Abstract:
A folded image intensifier viewer with an asymmetric viewing system and a single image intensifier and two eyepieces able to regulate the mutual distance between the eyepieces, after a beam splitter which divides the beam path into two extremely asymmetric parts, for each eyepiece a lens system is provided with at least one having at least two lenses.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a folded image intensifier viewer with an asymmetric viewing system and a single image intensifier and two eyepieces able to regulate the mutual distance between the eyepieces, after a beam splitter which divides the beam path into two extremely asymmetric parts. 
     DESCRIPTION OF THE RELATED ART 
     An image intensifier viewer, also called goggles, is known from Swedish Patent No. 450 671 (8503533-5). As is evident from this document such nightviewers have great advantages, when it is a question of size and above all through having the center of gravity able to be placed near to the head with a minimal construction in front, which makes such a nightviewer considerably more comfortable to wear than the previously known constructions of this type. 
     This known construction is shown in FIG. 1 which is taken from said patent document. FIG. 2A shows most of the viewing system with lenses 7 and 8, beam splitter 14, the two split beam parts thereafter, for the left eye mirror 16 and eyepiece 13A, and the for right eye prism 15 and eyepiece 13B. The outgoing beams of rays from the lens 8 are then each focused in an intermediate image in front of the two eyepieces 13A and 13B. It is evident that the mechanical beam for the right eye is longer than that for the left. The optical construction length is, however, very nearly the same for both beam parts, thanks to the optical path being reduced by means of a suitable choice of glass in the prism 15. Also shown is a single image intensifier 2 and an objective 1 for reproducing a scene on the intensifier 2. 
     This construction is rigid in that it is practically impossible to adjust the distance between the eyepieces to compensate for different distances between the eyes. This problem receives a sort of inadequate solution in the known construction through the egress pupils for the eyepieces being on the generous size so that a large number of users do not experience any great requirement for such an adjustment. The more an observer&#39;s distance between the eyes differs from the average distance of 66 mm, the worse, however, becomes the optical performance as the correction is optimized for the center of the egress pupils. Amongst others, the field of view vignette rings increase and the sharpness deteriorates. 
     However, in many cases for obvious reasons it is an urgent and sometimes imperative requirement, especially from military buyers, that such a possibility of adjustment nevertheless is present. Otherwise the binocular viewer cannot be used by everybody which in certain connections is an unacceptable state of things. The inconvenience of a fixed distance between the eyes consequently increases with reduced eyepiece pupils. Further difficulties therefore occur when one wishes to miniaturize image intensifier viewers, for example by means of smaller eyepieces and therefore reduced egress pupils. Furthermore, there are more often requirements for a larger field of view which often reduces the focal length of the eyepieces, and this is more and more difficult to combine with the large egress pupils which are necessary in order to be able to accept the lateral immovability of the eyepieces. 
     SUMMARY OF THE INVENTION 
     According to the invention these problems are eliminated through making one eyepiece movable. The requirements which must in this case be satisfied are partially the same as those for the known construction, viz. that the eyepieces must project equally far from the instrument casing and this also after adjustment of the distance between them, and that the magnification in the two eyepieces therewith must remain mutually equal. The latter presents special difficulty on the grounds that the construction is not symmetrical but in fact the left and right eyepiece systems are completely different, with, amongst others, different optical lengths, a difference which is corrected through using different optical constructions in the respective eyepiece channel, so that their so-called tele effect differs, and this is made possible through each part of the lens after the beam splitter containing a sufficient number of lens elements to achieve such an optical degree of freedom. 
     The above objects and advantages and others, which are evident from the continued description, are consequently achieved according to the invention through a binocular image intensifier viewer of the type mentioned in the introduction and in the following description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in the form of a non-limiting example of an embodiment and with reference to the Figures. 
     FIG. 1 shows schematically in a perspective view a known binocular image intensifier viewer with a fixed distance between the eyes. 
     FIG. 2A shows a part of the beam in the same image intensifier viewer but in a schematic plan view. 
     FIG. 2B shows a viewing system according to the invention with a variable eyepiece distance. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2A have already been described above. FIG. 2B can suitably be compared with FIG. 2A in order to see how the invention differs from that which is already known. 
     In the known construction in FIG. 2A, lens 7 is a collimation lens which reproduces the output plane of the light intensifier tube at an infinite distance. The lens 8 focuses the collimated beams from the lens 7 towards the respective eyepieces 13A and 13B. As is shown, the geometric/mechanical path for the right beam part is longer than that of the left here. Through choosing types of glass with a suitably high refractive index in the prism 15 the optical path is, however, reduced. 
     In FIG. 2B the lens 8 in front of the beam splitter 14 has been eliminated. The parallel beams from the lens 7 are instead focused in each beam individually by the lenses 8A and 8B. Instead of a lens 8 refracting for both paths, here in each part consequently an individual lens 8A and 8B have been inserted. This leads to that the collimated light from lens 7 now continues through the beam splitter and onto the lens 8A, permitting the important fact and the effect which is the aim of the invention, that this eyepiece 13A now can move laterally, as indicated by arrow 20, together with the mirror 16 and the lens 8A without causing the eyepiece focusing (13A), i.e. the dioptric adjustment, to need to be adjusted. Through this arrangement, viz. by moving the collimation lens 8 from a position in front of the beam splitter to one after the same, then unfortunately the available optical path through each of the channels is correspondingly shortened. With reference to the already earlier (FIG. 2A) shown certain mutual differences, the percentage differences (FIG. 2B) will therefore be increased. 
     A particular problem here is that it is mechanically tight; the distance between the eyepieces being defined by the human measurement of 66 mm. Furthermore, there must be sufficient space present for the different parallel beams of rays to be able to pass through the whole lens system without unacceptable vignetting. 
     In order to solve this problem, according to the invention optical solutions with several lenses are provided, so constructed that the system focal length in the respective eyepiece channels remains the same, even despite the optical and mechanical construction in the two channels being different. 
     Through a suitable choice of geometric configurations and refractive power distribution in the respective part systems in the beam path from the intensifier tube to the eyepieces, it has been shown to be possible, however, to still fit the system into the available space. 
     The solution which is suggested according to the example of an embodiment is consequently that the refracting lens 8 is swapped for two refracting lenses 8A,8B, placed after the beam splitter 14A (which can be a mirror or, as illustrated, a prism construction), complemented with each having a lens for correction of magnification and construction length. An example thereof is shown in FIG. 2B. The correction lens 9A is placed between the mirror 16, a reflecting unit, and the collimation lens 8A. Alternatively, a correction lens 9A&#39; may be placed intermediate mirror 16 and eyepiece 13A in lieu of lens 9A. The correction lens 9B is mounted close to the prism 15 at its output side, where it cannot obscure the common beam path between lens 7 and the beam splitter 14. Other positions are possible but it has shown that the aforementioned is advantageous and permits good correction and eyepiece fields of view greater than 50°. 
     As is known to the person skilled in the art, with the further degree of freedom which is obtained by several lenses, an improved correction of different image errors such as distortion, astigmatism and color errors is made possible. It is true that the phosphorus in the output surface of a light intensifier is green but it is not a pure spectral color and therefore an achromatization is still necessary. 
     In certain cases it is possible to have only one refracting lens in one part of the beam path wherewith one compensates to an equal magnification by means of the lens elements in the other part of the beam path. However, in general it is preferred to have at least one correction lens in each part of the beam path, as in that case one obtains several degrees of freedom during construction concerning geometry and magnification and naturally the reduction of reproduction errors. 
     It is clear that the magnification must be the same in the two systems as the wearer otherwise will have problems such as headache and double vision. It is necessary that the pictures also are similar concerning, for example, distortion and field of view bending. The increased degree of freedom concerning the design of the different lenses according to the invention is necessary in order to provide this. 
     As the skilled person knows, the calculation of the lenses and their positions is performed with the help of commercial computer problems, where one starts from a geometrically suitable configuration which is optimized through successive iterations. 
     It is normally desired in a binocular nightviewer that it obtains an magnification of 1:1, and this magnification which is achieved by the lens combination in the respective eyepiece branches is in this case dependent on the focal length for the objective which reproduces the night scene on the image intensifier tube&#39;s input plane. However, the invention is not limited to this, in itself, generally advantageous unitary magnification as it is also normal that, for example, a so-called teleconverter is placed in front of the objective on these instruments in order to thereby achieve other magnifications. 
     Unitary magnification can be achieved if the objective 1 and the eyepieces have the same focal length and the optical transport system from the image intensifier&#39;s output to the eyepieces has a unitary magnification. Otherwise, for achieving a unitary magnification the following conditions apply: 
     
         f(oc)=G·f(obj) 
    
     where f(obj) is the said objective&#39;s focal length, f(oc) is the focal length of the eyepiece, and G is the magnification between the intensifier tube&#39;s image on the phosphorous screen to the intermediate image in the image plane of the eyepieces.