A NIGHT VISION INSTRUMENT WITH ELECTRONIC IMAGE CONVERTER

Abstract: Method of designing a night vision instrument with an electronic image converter, comprising for example an image intensifier tube or a similar device. This night vision instrument (Fig. 3) may be located beside an ordinary telescope, sight, TV-camera or similar device (6) so that this same device may be used during night time. The combined system (Fig. 3) thus arranged is very compact, with the centre of gravity close to the user during handheld operation. The lightgathering objective lens consists moreover of only one mirror (1) and the image emerging on the screen (3) of the electronic image converter is facing the corresponding object being viewed. The image on the screen (3) is finally collimated so that it may pass into the entrance aperture of some other instrument (6) in parallel, and this image transfer is accomplished with three reflections or less and without any intermediate real image.

Description: A NIGHT VISION INSTRUMENT WITH ELECTRONIC IMAGE CONVERTER A night vision instrument consists of a high speed lens which reproduces an object on the photocathode of an image intensifier tube or similar device. The incident photons are in this manner transformed into electrons which are multiplied and focused onto a phosphorous screen or a similar device so that an image of the object is emerging. This image, which is created by electronic means, is viewed with the aid of for example an eyepiece or a microscope- The image is sometimes transferred optically into a telescope through which it can be viewed alternately with the ordinary daylight view. The optical relay system may consist of a colli ator lens (eyepiece) which creates an image of the phosphorous screen at an infinite image distance. This image is often in¬ verted by means of a fiber optical twister or by electronic technique inside the tube. The need of a large relative aperture makes it often suitable to use a specially designed catadioptric mirror objective of the cassegrain type, i.e. con- sisting of a large lightgathering mirror with a hole, and a smaller secondary mirror, usually applied on some correction lens.. The rays from the object will first pass beside the secondary mirror, then reflect twice and finally focus into an image, usually located close to the hole in the primary mirror. The central obstruction caused by the secondary mir¬ ror may thus be considerable. Another drawback is the relatively low reflection coefficient of the secondary mirror which is causing -"BUREA OMPI . WΪPO a reduction of the energy efficiency. Deformations of the secondary mirror due to mechani¬ cal or thermal disturbances may moreover reduce the sensitivity because the incident photons are spread over a larger area of the image intensifier photo- cathode. There are .thus good reasons to avoid the secondary mirror altogether. FIGURE 1 shows a commonly known and used construction drawn in approximately the correct proportions. The primary mirror 1 creates an image which is focused via the secondary mirror 2 on the photo- cathode 3. The image intensifier screen 4 is viewed through an eyepiece, or the optical information may be transferred into the daylight telescope 6 by means of the collimation objective 5. Comfortable handheld operation in this application is apparently inhibited by the fact that the centre of gravity is located too near the front lens. According to the invention is instead the photo- cathode of the image intensifier localized directly on the image plane of the primary mirror. The phosphorous screen is now viewed via suitable prisms and/or mirrors from the same direction as the object. Under, above or along the side of this night¬ vision system is emerging a natural space where for example a daylight telescope may be placed so that the image transfer from the nightvision instrument can be arranged in a straight way via the entrance aperture of the daylight telescope. The daylight telescope and the nightvision instrument fU E--f 0 P1 may consequently be positioned side by side so that they comprise a compact configuration with the centre of gravity far backwards, making it suitable for handheld use. All the drawbacks associated with a secondary mirror have moreover been avoided in this simple manner. FIGURE 2 shows the invention, again with the correct proportions. An image of the object is created by the mirror 1 on the image intensifier photocathode 3 and the screen 4 of the tube is viewed via prisms and/or mirrors and the collimation objective 5 through the daylight telescope 6. There is at the same time a possibility to photograph the image with a camera 13. Correction lenses 7,8 may, or may not, be inserted between the primary mirror 1 and the image intensi¬ fier tube 11 and even the window 9, which seals and protects the internal parts of the night vision instrument may be shaped into a lens. A comparison with Figure 1 reveals a further advan¬ tage: The central obstruction in Figure 2 is set by the external diameter of the image intensifier, about 40 to 55 mm. With an ordinary nightvision instrument, according to Figure l,is instead the obstruction (secondary mirror) diameter set by the geometry of the objective lens and the field of view. An obstruc¬ tion diameter of about 60 mm is for example required with a 100 mm objective focal length. Any increase of the objective focal length and the entrance aper¬ ture will however cause a proportional increase of the secondary mirror. According to the invention (Figure 2) on the other hand, will the image intensifier diameter remain unaltered and this advantage becomes more and more pronounced as the night vision instrument grows bigger. It has to be considered that the correction power of the secondary mirror is wanting when constructing an objective lens according to the invention but this correction may be achieved in other ways. All common- ly used elements for the construction of a mirror objective may otherwise be used, like aspherical and spherical mirrors, so called Mangin mirrors, spheri¬ cal and aspherical correction lenses and symmetrical- and null-lenses. A right angle roof prism 10 in combination with a mirror (or equivalent prism) as depicted in Figure 2 makes a suitable combination when transferring the image from the photocathode screen 4 into the day- light telescope 6. A non inverting image intensifier should be used in this case. If an inverting image intensifier is used, may the image transfer be arranged according to Figure 3, where instead a pentagon prism 14 followed by a mirror (or prism) is used. The mirror/prism in front of the daylight telescope 6 can be made partly transparent so that an ordinary daylight view of the object can be seen through the mirror 12, and a corresponding night scene composed of light which is reflected by the same mirror 12, may also be seen. The use of an ordinary type of image intensifier tube is presumed in the invention. The utility of the invention is however certainly not limited to this specific technique. Other electronic image converters working according to various principles and/or with different wavelengths like for example infrared (IR) rays are equally applicable in the invention. OΛ-'-P! 7. V1P ° -K

Claims: CLAIMS 1. A night vision instrument or similar device charac¬ terized of a catadioptric objective lens which consists of only one single mirror, and an electro¬ nic image converter which amplifies and/or trans¬ forms the frequency of the incident radiation. 2. A night vision instrument according to claim 1, characterized of that the image, which is created by the electronic image converter, is facing the corresponding object being viewed. 3. A night vision instrument according to claim 1 and 2, characterized of that the rays from the object are transferred by means of one single mirror, back into the direction of the object, via possible correction lenses, creating an image in the image plane of this optical system and where this image is converted into corresponding electri¬ cal information, which is processed by electronic technique and thereafter transformed back into an optical image which is finally transferred by means of suitable prisms and/or mirrors and lenses to the image plane of a telescope, camera, TV- camera, sight or similar device. 4. A night vision instrument according to claim 3, characterized of that the image transfer from the electronic image converter to the entrance of a telescope, camera, TV-camera or sight is performed by means of three reflections or less, like a pentagon prism or right angle roof prism or simi¬ lar mirror arrangement, and a flat mirror or corresponding prism arrangement. 1 • A night vision instrument according to claim 3, characterized of that the image transfer from the screen of the electronic image converter to the entrance aperture of the telescope, camera, TV- camera or sight, is performed without any inter¬ mediate real image. O PI

Inventors: VOGL GEORG, VOGL, GEORG
Applicants: VOGL GEORG, VOGL, GEORG
Application year: 1984
Main classification: G02B23
IPC codes: G02B23