GB2199154A - Stereoscopic sight - Google Patents

Abstract

A stereoscopic sight with respective viewing channels for the observer's two eyes has an objective lens (1), an optional image intensifier (2), and a collimating lens (3) in each channel. A prism block (6) with partial reflectors (7) and total reflectors (8) is disposed to spread the light in each channel in a direction towards the other channel. This increases the range of viewing positions and thereby permits a greater degree of observer head movement and/or interocular separation variation. The apparatus may be of periscopic form and may include means to converge the lines of sight from the observer's two eyes. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO OPTICAL APPARATUS This invention concerns improvements in or relating to optical apparatus, and relates more particularly to optical apparatus for low light level or night vision systems such as are used, for example, in night driving sights for armoured vehicles.

It t is known to provide a low light level or night vision system comprising an objective lens which focusses light from a source or object under view on to an image intensifier tube, and a magnifier or eye-piece lens through which an observer views the intensified image on the tube. It is further known to provided a two-channel version of such a system having a separate objective lens, image intensifier, and magnifier or eye-piece for each eye. An advantage of having such separate viewing channels for the two eyes is that a stereoscopic effect can be ac hi ev.ed.

This results from the images on the two image.

intensifier tubes repr.esentins the scene or object as viewed from two respective viewing positions which are spaced apart. The observer, by viewing one image with each eye, therefore receives the impression of a combined image with a three-dimensio na 1 or stereoscopic effect.

Such systems usually provide a rather limited exit pupil within which the eye can view the intensified image, and a rather limited eye-relief. This means the observer's eyes have to be kept near and in a very restricted positional relationship to the optics if a satisfactory view of the image is to be retained. In other words, the observer's permis,sible head movements are very l,imited.

Further, since the viewing positions for the two eyes are restricted, a particular apparatus may not be satisfactorily usable by different observers whose respective interocular spacings vary, unless some means of adjusting the spacing between the two viewing channels is provided. Such adjustment means adds to the complexity and cost of the system.

Broadly according to the present invention there is provided optical apparatus having a two channel viewing system such that an observer can view with each eye along a respective channel from a respective viewing position, the apparatus comprising in each channel means for providing a visual image, collimating means for collimating light from the visual image, and.a partial reflector and a further reflector arranged so that collimated light from the visual image is partially reflected by the partial reflector in a direction towards the other channel and is then reflected by the further reflector in a direction towards the viewing position to travel in parallel with collimated light from the visual image transmitted through the partial reflector towards the viewing position.With such an arrangement collimated light from the visual image in each channel is effectively spread in a direction towards the other channel so as to increase the range of the viewing positions and thereby permit a greater degree of head movement by the observer, and/or a greater variation in the interocular separation. Preferably said further reflectors.meet centrally of the two channels so that the respective viewing positions effectively abut one another. Said further reflectors are preferably total reflectors and said partial reflectors are preferably semi-reflectors.

Said partial and further reflectors are conveniently, but not necessarily, disposed at 45 degrees to the optical axis of the respective viewing channel, and the optical axes of the two channel 5 are preferably parallel.

Said partial and further reflectors are preferably provided at interfaces of a prism block, which preferably has light entry and exit faces orthogonal to the channel optical axes. The block may comprise end prisms of triangular cross-section which abut respective prisms of parallelogram cross-section with the partial reflectors provided at the interfaces between the triangular and parallelogram prisms, and with the further reflectors provided at the respective faces of the parallelogram prisms opposite those interfaces. Preferably there is provided a central triangular prism which abuts the further reflector faces of the parallelogram prisms, and conveniently the overall prism block is of rectangular parallelepiped form.

Means may be provided to' converge the lines of sight from the observer's two eyes and such means preferably c omp r is-es a n eb a t i v e lens extending across both channels, for example a plano-convex lens element having its planar face cemented to the output face of the prism block.

The invention is particularly applicable to low light level or night vision systems and the apparatus may therefore include in each channel image intenifier - means for producing an intensified image (which constitutes said visual image*) of a scene or object under view and objective lens means for focussing light from -the scene or object onto the image intensifier means.

The apparatus may in particular be used in a stereoscopic night driving sight, and may be of periscopic form.

Apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying- drawings, in which:- Figure 1 is a schematic representation of a low light level or night vision system, Figure 2 is a schematic side elevational view of a night driving sight using the system of Figure 1, and Figure s' 3 schematically show modifications to and 4 the system of Figure 1.

The sight shown in Figure 1 has two viewing channels with parallel respective optical axes 0, each channel comprising an objective lens or lens system 1, an image intensifier tube 2, and a collimating magnifier lens or lens system 3. The objective lens 1 focusses light from the scene or object under view onto the photocathode 4 of the image intensifier which produces an image of the scene or object of intensified brightness- on its phosphor 5.

Light emitted from this phosphor 5 is collimated by the collimator 3. The provision of two such channels, one for each eye, enables a stereoscopic effect to be ac h.i e ve d in the combined image from the two channels viewed by the two eyes E.

A prism block 6 is interposed between the viewing positions and the magnifying collimator lenses 3 so that the light tra.vels to the eyes E through this prism block. The prism block 6 has internal planar partially reflecting/partially transmitting interfaces 7 and totally reflecting interfaces 8 arranged so that in each channel collimated light from t-he lezs 3 is received by an interface 7 and is partially reflected thereby to a. parallel interface 8 which totally reflects it. Such light emerges from the prism block in parallel with light from the lens 3 that is transmitted through the partially reflecting interface 7.

Light from the collimating lens 3 enters the prism block 6 through a planar entry face 9 dispos-ed orthogonally to the optical axis 0, the interfaces 7 and 8 being disposed at 45 degrees -thereto. Light transmitted through the interface 7 emerges from the prism block 6 through a planar exit face 10 of equal size and parallel to the entry face 9, and light reflected by the interfaces 7 and 8 emerges through an exit face 11 of equal size and coplanar with the exit face 10.

The effect of the arrangement is that collimated light from the. visual phosphor 5 image in each channel is spread in -a direction towards the other channel. Particularly, in each channel light from the phosphor 5 visual image collimated by-the collimator 3 is partially reflected by the partial pre the ar7 7 in a direction towards the other channel and is then reflected by the further, total, reflector 8 in a direction towards the viewing position to travel in parallel with collimated light from the phosphor 5 visual image transmitted through the partial reflector 7 towards the viewing position. The image is therefore visible across the full area of the exit faces 10 and 11, i.e. over double the area of the entry face 9.The prism block 6 is symmetrical about a plane midway between and parallel ta the optical axes of the two viewing channels and the total reflectors 8 meet at their ends nearest the viewing positions at that central plane. Thus the exit face 11 of one channel is contiguous with the exit face 11 of the other channel and the two respective viewing positions for the two eyes effectively abut one another. Such effective visual 'filling in' of the space between the two channels is advantageous-in that head movement to the left tends to move the observer's right eye into this space and head movement to the right tends to move the left eye into it.It will be understood, of course, that if an observer's eye should move past the middle point P in Figure 1, then an image jump will-be experienced, particularly for close objects. However, the spacing between the channel axes and the apparatus components can be dimensioned so that, with head movement to the lef.t, the left eye retains a view through the respective exit face 10 for so long as the right eye can view through the respective exit face 11, and vice versa with head movement to the right.

The arrangement t-herefore enables th.e observer to retain a stereoscopic view whilst permitting a larger movement of 'the eyes relative to the optical system. It can also accommodate observers. having different respective interocular spacings without the need for adjustment.

The prism block 6 is constructed in a manner basically known per se from respective prism components of suitable transparent material, which is preferably glass but could be optical plastics. The partially and totally reflecting interfaces 7 and 8 are provided by applying suitable. coatings with the required reflection characteristics to appropriate faces of the prism components before they are cemented together to form the block. Conveniently the interfaces 7 are uniformly semi-reflecting so that half the Incident h; btS9; and half is reflect-ed to the totally reflecting interface 8, whereby the brightness or light intensity is substantially the same through each of the exit faces 10 and-ll.It will be seen in Figure 1 that for each channel the end of the partially reflecting interface 7 nearest the viewing position is aligned, as considered in a direction parallel to the optical axis 0,- with the end of the totally reflecting interface 8 nearest 'the collimator 3 so. there is no gap apparent between these interfaces when viewing in a direction parallel to the optical axis 0. Thus light rays parallel to the axis cannot pass directly from the entry face 9 to the exit face 11 without striking the partially reflecting interface 7.

As can be seen from Figure 1, the prism components which are assembled together to form the block 6 are, in cross-section, two 450, 900) 450 triangular end prisms 12, two 450, 1350 parallelogram prisms 13, and a central 450, 900, 450 triangular prism 14. The partial reflectors 7 are provided at the respective inte-rfaces between the triangular end prisms 12 and the parallelogram prisms 13, i.e. between the hypotenuse face of the e end prism 12 and the abutting long face of the parallelogram prism 13, and the further, total, reflectors 8 are provided at the respective long faces of the parallelogram prisms 13 opposite those interfaces, that is to say at the interfaces between th parallelogram prisms 13 and the central triangular prism 14 i.e. between the non-hyp tenu se faces of the central prism 14 and the abutting long faces of the paralle.logram prisms 13. The central triangular prism 14 does not in fact -perform any optical function in that no wanted light passes through it, and it could therefore be omitted. Preferably, however, it is provided to give mechanical strength and rigidity to the assembled prism block and to complete its conveniently rectangular cross-section and parallelepiped form. The exterior face 15 of the central triangular prism component 14, and the end faces 16 of the block, are not required to transmit light and are therefore preferably treated, e.g. ground and blackened, to prevent the passage of unwanted stray light, as are the end faces of the block parallel to the plane of Figure 1.

For ease of illustration and explanation Figure 1 shows the apparatus in 'straight-through' form with the optical axes 0 entirely in the plane of the drawing. In practice there may be bends or folds in the light path.

Figure 2 schematically shows a stereoscopic night driving sight of periscopic form. This has a -reflector 17 in or associated with each objec-tive lens or lens system 1 so as to effect a- 90 degree bend in the optical axis.

Incident light travelling generally horizontally is thereby caused to travel vertically and is focussed onto the ph otoc ath ode 4 of a vertically disposed image intensifer tube 2. There is also a reflector 18 in or associated with each collimating lens or lens system 3 to effect a 90 degree bend in the optical axis. Light from the phosphor -5 of the tube 2 which initially travels generally vertically is thereby caused to travel generally horizontally into the prism block 6 into w-hich the observer views generally horizontally.

As wilF be appreciated by those skilled in the art, the objective lenses or lens systems 1 (with associated reflectors 17 if any), the image intensifier tubes 2, and the collimating lens or lens systems 3 (with associated reflectors 18 if any) may take any suitable form, being components well known per se of which further description is unnecessary.

It will further be appreciated that the particular form of prism block 6 shown and described is given by way of illustration and example and other forms could be used, or the partial and further reflectors otherwise provided (e.g. by partially reflecting pellicles and simple mirrors). However, the form of prism block shown and describe-d is particularly preferred having a c-onven i ent rectangular parallelepiped shape with planar entry and exit faces orthogonal to the optical axes and planar interfaces disposed at 45 degrees to the optical axes. Nevertheless, other shapes could be devised by those skilled in the art and some departure from planarity of the faces and interfaces, and di f f.e rent angles may be acceptable.In particular, it will be appreciated that the interfaces 7 and 8 need not necessarily be at 450 to the optical axes. Also, although the further reflectors 8 are preferably total reflectors in order to maximise light ulilisation, less than 100% efficiency of reflection may be tolerable.

There could possibly be more than one partial reflector in each channel so that light reflected from the first partial reflector 7 is partially reflected and partially transmitted by another partial reflector located between and parallel to the first partial reflector 7 and the further reflector 8. However, plural partial reflectors in each channel is not generally desirable because of possible parallelism problems.

If desired, the spacing between the interfaces 7 and 8 may be such that there is some overlap of these- interfaces as considered in a direction parallel to the optical axis 0 so as to reduce or avoid 'direct view' by way of light rays angled to the axis 0. The extent of such overlap can be related to the angular field of view provided by the collimator 3 so as substantially to prevent or restrict the direct passage of light rays from the entry face 9 to the exit face 11. The purpose is to reduce or minimise brightness variations across the viewed picture and the prisms are in practice optimised to give the most even (in terms of brightness) picture to each eye.

This may involve varying the reflection/transmission characteristics over different parts of the interfaces 7, and the partially reflecting coatings may in particular be zoned or graded to improve the brightness uniformity.

Desirably there is associated with the prism block 6 means effective to converge the lines of sight from the observer'-s respective eyes-, so - as to acc-ommodate the natural - tendency of the eye muscles to cause convergence when viewing simultaneously with both eyes. Such means may comprise a respective prism 18 for each channel cemented to the exit faces 10 and 11 as shown in Figure 3, or may alternatively, and preferably, comprise a negative lens .19 of plano-concave form which extends across both channels, i.e. across the entire prism block, with its planar face cemented to the block output face which comprises the exit-faces 10 and 11 Fas shown in Fig-ure 4. Such a lens can, as well as converging the lines of sight, locate the viewed images at a convenient distance from the observer. This could alternatively be achieved by cementing separate plano-concave lenses to the output faces of the . respective prisms 18 in the Figure 3 arrangement, but this is more complicated than the single lens 19 arrangement of Figure 4.

It will.be understood that the night vision system or night sight shown and described could, if required, be augmented with a day vision system or day sight by providing a day light path to the observer's viewing position in a manner essentially known per se, for example involving beam splitter cubes- effective to combine the day and night paths.

Claims (18)

ctAIMS

1. Optical apparatus having a two channel viewing system such that an observer can view with each eye along a respective channel from a respective viewing position, the apparatus comprising in each channel means for providing a visual image, collimating means for collimating light from the visual image, and a partial reflector and a further reflector arranged so that collimated light from the visual image is partially reflected by the partial reflector in a direction towards the other channel and is then reflected by the further reflector in a direction towards the viewing position to travel in parallel with collimated light from the visual image transmitted through the partial reflector towards the viewing position.

2. Apparatus according to Claim 1 in which said further reflectors meet centrally of the two channels so that the respective viewing positions effectively abut one another.

3. Apparatus according to Claim 1 or Claim 2 in which said further reflectors are total reflectors.

4. Apparatus according to any preceding claim in which said partial reflectors are semi-reflectors.

5. Apparatus according to any preceding claim in which said partial and further reflectors are disposed at 45 degrees to the optical axis of the respective viewing channel.

6. Apparatus according to any preceding claim in which the optical axes of the two viewing channels are parallel.

7. Apparatus according to any preceding claim in which said partial and further reflectors are provided at interfaces of a prism block

8. Apparatus according to Claim 7 in which the prism block has light entry and exit faces orthogonal to the channel optical axes.

9. Apparatus according to Claim 7 or Claim 8 in which the prism block comprises end prisms of triangular cross-section which abut respective prisms of parallelogram cross-section with the partial reflectors provided at the interfaces between the triangular and parallelogram prisms, and with the further reflectors provided at the respective faces of the parallelogram prisms opposite those interfaces.

10. Apparatus according to Claim 9 in which there is a central triangular prism which abuts the further reflector faces of the parallelogram prisms.

11. Apparatus according to any of Claims 7 to 10 in which the overall prism block is of rectangular parallelepiped form.

12. Apparatus according to any preceding claim having means to converge the lines of sight from the observer's two eyes.

13. Apparatus according to Claim 12 in which said means to converge the lines of sight comprises a negative lens extending across both channels.

14. Apparatus according to Claim 13 and any of Claims 7 to 11 in which said negative lens is a plano-concave lens element having its planar face cemented to the output face of the prism block.

15. Apparatus according to any preceding claim including in each channel image intensifier means for producing an intensified image (which constitutes said visual image) of a scene or object under view and objective lens means for focussing light from the scene or object onto the image intensifier means.

16. Apparatus according to any preceding claim of periscopic form.

17. A stereoscopic night driving sight having apparatus according to any preceding claim.

18. Optical apparatus substantially as described herein with reference to any of the figures of the accompanying drawings.