WO2014124634A1

WO2014124634A1 - Device for generating light patterns comprising a one-dimensionally focussed illumination device

Info

Publication number: WO2014124634A1
Application number: PCT/DE2014/100039
Authority: WO
Inventors: Erik Lilienblum; Bernd Michaelis
Original assignee: Otto-Von-Guericke-Universität Magdeburg
Priority date: 2013-02-13
Filing date: 2014-02-06
Publication date: 2014-08-21
Also published as: DE102013002399B4; DE102013002399A1; EP2984444A1

Abstract

The invention relates to a device for generating light patterns, comprising at least one one-dimensionally focussed illumination device having at least one lamp and a reflector. The homogeneous light generated by the illumination device is present as bundles across a field of adjacent lenses running orthogonally to the illumination direction, such that parallel stripes of light can be imaged and specific one-dimensional light patterns can be generated.

Description

Device for generating light patterns with a one-dimensionally focused illumination device

The invention relates to a device for generating light patterns with a one-dimensionally focused illumination device.

So-called stripe patterns are needed in short-range photorammetry in order to find unique correspondences with homogeneous surface texture. A high light intensity is a prerequisite for short integration and measurement times. In addition, for the application of multi-step measuring methods must also

different stripe patterns can be generated.

The switching times between different patterns must be correspondingly short in order to limit the measuring time accordingly over several steps. With large measuring fields, the generated light patterns should also cover a corresponding area without losing intensity. The prerequisite for this is that a scaling of the pattern generation also requires a

Scaling the illuminance is possible.

In general, close-range photogrammetry with active illumination uses slide or video projectors, so-called beamers. The results, which are particularly useful here with multi-step methods, e.g. Phase shift or time correlation are sufficient for many practical applications. The procedural bases are e.g. described in Wiora, G .: Optical 3D Metrology: Precise Shape Measurement with an Extended Fringe Projection Method, University of Heidelberg, Dissertation, 2001.

Currently, measuring systems with one or two cameras and one projector are commercially offered by many companies. To solve metrological problems in industrial applications, projectors quickly reach their limits with regard to their maximum light intensity. The low light intensity results in relatively long integration times for image acquisition, which leads to long measurement times and lower measurement accuracy, for example in the case of oscillations. Because of the constant maximum output of a projector, the field of view can only be increased at the cost of a lower light intensity.

The simultaneous use of several projectors is generally not possible for metrological reasons, since the light patterns are superimposed between the projectors.

An apparatus for solving the above-mentioned technical problem with projectors is not known. One main reason for this is that the projected light is focused in a single lens and is also produced by only a single light source.

An alternative to using projectors is the use of laser light sources. However, laser light has the disadvantage that it causes speckle effects and at high intensity appropriate safety standards must be met.

In applications, laser light section methods are often used. The methods are also relatively robust in industrial use, but the measurement times are often even longer than when using projectors.

Laser light can also be used indirectly. In Schaffer M, Grosse M, Kowarschik R: High-speed pattern projection for three-dimensional shape measurement using laser speckies. Appl Opt. 2010 Jun 20; 49 (18): 3622-9. doi: 10.1364 / AO.49.003622 introduces a system that generates speckle patterns and then projects them through a lens to produce stochastic patterns. The system achieves very fast switching times and high light intensities.

However, at higher intensities, a high laser protection class quickly becomes necessary, which is problematic for many applications. In addition, the patterns are always stochastically distributed in two dimensions. The generation of one-dimensional patterns, ie stripe patterns, is not possible with this technique.

Another possibility for generating patterns of light is the use of individual light spots. With light spots, the technical task can be solved in terms of light intensity, scalability and switching speed, but the generated light patterns are relatively coarse structured and not one-dimensional, i. not strip-shaped.

This allows a photogrammetric evaluation of the patterns only in a very limited area. The coarsely structured patterns can lead to systematic measurement errors on unsuitable surfaces.

Against this background, the object of the present invention is to provide a device by means of which the aforementioned disadvantages can be overcome.

This object is achieved by a device according to claim 1 and the further advantageous embodiments according to the subclaims.

A device for generating light patterns is proposed, wherein the homogeneous light generated by a one-dimensionally focused illumination device is bundled over a field of juxtaposed lenses orthogonal to the illumination device such that parallel light strips can be imaged and specific one-dimensional light patterns can be generated. The light patterns are different

- at different heights,

and upon variation of the spatial distribution of the light sources in the illumination significantly different from each other.

In addition, the device allows the generation of light patterns with high intensity.

Although line illumination with very high light intensity is known from the field of line-based image acquisition systems. In many applications, line lighting is designed to bundle the light in one dimension in parallel.

The device comprises a field of parallel arranged lenses, preferably cylindrical lenses, which is brought into the beam path of a line illumination with LED's or fiber optics. The cylindrical lenses focus the homogeneous light from each individual illuminant, such as LED or optical fiber at the exit to small areas in the row direction. The light bundled in parallel by the reflector of the line illumination widens the areas focused by the cylindrical lenses into strips in the y direction.

In addition to cylindrical lenses, however, any types of lenses can be used in the device according to the invention for generating light patterns, such as, for example

spherical lenses, convex and / or concave type,

gradient lenses,

aspheric lenses and the like and combinations thereof.

The lens attachment produces exactly one light strip for each lamp in combination with each cylindrical lens of the lens attachment.

Depending on the focal length of the cylindrical lenses and the distance of the lens array from the line illumination, a light strip of different widths is imaged. The superimposition of the imaged light stripes results in a specific fringe pattern.

The stripe pattern corresponds to a one-dimensional brightness distribution, which essentially depends on the following variables:

1 . Distribution of the illuminants, i. from the positions in the row direction,

2. size of the lamps, for example, the width in the row direction,

3. Distribution of the lenses, i. from the positions in the row direction,

4. Size of the lenses, from the width in the row direction,

5. focal length of the lenses,

6. distance of the lenses to the illuminant,

7. Distance of the lenses to the target surface.

The generated stripe patterns are in principle suitable for stereoscopic evaluation with photogrammetric 3D measuring methods. Essentially, the device offers two distinct advantages over known measurement systems with projectors.

It is achieved a very high light intensity, since the light patterns are not caused by shading or distraction and no central bulb exists. Unlike projectors, the maximum Light output of all lamps used in principle only attenuated by the optical loss of the cylindrical lenses.

The system can be expanded in any dimension in one dimension without any restrictions of the light intensity. Thus, in contrast to state-of-the-art projectors, the measurement area can be increased without the light intensity decreasing proportionally.

It is provided that the device is designed such that the resulting light patterns are suitable for photogrammetric 3D surface reconstruction by means of single or multiple camera systems in a predetermined measurement volume.

Photogrammetric surface reconstruction is the main field of application for the device. The improvements over other pattern projections can result in higher measurement speeds, higher measurement accuracy and robustness against vibration and other external influences when 3D surveying large surfaces. In particular, they allow the use of line scan cameras. Among other things, this will create new opportunities for quality assurance based on 3D surface reconstructions in the industrial environment.

A further embodiment of the invention provides that the light patterns differ significantly from one another at different heights and with variation of the spatial distribution of the light sources in the illumination.

The generation of different light patterns is a basic requirement for the application of multi-step 3D measuring methods. By choosing a suitable spatial distribution, it is also possible to generate suitable light patterns. In a limited height range, it is also possible to generate classical phase shift patterns. Due to the dependence in the z-direction, the measurement volume can be decisively extended since the pattern generation then does not depend on a limited depth-of-focus range. It also enables multi-step measurement with just one camera.

According to a further advantageous embodiment of the invention, it is provided that the LEDs used in the illumination device are switched on and off, that for a given measurement volume different light patterns can be generated, which are suitable for photogrammetric 3D surface reconstruction with multi-step process in single or multi-camera systems.

Multistep photogrammetric measuring methods have a higher latrereal resolution compared to single-step measuring methods and are more robust in use against systematic measuring errors.

When using line scan cameras, however, different patterns are needed, which change their shape with each recorded line. Fast turnaround of patterns is therefore a prerequisite for time-efficient measuring procedures. The patterns can be changed very quickly when using LED's, since LED's as light sources are generally characterized by very short switching times.

Thus, the Vorichtung has another decisive advantage over projectors according to the current state of the art. The possibility that results from implementing multi-step 3D measuring methods with line scan cameras is an important prerequisite for many new application areas of SD measurement technology.

A further embodiment of the invention provides that optical fibers are used instead of the LEDs whose primary light sources can be quickly switched on and off. In addition to LEDs, optical fiber is a commonly used tool to construct line lighting. Each individual fiber is a small light source at its output. If adjacent light guides are fed by different primary light sources, it is also possible, when using light guides, to produce different patterns equivalent to the embodiment with LEDs.

Light guides have the advantage compared to LED's to have a much smaller emission area and be used as cold light. Therefore, there are applications in which the advantages of the device can only be used in conjunction with optical fibers

In a further embodiment of the invention, an assembly is provided which comprises a plurality or at least two devices of the aforementioned type, wherein the devices are arranged parallel to one another and a stripe pattern can be expanded orthogonal to a row.

The device can be used not only in conjunction with line scan camera systems but also with matrix camera systems. Since matrix camera systems are characterized by areal image acquisition, the extension of the stripe patterns orthogonal to line is necessary. By virtue of the already mentioned advantages of the device, it is thus also possible to decisively improve matrix camera systems for 3D measurement.

The manufacture of such a device can take place, for example, in the following steps:

- For the basic construction, a commercial line lighting with a reflector or a cylindrical lens can be used.

- An array of cylindrical lenses can be made by injection molding.

Commercially available grooved glass can also be used.

- The cylindrical lens array is mounted so that it lies in the beam path of the line illumination and generates a striped pattern when the line illumination is switched on.

- If you want to switch between different patterns, an electronic control must be built, which allows you to control all bulbs used individually or in groups. - If special patterns and pattern sequences are to be generated, then the spatial arrangement of light and cylindrical lenses must be changed accordingly or optimized. By way of example, embodiments of the invention are illustrated in the following figures and described in more detail.

1 shows schematically the construction principle of a line illumination in the side view,

Fig. 2: the schematic structure of the device as a row illumination with LED line in the front view, Fig. 3: schematically a line illumination with cylindrical lenses in the beam path and

4 shows schematically a system for generating one-dimensional brightness distributions

FIG. 1 schematically shows the construction principle of a row illumination in a side view. From a luminous means 1, light beams 3 are focused or parallelized via a one-dimensional reflector 2 in one direction, for example in the z-direction, to form a line light 3 '.

Alternatively, the focusing or parallelization of the light beams 3 can also be carried out via a cylindrical lens instead of the reflector 2. FIG. 2 diagrammatically shows an LED line 1 ', comprising a plurality of luminous means 1 and a reflector 2, in the front view. The high light intensity of such a line illumination is due to the fact that the light is emitted along a line over a large number of light sources 1, such as light guides or LEDs. Since the light beams 3 are not focused or parallelized in the line direction, the resulting line light 3 'is generally very homogeneous, which is also required for most applications. Due to the decentralized structure of the lighting means 1, a scaling in the row direction x without loss of light intensity is possible.

FIG. 3 schematically shows a device for generating light patterns in the form of strips 6. Here, luminous means 1, which form a row of luminous means, for example an LED row 1 ', a reflector 2 and a cylindrical lens array 5' are provided, the cylindrical lens array 5 'in the beam path of the line light 3' spaced from the light emitting means 1 'is arranged at a distance B and a field of parallel cylindrical lenses 5 comprises. The cylindrical lenses 5 focus the line light 3 'emitted by each individual illuminant 1, in each case on small regions in the row direction x. As a result of the line light 3 'parallelized by the reflector 2 of the line illumination, the areas focused by the cylindrical lenses 5 are imaged as strips 6 in the y-direction on an imaginary and schematically represented measuring surface 4. The resulting strips 6 are located at a distance O from the cylindrical lens array 5 '.

FIG. 4 schematically shows a system for generating one-dimensional brightness distributions 11.

Here are the seven sizes

2. size of the lamps, for example, the width in the row direction,

3. Distribution of the lenses, i. from the positions in the row direction,

4. Size of the lenses, from the width in the row direction,

5. focal length of the lenses,

6. distance of the lenses to the illuminant,

7. Distance of the lenses to the target surface. can be interpreted as inputs 9 of a system 10 whose output provides a one-dimensional light pattern or the one-dimensional brightness distribution 1 1. Only the two last variables, ie the distance B of the lenses 5 to the illuminant 1 or to the imaginary object surface 4 are scalar, the other inputs are vectors, ie for each illuminant 1 of the line illumination there is a position Lx1, Lx2 to Lxn and a Width Lb1, Lb2 to Lbn, and for each lens 5, there is a position Zx1, Zx2 to Zxn, a width Zb1, Zb2 to Zbn and a focal length Zf 1, Zf2 to Zfn.

The amount of input makes the system relatively complex. The inputs can be selected so that the resulting stripe patterns provide the best possible results for the photogrammetric method to be used. Because of the complexity of the system, multi-dimensional optimization techniques should be used. An important fixed edge size here is the measurement volume or the maximum height of the object surface to be measured.

Since the distance of the lenses 5 to the measuring object surface 4, which could lie in the xy plane in FIG. 3, for example, represents an input variable of the system, the fringe pattern 1 1 changes with the object height or with the distance O. For one-step measuring methods the device is thus already complete, since time-invariant light patterns are usually used here.

The significant change in the pattern as a function of the height of a measured object gives better possibilities for photogrammetric evaluation than when using projectors.

Thanks to the principle of line illumination, the system can be easily scaled, at least in the line direction, without losing its light intensity. Orthogonal to the line, a plurality of identical lighting can be arranged and operated in parallel, whereby a scaling is possible here as well. With identical patterns, the strips then merge.

A maximum light intensity is achieved in the proposed device not least by the fact that the light patterns are not caused by occlusion or shading, as in projection, but by bundling of light. A Loss of loss is recorded only on lenses and reflector. This power loss is relatively low.

For photogrammetric multi-step processes, time-variant patterns are needed. However, a time variance of the system can only be achieved by changing the input variables. If no mechanical effort is to be operated, only the distribution of the light source can be changed, which can be realized by switching on and off individual lamps. When using LEDs, this can be implemented by appropriate electronics. When using optical fibers, correspondingly different primary light sources are to be used, which can be switched on and off independently of each other.

By using LEDs short switching times can be guaranteed. Thus, the best conditions are given to use the device for multi-step processes.

The illustrated figures show the applicability of the developed device and demonstrate possibilities for their production, the invention being not limited to the indicated uses and materials and combinations thereof.

Claims

claims

Device for generating light patterns (6), comprising at least one one-dimensionally focused illumination device, with at least one light source (1) and a reflector (2), the homogeneous light (3, 3 ') generated by the illumination device being transmitted through a field (5'; ) of juxtaposed lenses (5) orthogonal to the illumination device is bundled in such a way that parallel light stripes can be imaged and specific one-dimensional light patterns (6) can be generated.

Apparatus according to claim 1, wherein a plurality of lighting means (1), for example LEDs (1) are provided and form a row of light sources (1 ') and wherein the light patterns (6) are at different heights and with variation of the spatial distribution of the lighting means (1) in clearly distinguish the lighting from each other.

Apparatus according to claim 1 or 2, wherein the device is designed such that the resulting light patterns (6) are suitable for photogrammetric SD surface reconstruction by means of single or multiple camera systems in a predetermined measurement volume.

Device according to one of the preceding claims, wherein the LEDs (1) used in the illumination device can be switched on and off so that different light patterns (6) can be generated for a given measurement volume, which can be used for photogrammetric 3D surface reconstruction with multi-step methods on or Multi-camera systems are suitable.

Device according to one of the preceding claims, wherein as the light source (1) optical fibers are used whose primary light sources are switched on and off quickly.

6. An assembly comprising at least two devices according to one of the preceding claims, wherein the devices are arranged parallel to each other, and a stripe pattern (1 1) orthogonal to a line is expandable.