Laser module comprising a micro-lens array

Abstract

Light emitting unit, in particular of or for a triangulation-based distance measuring device, for providing defined measuring light, in particular laser light, is disclosed. The light emitting unit comprising a light source for emitting light, in particular a laser light source for emitting laser light, and a beam forming assembly for shaping the light by affecting propagation of the light emitted by the light source, wherein the beam forming assembly is arranged and designed so that measuring light is provided in form of a light line having a midpoint and two opposite ends. The beam forming assembly comprises at least one micro-lens array, the at least one micro-lens array comprises a plurality of micro-lenses, wherein the micro-lenses are designed and arranged in joint manner next to each other with algebraic signs for curvatures of successive micro-lenses being opposite and so that a periodic structure is provided.

Claims

1. A triangulation-based distance measuring device comprising: a light emitting unit comprising a light source for providing measuring light in a form of a light line with defined intensity distribution across the line, a light receiving unit having a sensor for detecting measuring light reflected and received from an object to be measured; and a controlling and processing unit for deriving distance information based on the detected reflection, wherein the light emitting unit and the light detection unit are arranged with known spatial position and orientation relative to each other according to the Scheimpflug criterion, wherein: the light emitting unit comprises at least one micro-lens array that comprises a plurality of micro-lenses, wherein the micro-lenses are designed and arranged in joint manner next to each other with algebraic signs for curvatures of successive micro-lenses being opposite, and so that a periodic structure is provided, wherein periodicity is defined by at least two successive micro-lenses with opposite curvatures.

2. The triangulation-based distance measuring device according to claim 1, wherein the micro-lens array is a cylindrical micro-lens array having a plurality of cylindrical micro-lenses as micro-lenses arranged next to each other in a first direction across the array, in particular wherein the first direction corresponds to a crosscut perpendicular to an extension direction of the cylindrical micro-lenses.

3. The triangulation-based distance measuring device according to claim 2, wherein the cylindrical micro-lenses of the cylindrical micro-lens array are arranged and designed so that a periodic profile is provided on the surface of the array in the first direction, wherein the periodic profile has a wave-like shape.

4. The triangulation-based distance measuring device according to claim 2, wherein the profiles of the cylindrical micro-lenses of the cylindrical micro-lens array comprise at least partly circle-like shapes.

5. The triangulation-based distance measuring device according to claim 2, wherein the cylindrical micro-lenses of the cylindrical micro-lens array are designed and arranged next to each other so that a sinusoidal profile is provided in the first direction on the surface.

6. The triangulation-based distance measuring device according to claim 1, wherein the periodic structure comprises constant periodicity and amplitude across the array.

7. The triangulation-based distance measuring device according to claim 1, wherein the periodic structure comprises varying periodicity and/or amplitude across the array in depending on the desired properties of the light line.

8. The triangulation-based distance measuring device according to claim 1, wherein the periodic structure comprises varying periodicity and/or amplitude in the first direction, depending on the desired properties of the light line.

9. The triangulation-based distance measuring device according to claim 1, wherein a particular measuring range with respect to a light emitting direction is defined by periodical arrangement of the micro-lenses with a defined micro-lens to micro-lens pitch, wherein a maximum measuring range depends on the Talbot length defined at least by the micro-lens to micro-lens pitch, in particular wherein a minimum of the measuring range corresponds to half the Talbot length.

10. The triangulation-based distance measuring device according to claim 1, wherein the micro-lenses are arranged with a micro-lens to micro-lens pitch in a range of 20 m to 200 m.

11. The triangulation-based distance measuring device according to claim 1, wherein the micro-lenses are arranged with a micro-lens to micro-lens pitch of 150 m.

12. The triangulation-based distance measuring device according to claim 1, wherein a topographic micro-lens height regarding a surface of the micro-lens array is of at least 5 m.

13. The triangulation-based distance measuring device according to claim 1, wherein a topographic micro-lens height regarding a surface of the micro-lens array is between 40 m and 50 m.

14. The triangulation-based distance measuring device according to claim 1, wherein the micro-lens to micro-lens pitch is provided so that self-imaging of the micro-lens array in the laser line at a defined distance from the micro-lens array provides a Talbot pattern with light structures which when imaged onto an image sensor arranged for triangulation imaging the laser line are smaller than the pixel size of the sensor.

15. The triangulation-based distance measuring device according to claim 1, wherein the light source is embodied to be driven in a pulsed mode in such way that pulses with pulse durations of nanoseconds are provided on operation of the light emitting unit.

16. The triangulation-based distance measuring device according to claim 1, wherein the micro-lens array is represented by a pattern of convex and concave lenses.

17. The triangulation-based distance measuring device according to claim 1, wherein the light emitting unit comprises a further micro-lens array arranged between the light source and the micro-lens array is designed according to a micro-lens array of claim 1, the further micro-lens array provides translation-invariant positioning of the micro-lens array relative to the light source arranged to provide transition of the light emitted by the light source before reaching the micro-lens array is diminished by interaction of the light with the Fresnel cylindrical lens, wherein the light source comprises: a light-emitting diode, in particular comprising a spatial filter, in particular a masking with a slit, or by a laser source, wherein the emitted light is provided as laser light and the light beam is a laser beam, wherein the light source further comprises a collimation element having asymmetric light-emitting aperture, wherein the collimation element is designed so that a length of the light-emitting aperture in the first direction is significantly greater than in a second direction.

18. The triangulation-based distance measuring device according to claim 1, wherein the light source comprises a super-luminescent diode, or a multi-mode laser source.

19. The triangulation-based distance measuring device according to claim 1, wherein the micro-lenses are arranged with defined micro-lens to micro-lens pitch, wherein the pitch as imaged onto the sensor is equal to or smaller than a pixel size of the sensor, in particular wherein the pitch as imaged onto the sensor is equal to or smaller than a pixel size of the sensor.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Devices according to the invention are described or explained in more detail below, purely by way of example, with reference to working examples shown schematically in the drawings. Specifically,

(2) FIGS. 1a-b show a crosscut and a top view of a micro-lens array according to prior art;

(3) FIG. 2 shows an embodiment of a light emitting unit according to the invention;

(4) FIG. 3 shows a profile of a micro-lens array of a light emitting unit according to the invention;

(5) FIG. 4 shows an embodiment of a light emitting unit according to the invention;

(6) FIGS. 5a-b show a further embodiment of a light emitting unit respectively the light source according to the invention from different perspectives;

(7) FIG. 6 shows a further embodiment of a light emitting unit having a Fresnel lens according to the invention;

(8) FIGS. 7a-b show a traditional wrapping and a optimised wrapping resulting from a combination of a Fresnel lens and a micro-lens array;

(9) FIG. 8 shows a Talbot pattern which emerges by illumination of a periodical structure like a (cylindrical) micro-lens array according to the invention;

(10) FIG. 9 shows a surface measurement of an embodiment of a micro-lens array according to the invention; and

(11) FIG. 10 shows a working principle of a triangulation device the present invention relates to.

DETAILED DESCRIPTION

(12) FIG. 2 shows a light emitting 10 unit according to the invention. The light emitting unit 10 comprises a light source 11 and a micro-lens array 14. The light source 11 here may be designed as a light emitting diode (LED), a laser diode or a broad area laser (BAL) (e.g. with a collimation lens) or any other kind of suitable light source.

(13) Broad area lasers (BAL), also known as broad stripe, slab or broad emitter laser diodes have a gain volume which is much wider in one lateral direction (x-direction). Compared to single-mode laser diodes which have gain regions which are small in both directions, a BAL emits a very high optical power (in the order of 1-10 W). ABAL can be used in pulsed mode with pulses typically below 100 ns, but in some cases also with longer pulses or even in continuous mode

(14) In the wide direction, many spatial modes as well as longitudinal modes can coexist. In a narrow direction, preferably only one spatial ground mode propagates, and the laser can thus be focused down to a diffraction limited line focus. Furthermore, BALs are low cost and represent a suitable type of light source for this invention.

(15) Alternatively, a light source may be used with very small extent as to the vertical direction or comprising an additional optical element which provides a corresponding small extent (e.g. a LED masked by a very thin slit).

(16) Using an arrangement of the light source 11 and the micro-lens array 14 as shown provides for diffusion (spreading) of the light emitted by the light source 11 in one direction so that a line of light is provided as measuring light for triangulation measurement of an object. The optical element 14 provides a specific diffusing angle of the measuring light, here of about 27, i.e. an overall opening angle of about 54.

(17) Furthermore, the array of lenses 14 and the light source 11 are preferably designed and arranged so that the measuring beam is emittable in form of a basically continuous line regarding its extension in the wide direction (x-direction). For that, the pitch of the lens array 14 and the width of the laser diode can be chosen so that the projected diode width matches the diffraction angle of the lens array 14 and the emitted line thus becomes continuous without any dark spots which could otherwise occur if the lens array pitch is too fine or the laser width too small. A too coarse array pitch or wide laser could on the other hand cause bright spots where two projections overlap, so it is optimal to choose these parameters exactly so that there is no overlap, or multiples of 100% overlap.

(18) According to a specific embodiment, the configuration comprises one micro-lens array 14 in combination with a 10 mm exit aperture.

(19) In particular, prior to final diffusion by the micro-lens array 14, the light beam, in particular laser beam, is made wide enough to provide a large emitting surface. The initial width of the emitted (and collimated) beam may for instance be about 1 mm while after widening it may be 10 mm at the diffuser 14 (lens array). Several types of components can be used to widen the emitted beam, e.g. a further cylindrical lens, a lenslet array, a diffractive optical element, a Fresnel lens or some kind of computer generated or natural hologram. A further lenslet array may represent such beam spreading element, wherein the shown lenslet array 14 represents the beam diffusing element. If the source is a laser without collimation in a horizontal direction (slow axis), the beam may diverge fast enough that no extra optics is needed.

(20) Concerning the used light or laser diode and a possible focussing collimator, these may comprise asymmetric aperture. The aperture particularly is designed to be as large as possible along the line (to enhance efficiency), i.e. in a horizontal direction (slow axis), and additionally narrower across the line to increase and define depth of focus, as well as to improve focus quality. With a smaller NA it is possible to use a cheaper lens. Moreover, it is difficult to achieve a large depth of focus with only one lens without aperture since it would have to have a very short focal length. A lens with longer focal length with aperture improves pointing stability since magnification from laser position to beam angle is smaller. Since the aperture costs some (e.g. 60%) efficiency, it is beneficial to have a high power laser diode like a BAL.

(21) The micro-lens array 14 comprises a number of micro-lenses arranged next to each other, wherein successive micro-lenses have opposite directions of curvature, i.e. the sign or curvature is alternating from micro-lens to micro-lens. Hence, a wave-like surface profile is provided by that particular arrangement of lenses.

(22) Concave and convex like shaped lenses are alternating along a line across the lens array 14, in particular along the x-direction.

(23) By such arrangement of the micro-lenses a periodic structure is provided, which is free of optical irregularities on the surface (burrs, soiling, residues by manufacturing etc.). The amount of such irregularities is at least strongly reduced or avoided due to homogenous transition areas in the contact region of two successive micro-lenses.

(24) FIG. 3 shows a profile of a micro-lens array of a light emitting unit according to the invention. Such profile represents the shapes of successively arranged micro-lenses 141,142,143. As can be seen the lenses 141 and 143 have curvatures of identical directions (identical signs), wherein the lens 142 in-between has a curvature with opposite sign. The signs of curvature from micro-lens to micro-lens change in the contacting points (depicted by the dashed lines) of two successive lenses.

(25) Of course, it is to be understood that FIG. 3 shows only a comparatively small part of an entire micro-lens array. The structure further extends periodically at least in x-direction.

(26) Periodicity on the surface of the array is given by successively repeating sets of two lenses with opposite signs of curvature.

(27) The shape of the profile along the x-direction can also be described more general mathematically. The profile as shown represents a profile height (h) across a lateral position (x) on the surface. A function of the profile height and the slope of the profile in x direction (=the first derivation of the profile function) can be continuous functions without any jump discontinuities.

(28) For instance, the function of a profile height (h) according to one specific embodiment of the invention is

(29) $h=\frac{\frac{x^2}{R_0}}{1+\sqrt{1-\left(1+k\right)\frac{x^2}{R_0^2}}},$
wherein R.sub.0 is the radius of curvature and k is the conical constant.

(30) In particular, the radius of curvature is out of a range between 9 m and 20 m and the conical constant ranges between 1.5 and 1.1. Using such parameters, the pitches from micro-lens to micro-lens result between 20 m and 40 m and a preferred lens height between 5 m and 10 m.

(31) According to an embodiment of the invention the micro-lens array may comprise at least 150 pairs of micro-lenses, preferably more than 180 pairs, in particular wherein the lenses are cylindrical lenses.

(32) FIG. 4 shows an embodiment of a light emitting unit 10 according to the invention. The unit 10 comprises a light source 11 and a beam forming assembly 12. The light source 11 may be designed as a light emitting diode (LED), a laser diode or a broad area laser (BAL) (and a collimation lens) or any other kind of suitable light source.

(33) The beam forming assembly 12 comprises a cylindrical lens 15 and two lenslet arrays 13,14 (micro-lens arrays), wherein at least one of which comprises successively arranged micro-lenses with alternating signs of curvature (here shown with the micro-lens array 14). Using an arrangement of optical elements 13-15 as shown provides for diffusion (spreading) of the light emitted by the light source 11 so that a line of light is provided as measuring light for triangulation measurement of an object. The optical elements 13-15 provide a specific diffusing angle of the measuring light.

(34) The cylindrical lens 15 preferably has a focal length essentially equal to the distance to lenslet array 13.

(35) Focussing in the vertical plane is basically provided by the design of the laser source 11.

(36) It is beneficial to use cylindrical lens arrays 13 e.g. instead of single-surface large lenses since the tolerance on lateral positioning is much less strict. Having two diffusers (e.g. the two lenslet arrays 13,14 as shown) also reduces speckle by in effect converting several transversal modes to spatial incoherence. The first diffuser (which spreads the light to cover the second one) could have a pitch at least around five times smaller than the beam width to reduce effects of lateral positioning.

(37) Furthermore, the array of cylindrical lenses and the light source are preferably designed and arranged so that the measuring beam is emittable in form of a basically continuous line regarding its extension in the second direction. For that, the pitch of the first cylindrical lens array 13, the width of the laser diode and a laser collimation lens focal length can be chosen so that the projected diode width matches the diffraction angle of the lens array and the emitted line thus becomes continuous without any dark spots which could otherwise occur if the lens array pitch is too fine, the laser width too small or the collimator focal length too long. A too coarse array pitch, wide laser or short collimator focal length could on the other hand cause bright spots where two projections overlap, so it is optimal to choose these parameters exactly so that there is no overlap, or multiples of 100% overlap.

(38) The light emitting unit 10 as shown may provide a light line with a particular intensity distribution concerning the emitted light. Such distribution is provided by respective diffraction and collimation effects of the optical elements 13-15. The intensity may be lowest in the middle of the produced line. The light intensity (brightness) then increases towards the ends of the light line. The increase may particularly correspond to a growth of intensity according to a factor

(39) $\frac{1}{{\cos }^4\left(\right)},$
wherein represents a respective diffusion angle, i.e. the distance along the light line from the midpoint to a respective end. a is limited by the diffusion angle defined by the optical elements, e.g. finally by the cylindrical micro-lens array 14.

(40) As mentioned, also the lens array 14 is embodied as a cylindrical micro-lens array 14 which comprises multiple cylindrical micro-lenses arranges next to each other so that neighbouring lenses comprise inverted curvatures. Besides the opposite curvature of to successive lenses other (optical) properties, like e.g. focal lengths, may be identical.

(41) The micro-lens array 13 may also be embodied as a cylindrical lens array comprising alternating curvatures of successive lenses (not shown).

(42) FIGS. 5a and 5b show a further embodiment of a light emitting unit 10 respectively the light source 11 according to the invention from different perspectives. The optical elements 36 and 37 as shown may be directly and fixed arranged with the light source 11, wherein the light source may be understood as a laser source already being equipped with such components. FIG. 5a shows the laser source and a micro-lens array 34 with lenses of alternating signs of curvature in side view. The light emitting unit 10 comprises a laser diode 11 and a (collimation) lens 36. In addition, the light emitting unit 10 comprises an aperture 37 of asymmetric shape. As can be seen in side view, the element 37 comprises comparatively small aperture, e.g. a quite narrow slit for light transmission, for providing large depth of focussing in the first (vertical) direction. Such collimation element 37 further provides a large aperture in the second direction reduce losses for diffusion in the horizontal plane (FIG. 5b). As collimation and spatial limitation of the laser beam with respect to the vertical direction can so be provided in sufficient manner, no further cylindrical lens for focusing is needed.

(43) The focussing of the laser light in the vertical direction and diffusing in the horizontal direction leads to reduction of subjective speckles formation while providing a well defined line which is very suitable for triangulation measurements. Apart from lowering speckle noise and thus improving depth accuracy, the low-speckle illuminator also allows a much increased camera depth of field. The reason for this is that the speckle contrast no longer depends on the camera NA as it does with a fully coherent source.

(44) In addition, relating to detection of the produced laser beam, specific camera optics may be provided with the image sensor. The camera optics may comprise a camera lens which may also have asymmetric aperture since optical resolution along line (horizontally) may be more critical than across. This also results in realising reduced exposure time and thus improves eye safety. Such camera lens may be anamorphic (different magnifications regarding the first and the second direction) to e.g. obtain a wider field of view. Preferably, the camera comprises an intensity filter to provided proper filtering of incoming light.

(45) Due to the design of the lens array 34and in particular of the laser source 11, the lens 36 and the aperture 37intensity distribution of a line emittable with such arrangement may be adjusted having significantly greater intensities at its ends than in the centre of the line. In particular, intensity increase along the line is proportional to a factor

(46) $\frac{1}{{\cos }^4\left(\right)}\mathrm{or}\frac{1}{{\cos }^5\left(\right)}.$

(47) FIG. 6 shows a further embodiment of a light emitting unit 10 according to the invention.

(48) The unit 10 comprises a light source 11 and a beam forming assembly 12. The light source 11 may e.g. be designed as a broad area laser (BAL).

(49) The beam forming assembly 12 comprises a cylindrical Fresnel lens 16 and two lenslet arrays 13,14 (cylindrical micro-lens arrays), wherein at least one of the arrays comprises successively arranged micro-lenses with alternating signs of curvature (here shown for both arrays). Using an arrangement of optical elements (13,14,16) as shown provides for diffusion (spreading) of the light emitted by the light source 11 so that a line of light is provided as measuring light for triangulation measurement of an object. The optical elements provide a specific diffusing angle of the measuring light of about 23.

(50) The Fresnel cylindrical lens 16 is arranged for reducing or avoiding the generation of a Talbot pattern (see FIG. 8 and description below) at the object to be measured and consequently on a camera sensor for capturing an image of the object. The lens 16 introduces several phase shifts of at least 2 between different beam parts that add-up incoherently after passing the micro-lens array 14 in the measurement field. Preferably the optical path length difference is beyond the coherence length of the light source.

(51) For instance by use of a first lens array 13 with an diffusing angle of about 30 and a diffusion angle of about 23 of the second array 14 an optical path difference of 1.5 mm can be realised. Thus, temporal coherence of the laser light emitted with the light emitting unit 10 is reduced or removed.

(52) Having light emitted with lower coherence the occurrence of a Talbot pattern is reduced simultaneously. Hence, the generation of a so caused Talbot pattern along the laser line is getting less probable and surface measurements with such laser line becomes more precise and reliable.

(53) According to a further embodiment of the invention, the Fresnel lens can be combined with the second micro-lens array into a single micro-optical component by addition of the corresponding optical phase modulation functions. To avoid excessive phase jumps and acute angles produced by traditional modulo-2 wrapping, the wrapping algorithm can be optimized by shifting phase jumps left or right as to minimize the number of jumps and maximize the obtuseness of the edges on each side of each phase jump. This will come at the cost of a slightly increase total phase range, but greatly improves the manufacturability and resulting quality of the microstructure. In effect, this optimisation method will shift the jumps (i.e. Fresnel zone boundaries) to integer multiples of the micro-lens pitch.

(54) Such optimisation is shown with FIGS. 7a and 7b. FIG. 7a shows a traditionally occurring modulo-(2) wrapping comprising unwanted jumps 51 to be avoided and an acute angle 52. FIG. 7b shows the resulting wrapping with a desired obtuse angle 53 after having combined and adjusted a Fresnel lens and a micro-lens array as described.

(55) Alternatively, keeping the Fresnel zones intact, one could adapt the micro-lens pitch for each zone to also achieve obtuse angles at the phase jump. The micro-lens height should then also be adapted to maintain the same angular spectrum.

(56) In the case of very strong Fresnel lenses, the phase jump can be multiples of 2. Also in this case the same principle can be utilized to adapt the jump position to the micro-lenses or vice versa.

(57) According to a further embodiment, one or more additional (a-) cylindrical micro-lens array is positioned along the beam as to reduce the coherence further.

(58) According to a further embodiment, a second micro-lens array is designed to produce a line at an offset angle (e.g. 4520 degrees). Unlike for angle=0 (on-axis) where the path length difference is very small, at such large angles the path length differences between the left and right end of the array are significant and objective speckles (Talbot pattern) are hence significantly reduced. At the cost of power, the angularly offset line could also be obtained by masking an on-axis centred line. Alternatively, the beam from an on-axis line laser could be made off-axis by adding a blazed grating before or after the last micro-lens array.

(59) FIG. 8 shows a Talbot pattern (or Talbot carpet) which emerges by illumination of a periodical structure 44 like a (cylindrical) micro-lens array.

(60) The optical Talbot effect here is shown for monochromatic light. On the left of the figure the light can be seen diffracting through the lens array 44, wherein this exact pattern is reproduced 45 on the right of the picture in a defined distance away from the structural pattern (one Talbot length z.sub.t away from the array). Halfway between each edge and the middle (=secondary Talbot image 46), one sees the image shifted to the side (double-frequency 47 fractional image). Moreover, at regular fractions of the Talbot length sub-images are clearly seen. The Talbot length is defined by

(61) $z_t=\frac{2a^2}{},$
wherein a is the pitch between two successive micro-lenses.

(62) This pattern is repeated with period z.sub.t in distance z until the diffraction orders separate. It also extends up and down (along the laser line), but may be weaker further away from the centre due to longitudinal incoherence.

(63) With view to triangulation measurements results of respective distance measurements would become inaccurate in case a light line is detected which contains bright and dark regions of relatively large size compared to a pixel size of a detecting sensor unit. Therefore, in order to provide accurate measurements there are some possible approaches working around said problem.

(64) First, choose such a small period of micro-lenses so that the size of the occurring Talbot pattern is not of big problem. In particular, a lens-to-lens period is chosen which is smaller than a pixel resolution of camera in a respective triangulation sensor, e.g. smaller than 50 m.

(65) Alternatively, choose a large period and adjust working range to fit between z.sub.t and z.sub.t/2. As occurring patterns in the sub-images are comparatively small, they would not negatively influence accuracy of measurements in a significant way. However, as even double-frequency Talbot effects can be larger than camera pixel resolution respective design of the micro-lens array may have to be chosen as well.

(66) A further reduction of the Talbot effect can be achieved by reducing the coherence of the light. Using a Fresnel cylindrical lens one introduces several phase shifts of at least 2 between different beam parts that add-up incoherently after passing the micro-lens array in the measurement field. Preferably the optical path length difference is beyond the coherence length of the light source.

(67) According to a further embodiment of the invention (not shown) several Fresnel lenses might be used in a kind of optical relay setup (alternating collimation and focussing) to further increase the path length difference between different beam parts to reduce the Talbot-effect.

(68) In case the arranged cylindrical lens consist of a diffractive lens like a Fresnel-lens the profile of the micro-lens array might be added (addition of phase profiles) to generate a new combined diffractive element providing the optical response of the cylindrical lens and of the micro-lens array in one part and in one diffractive structure.

(69) Moreover, by driving the light source (laser diode) in a very short pulse mode at a high pulse repetition rate preferably with a pulse width of a few nanoseconds the emitted spectrum will be broadened because many longitudinal modes start to oscillate similar to an SLED device. This effect will also reduce the temporal coherence of the light source and by this will diminish the visibility of the Talbot-effect in the measurement range.

(70) According to a related embodiment of the invention, the light source is embodied to be driven in a pulsed mode in such way that pulses with pulse durations of nanoseconds are provided on operation of the light emitting unit.

(71) FIG. 9 shows a surface measurement of an embodiment of a micro-lens array according to the invention. The design of such micro-lens array is sinusoidal meaning a periodic pattern of convex and concave acylindrical lenses. The chart shows a measured height in micrometers over respective lateral positions the measurements are taken (in micrometers as well). As can be seen, according to the shown embodiment the height between the lowest points and the highest points in the profile is in a range of 70 to 90 m.

(72) Of course, according to alternative embodiments of the micro-lens array (not shown) there might be different heights and/or distances from lens to lens. E.g. the lenses may be provided with alternating radius sign between 7 m and 18 m, i.e. with heights of about 15 m to 36 m.

(73) According to a further embodiment of the invention (not shown) the cross section of the cylindrical micro-lens array may be in form of curvature-alternating parts of spheres. In particular, half-spheres are arranged next to each other with opposite curvatures (e.g. convex and concave in turns).

(74) FIG. 10 shows the working principle of a triangulation device 1 according to the invention. The device 1 comprises a light emitting unit 2 and a light receiving unit 3, e.g. a camera, the relative positions and orientations of which are known. In other words, such laser triangulation is to send out light in one known direction from a known position and receive from a known position and measure the angle of the incoming light.

(75) The light emitting unit 2 comprises a light source which may be represented by a laser diode for emitting laser light. Furthermore, the light emitting unit comprises an optical unit for forming the emitted laser light so that a defined measuring beam 4 can be emitted. Such measuring beam is focussed according to a first direction (vertically) and diffused with respect to a second direction (horizontally), orthogonal to the first direction. By doing so, a laser line can be produced and projected onto an object 5 to be measured.

(76) The light receiving or detecting unit 3 comprises an optical assembly (e.g. imaging lens) as well to form and direct the reflected light 6 to an image sensor of that unit. The sensor preferably is designed as a CCD or CMOS sensor providing a pixel-array in form of a line or an area. The sensor is also preferably tilted according to the Scheimpflug criterion so that camera's object plane coincides with the illuminated plane so that all illuminated points are imaged sharply onto the sensor. The image sensor is designed being at least sensitive for light of a wavelength of the measuring light 5. The pixels of the image sensor are exposed by the incoming reflected light 6 and a course of the line at the object 5 can be derived based on the illuminated pixels of the sensor. That allows determining distances to the object's surface based on the knowledge of the relative positions of emitter 2 and detector 3 and the detected line, in particular based additionally on the properties of the optical assembly and the position of the detected line on the image sensor.

(77) According to an alternative embodiment not shown here, the emitted beam 4 is emitted in a direction perpendicular to the housing, allowing to place an additional receiving unit at the left side of the emitting unit 2 in order to generate additional measuring data. There also can be arranged a third receiving unit 3 placed beside the emitting unit 2 at the same distance than the first one (and/or second one) or at different distances to achieve a higher robustness against the detection of objects with a strong contrast change (introducing a shift of the detected intensity centroid) or the detection of edges causing shadowing effects.

(78) By moving the triangulation device 1 over the object 5, continuously receiving the reflected light 6 and processing signals provided by the image sensor, the surface of the object 5 can be measured in its entirety. In particular, such scanning is performed by a coordinate measuring machine (either motorized or hand-held) carrying the triangulation device 1 and moving it along a desired measuring path.

(79) According to the invention, the light emitting unit 2 comprises a micro-lens array having alternating curvatures of successively arranged micro-lenses, e.g. according to any embodiment of a respective micro-lens array described herein above in context with the invention.

(80) Although the invention is illustrated above, partly with reference to some specific embodiments, it must be understood that numerous modifications and combinations of different features of the embodiments can be made and that the different features can be combined with each other or with triangulation principles and/or coordinate measuring machines known from prior art.