Abstract:
A device is part of an optical system ( 1 ) that works with an intermediate image and comprises stop( 5 ) with associated aperture ( 6 ). The device comprises a light-reducing or light-absorbing arrangement ( 12 ) located at the side of the aperture. The arrangement is arranged to reduce considerably or eliminate completely light ( 11 ) incident to the side of the aperture, and thereby considerably reduce beam reflections. In this way, there is less likelihood of the optical system being detected when this is in use.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage Entry of PCT/SE02/01190 filed on Jun. 19, 2002, which claims priority under 35 U.S.C. §119 to Swedish Application 0102332-4, filed on Jun. 29, 2001. 
     BACKGROUND 
     This disclosure relates to a device for an optical system that works with an intermediate image and comprises a stop with an associated aperture. 
     In various advanced applications, for example military applications, where sights, telescopes, electro-optical TV cameras, etc are used, the use of optical systems of this type is already known. The optical system can work, for example, within the visual range and the infrared range, that is with wavelengths within the range 0.45-14 micrometres. In connection with this, reference is made to the patent literature and the known systems on the market. By light is meant in this application at least a wavelength range 0.3-30 μm, comprising visible light and such wavelengths within the ultraviolet and infrared range that can be handled by optical technology used for the visible range. 
     With this type of optical system there is a pronounced desire to be able to design the system in such a way that it does not reveal itself during use. The problem is here that there are difficulties in avoiding stray light and reflections that can be detected by detection equipment, for example laser radar. The invention intends to solve this problem and proposes in this respect that the stray light and reflections in question should at least be reduced considerably. In connection with this, beam-reflecting surfaces (mirror surfaces) are used in one embodiment, and there is a desire to be able to arrange these surfaces in such a way that an optical system is obtained that is exceptionally difficult to detect. The invention also intends to solve this problem. 
     SUMMARY 
     What can primarily be considered to be characteristic of a device of the type described by way of introduction is that it comprises a light-reducing and light-absorbing arrangement located to the side of the aperture, which arrangement is arranged to reduce considerably or eliminate completely light incident to the side of the aperture, and thereby considerably reduce or eliminate the effects of stray light and reflections. 
     In one embodiment, the light-reducing or light-absorbing arrangement comprises a light-reflecting surface arrangement with a surface coating or surface coatings that give low reflection of the light incident to the side of the aperture. For this purpose, the surface arrangement can comprise mirror surfaces set at an angle to each other that results in the reflection or reflections being obtained in controlled directions. The mirror surfaces in question can be provided with coatings that give a low reflection, the respective reflecting surfaces being arranged so that a reduction is obtained by, for example, 10-2. In one embodiment, two or more mirror surfaces are arranged so that they produce at least two or three reflections of the light incident to the side of the aperture. 
     The device that considerably reduces the stray light or reflections can be designed as a unit that is arranged with an opening extending centrally, the mouth of which acts as an aperture. The unit is arranged with unique characteristics and reference is hereby made to the subsidiary claims below. 
     By means of the device that is proposed above, a technically simple solution is obtained to the problems mentioned by way of introduction. As the invention utilizes known means and means that do not require the optical system as such to be redesigned for the implementation of the invention, economic benefits are also obtained. One and the same system can be provided with “light traps” of various designs and performance and in this way the system can be arranged for a stringent requirement concerning the elimination of stray light and reflections, or requirements that it will work with the system as such. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A currently preferred embodiment of a device that has the significant characteristics of the invention will be described in the following with reference to the attached drawings in which 
         FIG. 1  shows in longitudinal section and in outline an example of an optical system with intermediate image and where a light trap is arranged in association with an aperture incorporated in the system, 
         FIG. 2  shows in vertical section a mirror surface arrangement for dealing with the unwanted beams in question and for eliminating the effects of the same, and 
         FIG. 3  shows an example of the design of the light trap, viewed in perspective and from above. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , an optical system with so-called intermediate image function is shown in outline by  1 . The system can comprise one or more first lenses or optical elements  2  and one or more second lenses or optical elements  3 . The system also comprises a detector surface  4 . In addition, there is a stop function  5  that comprises an aperture  6 . The ordinary and required beam path is represented by solid lines  7 ,  8  and  9 ,  10 . The function and the design of this type of optical system is already well known and will therefore not be described here in greater detail. 
     According to the invention, an arrangement is arranged in association with the said aperture that at least considerably reduces the beam or the stray light that goes from the lens or the lens arrangement  2  to the side of the aperture  6 . An example of such stray light is represented by a broken line  11  in  FIG. 1 . The arrangement for the reduction of the stray light  11  incident to the side of the aperture  6  comprises a unit described in greater detail below. It is characteristic of the unit  12  that it works with a mirror surface or mirror surfaces  13 ,  14  which in the embodiment are coated with a surface coating that considerably reduces the reflected beam. In the embodiment shown, the beam  11  is incident to a mirror surface  14 , which in turn reflects the beam towards a second mirror surface  13 . The said reflected components of the beam are indicated in  FIG. 1  by  11   a  and  11   b . With the reflection by the first mirror surface  14 , there is thus an exceptional reduction of the beam intensity, for example a reduction by 10-2. A corresponding reduction occurs when the beam is reflected by the second mirror surface  13 , which means that the beam  11   b  leaving the unit  12  is considerably reduced in comparison with the incident beam  11 . It is recognized that the mirrors  13  and  14  can be arranged at different and suitable angles of inclination in relation to each other and in relation to the incident beam  11 . In each individual design, it is thus possible to build in a level of reduction simply by the use of mirrors and their surface coatings, and also to obtain a controlled emission of the reflected beam  11   b  out of the unit  12 . The number of surfaces can also vary and, in a preferred embodiment, a mirror arrangement is used that gives 2, 3 or more reflections in the V-shaped channel of the unit. The unit  12  is also provided with a central opening  15 , at one end of which, more specifically the end facing towards the lens  2 , the mouth forms the aperture  6 . 
       FIG. 2  shows the construction of an embodiment with reflective surfaces  13 ′,  14 ′. The device corresponding to the unit  12  according to  FIG. 1  is here divided into a first and a second unit  20 ,  21 . The first unit  20  is in the shape of a pyramid with an opening  15 ′ that goes right through, which opening is the shape of a truncated pyramid, the sides of which form angles λ to the longitudinal axis  17 . The unit  20  comprises four outer surfaces that constitute mirror surfaces,  14 ′. The second unit  21  is designed as an internal truncated pyramid, around the longitudinal axis  17 , where the four inner surfaces  13 ′ are mirror surfaces. In  FIG. 2  only one surface  13 ′ and one surface  14 ′ are shown for the sake of clarity. The outer part of the first unit  20  and the inner part of the second unit  21  meet at the lower part of the device. In this way, a V-shaped channel is produced with reflective edges. The pyramid does not need to be limited to having four sides. Alternatively, the pyramid can be replaced by some other shape or by a cone. The reflective surfaces  14 ′ of the unit  20  form the angle α to the longitudinal axis  17 . α can be of the order of 30°. The reflective surfaces  13 ′ of the unit  21  form the angle β to the longitudinal axis  17 . The angle β can be of the order of 10°.  FIG. 2  shows an alternative incident beam in relation to the embodiment according to  FIG. 1 . In this case, the incident beam is indicated by a broken line  18 . The beam is reflected towards a surface in question  14 ′ and continues according to  18   a , is reflected to a surface in question  13 ′ and continues according to  18   b  and is reflected again towards the surface  14 ′ and continues out of the device according to  18   c . It is recognized that a considerable reduction can be obtained in the incident beam by means of the three reflections, twice on surface  14 ′ and once on surface  13 ′. In the figure, the incoming beam is at an angle γ and the outgoing beam is at an angle δ to the longitudinal axis  17 . The two angles are different and can be varied in the arrangement. 
       FIG. 3  shows the first and the second unit  20  and  21  with the mirror surfaces  14 ′ and  13 ′ respectively. In one embodiment, the embodiment described above as a truncated pyramid is, in principle, interchangeable with cone shapes for the unit, surfaces, openings, etc. It is also recognized that combinations of the two embodiments can be used. It is also recognized that the surface arrangement in question can be arranged in such a way that a practically total local elimination of the stray light or the incoming beam  11 ,  18  is achieved. Respective mirror surfaces can be provided with a coating of a known type, for example an absorbent layer coated with an anti-reflective coating, a so-called “black mirror”. 
     The invention is not limited to the embodiments described above by way of example, but can be modified within the framework of the subsequent claims and concept of the invention.