GB2236867A

GB2236867A - Photochromic filter in sighting telescope with laser

Info

Publication number: GB2236867A
Application number: GB9013537A
Authority: GB
Inventors: Michael Andrew Hamill
Original assignee: Wallop Defence Systems Ltd
Current assignee: EST Defence Company UK Ltd
Priority date: 1989-06-16
Filing date: 1990-06-18
Publication date: 1991-04-17
Also published as: GB9013537D0

Abstract

A sighting telescope receives light from a target so as to provide a target image in the eyepiece E. A CRT display image 12 may be internally generated and combined via beam splitter 5, CRT prism 13, lenses 14 and mirror 15 with the target image. A laser 11 directs light out through the objective part 2 which includes a photochromic filter 1. The light path extends from the front end F to B and continues from C to E. <IMAGE>

Description

SIGHTING TELESCOPE The invention relates to a sighting telescope incorporating a laser rangefinder for both sighting and defining accurate range to a given target. Such telescopes have application in surveying equipment, gun sights etc.

Such telescopes may incorporate an internally generated display for giving numerical and/or other information. Such a display may be derived from light emitting diodes (LED's), liquid crystal displays (LCD's) or cathode ray tubes (CRT's). Difficulty is often found in observing such displays when the intensity of light entering the telescope is at a high level ( 1000 Lur). Hitherto, attempts to provide filters to alleviate this problem have met with the difficulty that the output of the laser through the objective part of the telescope is unduly interfered with by the filter. A particular problem with many telescopes is a space constraint within the optical head of the instrument which precludes the use of a switchable broad band filter. The present invention provides a solution.

According to the invention there is provided a sighting telescope having a light path with an objective part and an eye-piece whereby a target may be viewed; a laser for generating a laser light beam; a beam injection arrangement whereby the laser beam is directed out through the objective part towards a target while the target is being viewed; and a filter assembly in the objective part which includes photochromic material having the characteristics of any of the range of the Reactolite Rapide materials.

Preferably there is provided a device for generating a display to be viewed in conjunction with the image of the target and there is an image combining arrangement for combining the display image and the target image.

Reactolite Rapide materials are alumino phosphate based materials, the crystal structure being very much smaller than found in silver oxide doped glass. It is thought that this accounts for the phenomenon utilised by the invention. "Reactolite Rapide" is a registered Trade Mark of Pilkington Brothers plc and the materials are made by that company.

Preferably the filter assembly comprises a crown glass window to which is cemented a Reactolite Rapide glass filter. It is found that the refractive index of BK7 crown glass matches that of the filter particularly well and this is the preferred glass.

The preferred laser used in conjunction with the invention is an Nd:YAG type.

It is preferable that the filter assembly should form part of the front element of the telescope: it could be incorporated at any suitable place in the combined optical and laser light path before the injection point of the CRT image.

Furthermore, it is possible that a lens or a beam splitter of the telescope may be made of Reactolite Rapide glass, thereby providing the filter. Spectral response would, however, degrade the further back the component is in the optical train.

The invention will further be described with reference to the accompanying drawings, of which: Figure 1 is a schematic diagram of a telescope incorporating the invention; Figure 2 is a side elevation of the filter assembly described in Figure 1; Figure 3 is a diagram illustrating a filter test procedure; and Figure 4 is a graph showing test results.

Referring to Figure 1 there is illustrated a sighting telescope schematic optical layout for a gun which has a light path LP extending from the front (objective) end F of the telescope to the eye-piece E.

For ease of drawing, the light path has been broken in the drawing but it is to be understood that the light path continues from B to C.

The light path includes a front filter assembly 1; an adjustable objective lens cell 2; a beam-combining mirror 3 which allows beam injection in a manner to be described; an illuminated graticule 4; a beam splitter 5; a field lens 6; a four mirror optical hinge 7; a relay lens 8; a narrow field of view lens 9; a wide field of view lens 10; a laser 11; and the eye-piece E.

Mirror 3 is of the beam-combining type and allows the incoming light from a target to be directed to the eye-piece while allowing the Nd:YAG Laser 11 to emit a beam through the objective part of the light path which consists of the mirror 3, the objective lens cell 2 and the filter assembly 1.

An internally generated display produces, on a cathode-ray tube 12, aiming mark and graphical information relative to range and ammunition selected.

This display is visible in the eye-piece via the beam-splitter 5; a CRT prism 13, lenses 14 and a mirror 15.

When there is high incident illumination it would be difficult to see the CRT display. It is because of this that the invention provides the filter assembly 1 which is photochromic. Figure 2 illustrates this assembly which comprises a plane window 16 of BK7 crown glass to which is cemented a Reactolite Rapide glass filter 17. This is a photochromic glass material of rapid response characteristics. Tests have shown that unlike other photochromic materials, particularly those incorporating silver oxide crystals, this material does not unduly affect the transmission of the laser beam.

Figure 3 shows a test arrangement in which a Reactolite Rapide lens 20 (in this instance) was situated at the output of an Nd:YAG Laser- 21. An energy monitor 22 measured the beam intensity passing through the lens and displayed the result on display 23. An ultra-violet light source 24 was provided.

In the test the lens was illuminated for 5 minutes by the ultra-violet light source 24 to saturate it to its most obscure. The source 24 was removed and the lens allowed to return to its least obscure level under low light conditions. During this time laser light through the lens was measured. The results are shown graphically in Figure 4. It will be seen that impedance to laser light is constant throughout the photochromic range at about 9%.

Claims

1. A sighting telescope having a light path with an objective part and an eye-piece whereby a target may be viewed; a laser for generating a laser light beam; a beam injection arrangement whereby the laser beam is directed out through the objective part towards a target while the target is being viewed; and a filter assembly in the objective part which includes photochromic material having the characteristics of any of the range of the Reactolite Rapide materials.

2. A sighting telescope as claimed in claim 1 wherein there is provided a device for generating a display to be viewed in conjunction with the image of the target and there is an image combining arrangement for combining the display image and the target image.

3. A sighting telescope as claimed in either of the preceding claims wherein the filter assembly comprises a crown glass window to which is cemented a Reactolite Rapide glass filter.

4. A sighting telescope as claimed in claim 3 wherein the crown glass is BK7 crown glass.

5. A sighting telescope as claimed in any of the preceding claims wherein the laser is of the Nd:YAG type.

6. A sighting telescope as claimed in any of the preceding claims wherein the filter assembly is part of the front element of the telescope.