow-band pattern projection and recording the accuracy of the 3D reconstruction without sacrificing speed to further increase.

At very high measurement accuracies (relative measurement accuracy much better than 1.0 · 10 ^{-4 of} the 3D points), 3D acquisition rates higher than 10 Hz, ie more than 10 3D images per second, should be achievable.

This object is achieved by a method for 3D measurement of objects in which at least one or several different spectrally narrow-band optical patterns are projected onto the object surface to be measured three-dimensionally and the at least one projected pattern in its change or the Projected different patterns are detected in their change of at least one detector as location-different corresponding image pattern of the object surface to be measured and evaluated to obtain 3D information of the object. In this case, according to the invention, the light waves of the respective spectrally narrow-band optical pattern at least during the exposure time of the at least one detector additionally changed by a phase variation and the spectrally narrowband optical pattern projected respectively with their light wave changes to the object and from this by the at least one detector as location different corresponding image pattern detected the object surface to be measured.

The said phase variation of the pattern projection and evaluation is achieved by a phase-changing optical element, in particular a diffuser, caused in the beam path of a spectrally narrow-band projector.

In the subclaims advantageous embodiments of the invention features are listed.

Due to the continuous phase variation of the light wave of the optical pattern during the exposure time on the detector an averaging over many different subjective Specklerausch distributions, so that in the sum then forms only an intensity shift in the resulting images, which is for the point assignment in contrast to a individual subjective Specklerauschverteilung has no negative effects and thus has no effect on the accuracy of measurement. This makes it possible to achieve the same accuracies with spectrally narrow-band optical patterns as when lighting with spectrally wideband sources and, moreover, can take advantage of the spectrally narrow-band optical patterns, such as high achievable light output and high projection rate (down to the kHz range) ,

The invention will be explained below with reference to an embodiment shown in the drawing for fast and highly accurate 3D measurement of objects with suppression of interfering subjective speckles.

The figure shows a schematic representation of an apparatus for performing the method according to the invention with a spectrally narrow-band projector, in whose beam path a phase-changing optical element for the pattern projection is arranged.

From an object 1 the surface should be measured and reconstructed in three dimensions with high precision. For this purpose, patterns of a projector 2 on the surface of the object to be measured 1 projected. The patterns are each preceded in turn by a spectrally narrow-band pattern generation source 3 of the projector 1 on a diffuser 4 intermediately. The diffuser 4 is on a rotatable engine 5 attached.

Using an imaging optics 6 become the inter-imaged patterns of the diffuser 4 on the object 1 displayed. The scattering disc located in the intermediate image plane 4 is due to the rotating motor 5 permanently set in motion, so that the intermediate image of the pattern generation source 3 at any time a different part of the lens 4 illuminated. The rotation leaves the lens 4 permanently in the intermediate image plane. In this way form the engine 5 and on this fixed and thus moving motor lens 4 an apparatus for the intermediate imaging of patterns with a statistical surface structure for the suppression of subjective speckles according to the invention.

With two synchronized cameras 7 and 8th , which have been previously calibrated with respect to the inner and outer parameters of the stereo construction, from different locations becomes a number of, for example, twelve images of those on the object 1 projected patterns. For each of the twelve camera image pair recordings, for example, a different pattern of the spectrally narrowband pattern generation source is generated 3 on the diffuser 4 intermediately. The diffuser 4 is continuous, but at least during the exposure time of the cameras 7 and 8th adjusted. Due to the continuous rotation of the lens 4 Different statistical phase variations are impressed on the light wave of the respective optical pattern, so that at each instant during the exposure time a different subjective noise component is generated and by the temporal averaging of the individual noise components a noise-reduced intensity image from the cameras 7 . 8th is detected. This interferes with the coherent formation of subjective speckles and effectively suppresses their negative impact on 3D reconstruction. The detection area of the synchronized cameras 7 . 8th is represented by dashed lines.

This sequence from the mentioned image pairs becomes the known three-dimensional reconstruction of the surface of the object 1 to a computer (not shown for reasons of clarity) transmitted. In the computational evaluation of the said stereo image sequence, homologous points are assigned to one another in a known manner using the established method of time correlation and 3D points in space are determined therefrom with the aid of the likewise known calibration parameters, which can now be further processed depending on the application. By the movement of the engine 5 can be an automated, continuous, statistical change in the optical path length for each pixel of the cameras 7 and 8th be guaranteed. This makes it possible to suppress the subjective speckles which usually occur parasitically during spectral narrow-band projection, so that a fast (high pattern projection rate) and highly accurate recording of the object 1 for 3D reconstruction is achieved.

For the spectral narrow-band pattern projection and detection, a laser is used as the light source which emits light with a spectral width of <100 pm at a center wavelength of 532 nm. The resulting coherence length greater than the object surface roughness would, without the invention, result in the formation of subjective speckles upon imaging of the pattern illuminated Surface of the object 1 with the cameras 7 . 8th cause. By the proposed phase variation of the light waves from each of the object 1 Projected patterns become these subjective speckles in the camera capture of the pattern images of the object 1 avoided.

As a pattern known pattern for 3D reconstruction can be projected. In particular, the method of the aforementioned HSP can be used to realize the pattern generation source.

By way of example, the projection of patterns with diffractive-optical elements, statistical patterns or stripe patterns may also be mentioned. Other systems for the statistical variation of the phase position in the illumination beam path are conceivable.

Instead of the laser in the pattern generation source 3 Other spectrally narrowband bulbs, such as laser diodes, would be useful.

The pattern generation source may also be implemented as a pattern shifter or pattern changer.

It is also conceivable that for multiple projection spectrally different patterns multiple projectors 2 each with spectrally different pattern generation source 3 are provided or that the projector 3 several spectrally different pattern generation sources 3 contains. In this way, multiprojection patterns of different wavelengths (center frequency and spectral bandwidth) on the object 1 be imaged.

LIST OF REFERENCE NUMBERS

1

object

2

projector

3

Pattern generation source

4

diffuser

5

engine

6

imaging optics

7, 8

camera

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 19623172 C1 [0003]
• DE 102006060108 A1 [0006]
• DE 000019806288 A1 [0006]
• DE 60300824 T2 [0007]
• DE 19608632 A1 [0007]
• DE 10308383 A1 [0008]

Cited non-patent literature

• W. Schreiber and G. Notni: Theory and arrangements of self-calibrating whole-body three-dimensional measurement systems using the fringe projection technique, Optical Engineering 39, 2000, 159-169 [0003]
• J. Gühring, Dense 3-D surface acquisition by structured light using off-the-shelf components, video-metrics and optical methods for 3D shape measurement 4309, 2001, 220-231 [0003]
• A. Wiegmann, H. Wagner, R. Kowarschik: Human Face Measurement by Projecting Bandlimited Random Patterns, Optics Express 14, 2006, 7692-7698 [0003]
• M. Schaffer, M. Grosse, B. Harendt, R. Kowarschik: High-speed three-dimensional shape measurements of objects with laser speckles and acousto-optical deflection, Optics Letters 36, 2011, 3097-3099 [0004]

Claims (7)

Method for 3D measurement of objects, in which at least one or more different spectral narrow-band optical patterns are projected onto the object surface to be measured three-dimensionally and the at least one projected pattern in its change or the projected different patterns in their alternation be detected by at least one detector as location-different corresponding image pattern of the object surface to be measured and evaluated to obtain 3D information of the object by the light waves of each spectrally narrow-band optical pattern at least during the exposure time of the at least one detector are additionally changed by a phase variation and the spectrally narrow-band optical pattern projected in each case with their light wave changes to the object and by the at least one detector as a location different corresponding Bil Dmuster the object surface to be measured are detected. A method according to claim 1, characterized in that the pattern projection and evaluation strip patterns are used, which are moved in its sequence on the object. A method according to claim 1, characterized in that for the pattern projection and evaluation statistical patterns are used, which are moved in its sequence on the object. Method according to one or more of claims 1 to 3, characterized in that a plurality of spectrally different pattern projections on the object. Device for carrying out the method according to one or more of claims 1 to 4, characterized in that in the beam path of at least one spectrally narrow-band projector ( 2 ) with a high-frequency pattern generation source ( 3 ) at least one of the light waves of each on the object to be measured ( 1 ) to be projected pattern in its phase changing optical element ( 4 ) is arranged. Device according to claim 5, characterized in that as the light waves of each of the objects ( 1 ) to be projected pattern in its phase changing optical element a lens ( 4 ) is provided. Device according to claim 5, characterized in that the at least one spectrally narrow-band projector ( 2 ) with the high-frequency pattern generation source ( 3 ) As a spectrally narrow-band light source, a monochromatic light source, in particular a laser or a laser diode contains.

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