Abstract:
In order to improve target illumination a light emitter comprises several partial light sources ( 3   a   , 3   b   , 3   c   , 3   d ), the partial beams ( 5   a   , 5   b   , 5   c   , 5   d ) of which are collected by means of a beam collector optic ( 2 ) and directed to the aperture of a collimator ( 1 ). The beam collector optic ( 2 ) comprises two half-mirrored sheets ( 12   a   , 12   b ), which each direct two similarly polarised partial beams ( 5   a, b;    5   c, d ) side by side onto a polarisation cube ( 13 ), where the pairs of partial beams which are polarised perpendicular and orthogonal to each other are overlaid.

Description:
DESCRIPTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to an optical rangefinder as used, for example, in surveying plots of ground and structures.  
           [0003]    2. Prior Art  
           [0004]    Optical rangefinders of the generic type have long been known. However, the luminance of the usually used emitters having a laser diode as a light source is as a rule lower than will be permissible from the point of view of eye protection. Moreover, the light beam emitted by an individual laser diode usually has a disadvantageous, very elongated cross-section, which can lead to insufficient focusing onto the target and consequently to an inadequate luminous flux and measuring errors. For these reasons, the range and accuracy of measurement and reliability of measurement of rangefinders of the generic type will be less than desirable and in principle also possible.  
         SUMMARY OF THE INVENTION  
         [0005]    It is the object of the invention to provide an optical rangefinder of the generic -type whose emitter has a high luminance and ensures good target illumination and high luminous flux at the target.  
           [0006]    The advantages achieved by the invention lie in particular in a decisive improvement in the range, i.e. in the maximum measured distance or, for a given range, an increase in the accuracy of measurement. These advantages are achieved with a relatively small emitter optical system, which makes it possible to keep the dimensions of the entire device small. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The invention is explained in more detail below with reference to figures which merely represent the examples.  
         [0008]    [0008]FIG. 1 a  shows a diagram of the emitter of a first embodiment of a rangefinder according to the invention,  
         [0009]    [0009]FIG. 1 b  shows the beam cross-section of the emitter according to the first embodiment at the target,  
         [0010]    [0010]FIG. 2 a  shows a diagram of the emitter of a second embodiment of a rangefinder according to the invention,  
         [0011]    [0011]FIG. 2 b  shows the beam cross-section of the emitter according to the second embodiment at the target,  
         [0012]    [0012]FIG. 3 shows a diagram of the emitter of a third embodiment of a rangefinder according to the invention,  
         [0013]    [0013]FIG. 4 shows a diagram of the emitter of a fourth embodiment of a rangefinder according to the invention,  
         [0014]    [0014]FIG. 5 schematically shows the design of the emitter of a fifth embodiment of a rangefinder according to the invention and  
         [0015]    [0015]FIG. 6 shows the aperture of the collimator of the fifth embodiment of the rangefinder according to the invention and the beam cross-section at said collimator.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    An optical rangefinder according to the invention has an emitter and a receiver which, for example, may be composed in a known manner of an optical system and avalanche photodiodes as well as an electronic control and evaluation unit, likewise of known design, which controls the emission of light pulses by the emitter and evaluates the output signal of the receiver which receives the reflected light pulses. The distance measurement can be effected by transit time measurement or by the phase comparison method. Here, “light” is always to be understood as meaning that it is not limited to the visible range of the spectrum but also includes at least the infrared range.  
         [0017]    The emitter of a rangefinder according to the invention may have large differences in its basic design. In every case, however, it has (FIG. 1- 5 ) a collimator  1  and a light source arranged before said collimator and consisting in each case of at least two partial light sources, as well as a beam-collecting optical system  2  arranged in the beam path between the light source and the collimator. Each partial light source contains a laser diode, which is usually an edge emitter, or a plurality of such laser diodes arranged in succession in the direction of the emission edges. The wavelength common to all laser diodes is the infrared range, preferably from 850 nm to 980 nm, or 1550 nm. The cross-section of the light beam emitted by a laser diode can in each case be reduced parallel to the emission edge and increased transverse thereto by superposition by means of a beam-forming optical system arranged very close to the emission edge and based on light diffraction or refraction, and the light beam can thus be more highly focused.  
         [0018]    With wavelengths from 850 nm to 980 nm, the beam can be focused to a very narrow beam, which permits distance gradation with high lateral resolution. Wavelengths of about 1550 nm are also very advantageous because the upper limit of the permissible individual pulse energy of about 8 mJ, which is determined by eye safety, is a factor of about  16000  higher than at wavelengths from 630 nm to 980 nm. The at least partial use of this factor, which is possible according to the invention, permits a very substantial increase in the range or—at a given range—in the accuracy of measurement.  
         [0019]    According to the first diagram (FIG. 1 a ), the light source consists of two partial light sources  3   a,b  arranged directly side by side. The beam-collecting optical system  2  is in the form of a collecting optical system  4  which is common to both partial light sources and collects partial beams  5   a,b  emitted by said partial light sources in the object plane  6  of the collimator  1 . They enter the aperture of the collimator  1  in such a way that each individual partial beam  5   a,b  substantially fills the aperture and therefore their cross-sections substantially overlap there. At the collimator  1 , the partial beams  5   a,b  then diverge again, but only to such an extent that their cross-sections lie directly side by side at the target (FIG. 1 b ). The light beam emitted by the emitter has, at the target, a luminous flux which is approximately twice as great as that of a light beam emitted by an individual laser diode.  
         [0020]    According to the second diagram (FIG. 2 a ), the partial light sources  3   a,b  are further apart. A separate collecting optical system  4   a;b  which once again collects the partial beam  5   a  or  5   b  emitted by it in the object plane  6  of the collimator  1  is coordinated with each partial light source. The partial beams  5   a,b  are incident side by side on the aperture of the collimator  1 , in such a way that they substantially completely fill said aperture and then form a parallel light beam of circular cross-section at the collimator  1  (FIG. 2 b ), which therefore has, at the target, not only approximately twice as high a luminous flux as a light beam emitted by individual laser diode, but also a more advantageous cross-section.  
         [0021]    According to the third diagram (FIG. 3), the partial light sources  3   a,b  are arranged opposite one another, and the beam-collecting optical system  2  has collecting optical systems  4   a;b  arranged after said partial light sources, as well as a prism  7  arranged between said optical systems and in the form of a diverter element. The prism  7  has two reflection surfaces  8   a,b  which divert the partial light beams  5   a,b  emitted by the partial light sources  3   a,b  into a further, common collecting optical system  9  which collects them, in a manner similar to the light source according to the first diagram, in the object plane  6  of the collimator  1  and directs them to its aperture, where they substantially overlap. The beam cross-section corresponds to FIG. 1 b . The use of a diverter element makes it possible to arrange the partial light sources  3   a,b  relatively far apart, which facilitates the cooling of the laser diodes.  
         [0022]    The fourth diagram (FIG. 4) corresponds substantially to the third one. The difference is in particular in the fact that the collecting optical systems  4   a,b ,  9  are adjusted differently, in such a way that, similarly to the light source according to the second diagram, there is virtually no overlap of the partial light beams  5   a,b , but the aperture of the collimator  1  is filled by the partial beams  5   a,b  incident on it side by side, and they then propagate at said collimator substantially parallel in such a way that the beam cross-section corresponds to FIG. 2 b .  
         [0023]    [0023]FIG. 5 shows, likewise schematically but in more detail, the structure of the emitter of a rangefinder according to the invention, which corresponds to a further diagram. In addition to a collimator  1  and a beam-collecting optical system  2 , it contains partial light sources  3   a,b , which emit partial beams  5   a,b  of the same polarity which are oriented perpendicular to one another, and two further partial light sources  3   c,d , which likewise emit partial beams  5   c,d  which are oriented perpendicular to one another and whose polarization is orthogonal to that of the partial beams  5   a,b . The partial light sources  3   a,b,c,d  consist in each case of a laser diode  10  and a cylindrical lens  11  arranged a small distance away from said laser diode. A collecting optical system  4   a;b;c,d  is arranged after each of the partial light sources  3   a,b,c,d .  
         [0024]    A plate  12   a  which is half-mirrored and half-transparent so that it transmits the partial beam  5   a  but diverts the partial beam  5   b  in a direction parallel to the partial beam  5   a  is arranged, as a first diverter element, after the collecting optical systems  4   a,b . The second diverter element arranged after the collecting optical system  4   c,d  is, like the first diverter element, in the form of a half-mirrored and half-transparent plate  12   b  which transmits the partial beam  5   c  while it diverts the partial beam  5   d  in a direction parallel to said partial beam  5   c . The partial beams  5   a,b  on the one hand and  5   c,d  on the other hand each reach, directly side by side, a polarization cube  13 , where the pairs of partial beams meet at right angles. The partial beams  5   a,b  in each case partially overlap with the partial beams  5   c,d  which are polarized orthogonal to them and are collected by a further collecting optical system  9  in the object plane  6  of the collimator  1  and directed onto its aperture, which is substantially filled in this way (FIG. 6). In the centre of the cross-shaped light spot is an approximately square spot of twice the luminance, where in each case two partial beams of orthogonal polarization overlap.  
         [0025]    Various modifications of the examples described are possible. Thus, in particular in the case of diagrams  1  to  4 , more than the two partial light sources shown can be used. The collection of partial beams by diverter elements can be cascaded for increasing the number of partial light sources, etc. Finally, it is also possible to use laser diodes having wavelengths of, in particular, from 600 nm to 1000 nm and in particular from 630 nm to 980 nm, which are outside the above-mentioned ranges.  
         [0026]    List of Reference Numerals  
         [0027]    [0027] 1  Collimator  
         [0028]    [0028] 2  Beam-collecting optical system  
         [0029]    [0029] 3   a,b,c,d  Partial light sources  
         [0030]    [0030] 4 ,  4   a,b,c,d  Collecting optical systems  
         [0031]    [0031] 5   a,b,c,d  Partial beams  
         [0032]    [0032] 6  Object plane of 1  
         [0033]    [0033] 7  Prism  
         [0034]    [0034] 8   a,b  Reflection surfaces  
         [0035]    [0035] 9  Collecting optical system  
         [0036]    [0036] 10  Laser diode  
         [0037]    [0037] 11  Cylindrical lens  
         [0038]    [0038] 12   a,b  Plates  
         [0039]    [0039] 13  Polarization cube