GB2279154A

GB2279154A - Optical viewing apparatus

Info

Publication number: GB2279154A
Application number: GB8918633A
Authority: GB
Inventors: Hillary Gil Sillitto
Original assignee: GEC Marconi Avionics Holdings Ltd
Current assignee: Leonardo MW Ltd
Priority date: 1989-08-15
Filing date: 1989-08-15
Publication date: 1994-12-21
Anticipated expiration: 2009-08-15
Also published as: GB8918633D0; GB2279154B

Abstract

The apparatus permits an external target to be viewed along a sight-line 11 by a sensing device and for the projection of a beam of laser radiation 12 towards the target. The apparatus includes a first partially- reflecting member 10 partially transparent to radiation at a wavelength to which the sensing device is sensitive and inclined at an angle to the sight-line 11. The member 10 has a viewing area through which the target may be viewed and a laser output area through which the laser beam passes. A second partially- reflecting member 13 receives radiation from the first partially-reflecting member (10). A retro-reflector 14 is provided for receiving laser radiation reflected from the second partially-reflecting member 13 and for deflecting such radiation by way of the second partially-reflecting member 13 onto the viewing area of the first partially- reflecting member 10 for reflection into the field of view of the sensing device on the sight-line 11. This radiation accurately represents the direction in which the transmitted laser radiation is being projected regardless of alignment errors of the laser source or of the optics. Fixed polariser 17 and adjustable polariser 18 act as intensity control means. <IMAGE>

Description

OPTICAL VIETV & APPARATUS This invention relates to optical viewing apparatus of the type used for optical aiming or tracking systems, and in particular to such apparatus which provide a visible aiming mark for a laser beam projected through the apparatus.

In visual or television-type optical aiming and/or tracking devices it is common to use a laser beam projected along the optical axis of the apparatus towards a target. This may be for the purpose of range-finding or target designation, for example. It is necessary that the observer using the apparatus shall be provided with a suitable aiming mark accurately aligned with the laser beam so that the latter may be accurately directed towards the target. If the aiming mark takes the form of an illuminated graticule where the source of illumination is not the laser itself, then there are clearly problems in achieving and maintaining the necessary alignment.

If the laser itself acts as the graticule then the problem becomes one of providing the observer with sufficient illumination without the risk of eye damage. This has been avoided in existing apparatus by the use of expensive dichroic dielectric beamsplitters which need to be contained in a protected environment.

These problems were overcome by the invention claimed in our British patent No. 2178863, which used a viewing window of the apparatus to divert a small part of the laser radiation back to an observer to form an aiming mark. This involved introducing attenuation into the observer's sighting path which, in some instances, could be a disadvantage.

It is in object of the invention to provide optical viewing apparatus which overcomes this and other problems.

According to the present invention there is provided optical viewing apparatus having means for permitting the viewing of an external target along a sight-line by a sensing device and for projecting a beam of laser radiation towards the target, which apparatus includes a first partially-reflecting member partially transparent to radiation at a wavelength to which the sensing device is sensitive arranged at an angle to the sight-line and having a viewing area through which the target may be viewed by the sensing device and a separate laser output area through which the beam of laser radiation may pass, a second partially-reflecting member arranged to receive radiation reflected from the first partially-reflecting member, and a retro-reflector operable to receive laser radiation reflected from the second partially-reflecting member and to deflect said radiation by way of the second partially-reflecting member onto the viewing are of the first partially-reflecting member for reflection into the field of view of the sensing device on the said sight-line to form an aiming mark.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a perspective schematic view of one embodiment of the invention; Figure 2 is a schematic side view of the arrangement of Figure 1; and Figure 3 is a schematic side view of an alternative embodiment of the invention.

Referring now to Figures 1 and 2, a first partially-reflecting member in the form of a beam-splitter 10 is tilted at an angle of about 450 to a sight-line 11 passing through a viewing area of the beam-splitter. The beam-splitter 10 is arranged to reflect a small proportion of the radiation incident upon it, say between 1% and 10%. A beam of visible laser radiation 12 is directed through a laser output area of the beam-splitter along an axis parallel to and laterally displaced from the sight-line 11. The laser radiation reflected from the beam-splitter 10 falls on a second partially-reflecting member in the form of a second beam-splitter 13. This is arranged generally at 900 to the first beam-splitter 10 and again reflects a small proportion of the radiation incident upon it.Radiation from the second beam-splitter 13 enters a corner cube retro-reflector 14 which returns the radiation to the second beam-splitter 13 along a parallel but laterally-displaced path. Further reflection from the beams-splitter 13 and 10 respectively directs the radiation along the sight-line 11 to a sensor, in this case the eye 15 of an observer viewing through one half of a binocular sight 16.

A variable attenuator is introduced between the second beam-splitter 13 and the corner cube 14 in the form of a fixed polariser 17 and a rotatable polariser 18.

One surface of each of the beam-splitters 10 and 13 is coated with an anti-reflection coating so that ony a single reflection takes place.

The effect of the viewing apparatus described above is to produce in the field of view of the observer a bright aiming spot of light which is exactly aligned with the laser beam, since it is produced by that beam and the optical arrangement produces exactly 1800 deviation regardless of small misalignments of any optical element. The intensity of the spot is reduced to an eye-safe level by the losses introduced by the two reflections from each of the beam-splitters 10 and 13.

Further attenuation may be introduced busing an attenuating material for the corner cube reflector 14.

Variable attenuation also needs to be provided to adjust the brightness of the aiming spot as ambient light levels change. This is the function of the fixed polariser 17 and the adjustable polariser 18. The latter may be rotated to give the required brightness of the aiming spot.

The optical components described above may be enclosed in a sealed housing, with a viewing window 19 forming the front window of the housing and arranged for the pair of binoculars 16 to be attached to the rear of the housing 20. Such an arrangement prevents degredation of the optical elements due to dust and other pollutants.

The embodiment described was arranged for direct binocular viewing by an observer. Clearly monocular viewing may be provided with very little change to the apparatus described. The direct viewing by an observer may be replaced by remote viewing by using a sensing device such as a television camera or a thermal imaging device. In such cases the eye-damage problem does not arise, though care will still have to be taken to ensure that the camera or imager is not itself damaged. The wavelength of the laser output will depend upon the sensor being used. For a thermal image the laser output should be in the infra-red region.

Contamination of the outer surface of the viewing window will affect the aiming spot, and this will give an indication that cleaning of the window is necessary.

The beam-splitters 10 and 13 must have optically flat surfaces, though these surfaces do not need to be parallel to one another. The corner cube reflector 14 must be made so as to produce exactly 180 deviation to provide the accurate relationship between the aiming spot and the laser beam.

As has been described above, the arrangement of Figures 1 and 2 involves two reflections of the laser beam from each of the two beam-splitters 10 and 13 before it enters the observers sight-line. The actual attenuation introduced depends upon the reflectivity of each beam splitter, and attenuation factors of the order of 105 to 108 are easily obtained. This reduces the intensity of the laser beam reaching the observer's eye to a perfectly acceptable and eye-safe level. However, if still further attenuation is required a reflector 30, which may have a low reflectivity, may be located between the first beam-spltter 10 and the corner-cube 14 as shown in Figure 3. As shown the third reflector 30 is placed in such a position that the second beam-splitter 13 is arranged substantially parallel to the first beam-splitter 10.

The polarisers 17 and 18 should be of such quality and surface finish as to reduce any uncontrolled transmitted wavefront error or scatter to a minimum. The dimensions of the corner cube reflector 14 have to be such as to provide the necessary lateral offset between the transmitted laser beam 12 and the sight-line 11.

It may be necessary to enable the observer to view an external target using the aiming mark formed on the beam-splitter 10 without transmitting laser radiation towards the target. This may be achieved by locating a movable shutter 21 in the path of the laser beam 12 after the beam has passed through the first beam-splitter 10. The shutter carries an absorbing material. The shutter 21 can be moved away from the beam path to direct laser radiation at the target.

The relative alignments of the various optical elements of the apparatus may be varied.

Claims

What we claim is:

1. Optical viewing apparatus having means for permitting the viewing of an external target along a sight-line by a sensing device and for projecting a beam of laser radiation towards the target, which apparatus includes a first partially-reflecting member partially transparent to radiation at a wavelength to which the sensing device is sensitive arranged at an angle to the sight-line and having a viewing area through which the target may be viewed by the sensing device and a separate laser output area through which the beam of laser radiation may pass, a second partially-reflecting member arranged to receive radiation reflected the first partially-reflecting member, and a retro-reflector operable to laser radiation reflected from the second partially-reflecting member and to deflect said radiation by way of the second partially-reflecting member onto the viewing area of the first partially-reflecting member for reflection into the field of view of the sensing device on the said sight-line to form an aiming mark.

2. Apparatus as claimed in Claim 1 in which the retro-reflector includes a corner cube reflector.

3. Apparatus as claimed in Claim 2 in which the corner cube reflector is made from a material which attenuates the laser radiation passing through it.

4. Apparatus as claimed in any one of Claims 1 to 3 which includes intensity control means for varying the intensity of the laser radiation reflected into the field of view of the sensing device relative to the intensity of illumination of the target as viewed by the sensing device.

5. Apparatus as claimed in Claim 4 in which the intensity control means includes a first polariser located between the second partially-reflecting member and the optical deflection means and a second polariser rotatable relative to the first polariser and located adjacent thereto.

6. Apparatus as claimed in any one of Claims 1 to 5 which includes an optical element of low reflectivity located on the optical path between the first partially-reflecting member and the optical deflection means.

7. Optical viewing apparatus substantially as herein described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. Optical viewing apparatus having means for permitting the viewing of an external target along a sight-line by a sensing device and for projecting a beam of laser radiation towards the target, which apparatus includes a first partially-reflecting member partially transparent to radiation at a wavelength to which the sensing device is sensitive arranged at an angle to the sight-line and having a viewing area through which the target may be viewed by the sensing device and a separate laser output area through which the beam of laser radiation may pass, a secpnd partially-reflecting member arranged to -receive radiation reflected from the first partially-reflecting member, and a retro-reflector operable to receive laser radiation reflected from the second partially-reflecting member and to deflect said radiation by way of the second partially-reflecting member onto the viewing area.of the first partially-reflecting member for reflection into the field of view of the sensing device on the said sight-line to form an aiming mark.