Patent
1985-03-05
1986-12-16
Henry, Jon W.
350502, 350538, 350557, 350 12, G02B 2304, G02B 2305, G02B 2312
Patent
active
046292950
DESCRIPTION:
BRIEF SUMMARY
A night vision instrument includes 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 emerges. 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 collimator lens (eyepiece) which creates an image of the phosphorous screen at an infinite image distance. This image is often inverted 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. consisting 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 mirror may thus be considerable. Another drawback is the relatively low reflection coefficient of the secondary mirror which causes a reduction of the energy efficiency. Deformations of the secondary mirror due to mechanical or thermal disturbances may moreover reduce the sensitivity because the incident photons are spread over a larger area of the image intensifier photocathode. There are thus good reasons to avoid the secondary mirror altogether.
FIG. 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 photocathode 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 the photocathode of the image intensifier is 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 nightvision 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 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.
FIG. 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 intensifier 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 FIG. 1 reveals a further advantage: The central obstruction in FIG. 2 is set by the external diameter of the image intensifier, about 40 too 55 mm. With an ordinary nightvision instrument, according to FIG. 1, the obstruction (secondary mirror) diameter is in
REFERENCES:
patent: 3173012 (1965-03-01), DeWinter
Scientific American, Amateur Telescope Making Advanced, pp. 516-517, 1946.
Henry Jon W.
Simrad Optronics A/S
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