Abstract:
In a binocular viewing device for observing one image with both eyes simultaneously, the image is observed directly via a beam splitter with a first ocular, and the image is observed indirectly with a second ocular via an optical imaging system which forms an intermediate image to which the second ocular is adjusted. The optical imaging system may comprise a collimator objective, a telescope objective and an image-inverting deflecting prism.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a binocular viewing device for viewing an image with both eyes simultaneously. The device includes a first and a second ocular, a beam splitter and a deflecting prism. 
     Such viewing devices are used to form separate images for each eye, in one image plane, such as for example in connection with the use of a microscope. Known viewing devices employ a plurality of rhomboid deflecting prisms and a beam splitter. One ocular is adjusted to the image plane via one deflecting prism and the beam splitter, and the other ocular via two deflecting prisms and the beam splitter. Adjusting the viewing device to the observer&#39;s pupillary distance is then effected by rotation via a periscopic transmission. However, this construction leads to a bulky and heavy viewing device which is not flat. Due to the size of the deflecting prisms, the device is made of two parts which are arranged at an angle relative to each other, and the parts should be rotatable relative to each other so to enable adjustment to the observer&#39;s pupillary distance. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a viewing device, of the type described above, which is simple, small, of a flat construction and simply adjustable to the observer&#39;s pupillary distance. According to the invention the viewing device includes a first ocular which is adjusted to the image via the beam splitter only. Between the beam splitter and the second ocular there is arranged an optical imaging system for forming an intermediate image in an intermediate image plane, to which the second ocular is adjusted. This viewing device is of an asymmetrical construction, allowing a number of deflecting prisms to be dispensed with. In this way a compact and flat construction is possible. 
     In a preferred embodiment of the device according to the invention the optical imaging system comprises a collimator objective, whose object focal plane (via the beam splitter) substantially coincides with the image to be observed, a telescope objective arranged in front of the collimator objective and having substantially the same focal length as the collimator objective, and an image inverting prism. The second ocular is adjusted to the focal plane of the telescope objective, which focal plane forms the intermediate image plane. Such objectives can be of very simple and compact construction, so that the complete viewing device can be very small. 
     According to a further aspect of the device according to the invention, the telescope objective, the prism and the second ocular constitute a single structural unit which can be shifted relative to the collimator objective for adjusting the oculars to the observer&#39;s pupillary distance. Such a construction provides a very convenient adjustment facility, because the path of the beam between the collimator objective and the telescope objective is substantially parallel. 
     In order to minimize the influence of a slight tilting of the pupillary distance-adjusting unit relative to the beam splitter the first ocular and the collimator objective during adjustment, the telescope objective may have the same focal length as the second ocular. Thus, the parallelism of the viewing device for both eyes is maintained even in the case of a slight tilt. 
     The viewing device according to the invention can be used advantageously in a viewing apparatus which further comprises an objective and an image intensifier. Suitably, an image twister comprising a bundle of glass fibers is arranged between the image intensifier and the binocular viewing device. When observing terrestrial scenes by means of the viewing device, it is necessary that the image to be viewed is erect in the image plane. For other uses, such as microscopes, where the image orientation is irrelevant, the image to be observed may remain inverted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole drawing FIGURE is a schematic diagram of an embodiment of a device according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An objective 1 forms an inverted image, of a scene to be observed, on the cathode 10 of an image intensifier tube 2. An image twister 3, for example comprising a bundle of glass fibers, in the image intensifier tube 2 ensures that the image formed in the image plane 11 is not inverted. 
     The image in the image plane 11 is viewed directly with the right-hand ocular 5&#39;, that is it is viewed only via the beam splitter 4. This beam splitter transmits approximately 50% of the light emitted by the image plane and reflects the remainder of the light. As the partly reflecting surface of the beam splitter 4 makes an angle of 45° with the axes of the image intensifier tube and of the ocular 5&#39; (which axes substantially coincide with each other) the direction of the reflected light is perpendicular to the axis of the ocular 5&#39;. 
     The reflected light beam is received by a collimator objective 9. This objective constitutes one structural unit with the image intensifier tube 2, the twister 3, the beam splitter 4, and the ocular 5&#39;. 
     When the collimator objective 9 is adjusted so that its focal plane coincides with the image plane 11, the beam will leave the collimator objective 9 as a substantially parallel beam. 
     This beam enters the telescope objective 8, which effectively has the same focal length as the collimator objective 9, in order to obtain a unity imaging scale. The objectives 8 and 9 may be of identical construction. 
     The beam emerging from the telescope objective 8 is incident on a pentagonal-roof prism 8 (a so-called Goulier prism) and is deviated through 90°. Due to the double reflection inside this prism the image is simultaneously inverted. The intermediate image plane 7 may be situated in the prism 6. 
     The beam which exits from the prism 6 is incident on the ocular 5, which is adjusted to the intermediate image plane 7 and which corresponds to the ocular 5&#39;. The intermediate image plane 7 should therefore be disposed in the same plane as the image plane 11. Image plane 7 may be slightly tilted with respect to image plane 11 because the telescope objective 8, the prism 6 and the ocular 5 form one structural unit, which is arranged to be slidable relative to the structural unit comprising the elements 2, 3, 4, 5&#39; and 9. 
     Since the light beam between the collimator objective 9 and the telescope objective 8 is substantially parallel, the intermediate image plane 7 is not shifted when the two units are moved relative to each other in order to adjust the two oculars 5 and 5&#39; to the observer&#39;s pupillary distance. Since the collimator objective 9 and the telescope objective 8 have the same focal length, the same image with the same size as in the image plane 11 is formed in the intermediate image plane 7. As a result of this, the image viewed via the two oculars 5 and 5&#39; is always the same, independent of the shift of the two units relative to each other. 
     Thus, a night-viewing device is obtained with which one can observe a scene, whose intensity has been increased by an intensifier tube, with both eyes, although the expensive image-intensifier tube need only be provided once. (The stereoscopic impression which would be obtained by using two image intensifier tubes would be negligible). 
     In order to obtain a compact, flat and light construction, a pentaprism is selected for the element 6. As a result of this, it is not possible to arrange a field lens in the intermediate image plane 7. For small diameter oculars 5, it is not possible to observe the entire intermediate image at the left side, but only a central portion thereof. If the observer adds the central portion of the left-hand image to the complete right-hand image, then, according to the Schrodinger equation for binocular intensity interaction, a uniform overall image impression B G  is obtained, which is given by 
     
         B.sub.G =(B.sub.H.sup.2 +B.sub.D.sup.2)/(B.sub.H +B.sub.D) 
    
     where B H  is the brightness of the light image areas and B D  the brightness of the corresponding dark picture areas. 
     This effect also leads to a reduction of vignetting in the case of large pupillary-distance settings, when the distance between the collimator objective and the telescope objective is increased and the telescope objective need not be arranged obliquely for this.