GB2292227A

GB2292227A - Binocular magnifier having concave mirror

Info

Publication number: GB2292227A
Application number: GB9416118A
Authority: GB
Inventors: Ian Marshall
Original assignee: VIRTUALITY
Current assignee: VIRTUALITY
Priority date: 1994-08-10
Filing date: 1994-08-10
Publication date: 1996-02-14
Also published as: GB9416118D0

Abstract

An optical system for magnifying a single object 1 and projecting two images at infinity conjugate into each eye of an observer, which forms two intermediate aerial images 4 (only one shown) near to two eyepieces 5 - 7. The eyepieces collimate the aerial image and project it into the observer's eyes. The aerial image is projected by a concave mirror 3, is divided into the two optical paths by two beamsplitters 2, 8 angled at 90 DEG , and reflected into the eyepieces comprising one or more lenses 6 by two plane mirrors 10. The object may be a liquid crystal display or cathode ray tube. <IMAGE>

Description

Binocular Magnifier.

This invention relates to the need to magnify an image from a single small object and project it into both eyes of an observer. It can be applied in a wide range of industries where such objects need to be seen comfortably at high magnification, and where the images must be projected at a near infinity conjugate. The invention offers advantages over conventional optical designs by providing a high magnification of a small object, compact design and wide field of view, whilst using low cost components.

Various kinds of optical systems are known for magnifying objects such that they can be seen comfortably by an observer. Common examples that project into one eye only are video camera viewfinders that magnify images on Cathode Ray Tubes or Liquid Crystal Displays. These are usually called monocular devices. The eyepieces in binoculars magnify aerial images formed internally in the binoculars by other lenses to project them into two eyes.

This invention relates to the binocular magnification of single objects that are small compared to the interpupillary separation ofthe eye, being typically less than 1 inch across. It is also related to the requirement that the projected field of view is greater than 30". This requires that the optical system has a focal length of less than 5Omm, though more typically 20mm. Therefore, a conventional 'bi-ocular' magnifier, which incorporates a single lens assembly through which each eye looks along a separated optical path, is not feasible. This is because the separation of the eyes is too large in comparison to the maximum viable focal length. The image must therefore be divided by some beamsplitting device and separately projected into two halves ofthe optical system.

It is intended that the invention is useful in supplying products to the consumer market, where it may be manufactured in large quantities but at very low cost.

Hence, achieving a design that is cheap to make is of particular importance in determining the viability ofthe optical design.

Another important requirement is that the misalignments between the two projected images, and their respective aberrations, must be sufficiently small to allow an observer to use the device for long periods of time without straining his vision. This requires that all the aberrations that can cause such problems are properly compensated in the design, that the device is fabricated to a high quality, and that any adjustment to compensate the observer's vision defects or match the device his interpupillary separation of his eyes must not change the relative magnification and alignment ofthe two images.

It is also important that any such magnifier is simple to operate and has a simple mechanical structure. Therefore, it is desirable that the magnifier does not require adjustment either to match the observer's interpupillary separation, or to adjust focus.

It is therefore an advantage ofthe invention that it can be designed to project images over sufficiently large exit pupils to remove any need for the said interpupillary adjustment, and with sufficient eye relief, that is the distance from the last optical surface to the user's eye, such that he can wear spectacles. This would remove the need for any adjustment in normal use. The invention therefore usefully improves on the state-of-the-art by offering a cheap optical system that can be used without adjustment.

The conventional way ofdesigning an optical system to meet this requirement would be to incorporate a lens that projects the image through a beamsplitter to form an aerial intermediate image near two eyepieces, which then collimate the images and project them into the observer's eyes. The aerial image can usefully be magnified to increase the field of view seen through the eyepieces. However, in a compact form, the distance from the object to the relay lens and from the relay lens to the aerial image must be small, and hence the ray angles traversing the relay lens must vary by large amounts. This makes the design of the relay lens more complex, as its construction must compensate aberrations over a wider field of view. There is a resultant trade-off between cost and size, such that a conventional design is either too expensive or too large.This invention provides a lower cost design in a smaller space than the aforesaid conventional approach.

Another conventional technique uses concave mirrors in the eyepiece that reflect and collimate the images that are projected onto them by semi-reflecting beamsplitters. As with conventional eyepieces, a magnified aerial image of the object is formed near to the eyepieces by a relay lens. In a typical system, this image intersects a semi-reflecting beam splitter that reflects it at 900 to its previous direction to the concave mirror, which in turn reflects and collimates the image. The reflected image then passes back through the said semi-reflecting beamsplitter to be seen by the observer.

In this case, the concave mirror introduces a large amount of the aberration called Petzval Curvature, but of an opposite sign to that usually introduced by the relay lens. There is therefore a useful degree of aberration compensation between the relay lens and concave mirror, such that the relay lens design is simplified. Other conventional forms are known that incorporate more than one semi-reflecting beamsplitter. However, for a simple system that does not incorporate selective polarising optics, each time the image intersects the semi-reflecting beam splitter, 50% ofthe light is lost. Because the relay lens must also incorporate a beamsplitter to divide the light paths, the light transmitted to the eye is less than 12.5% ofthe original image brightness. This conventional design therefore suffers from low brightness.Furthermore, even though the relay lens design in such a system is simplified by the aberration compensation between the concave mirror and the relay lens, it is still more complex than that incorporated in this invention.

This simpler relay optic design is achieved in this invention by integrating a beamsplitter and concave mirror into a sub-assembly that both performs the function of a relay lens and divides the image into two optical paths.

According to the present invention, the light from an object that is projected into one oftwo eyes traverses a semi-reflecting beamsplitter (any reflected light is unwanted and is blocked) and impinges on a spherical or aspherical concave mirror that reflects and focuses an image ofthe object. The use of a concave mirror presents the cheapest way of forming an aerial intermediate image that is substantially free of chromatic aberration, and is substantially different from the aforesaid conventional use of concave mirrors in eyepieces because the concave mirror receives light from a close conjugate and refocuses it to a second close conjugate. The aberrations introduced by such a mirror are therefore qualitatively and quantitatively different than when used conventionally to collimate the image. The primary aberrations called spherical aberration, coma, distortion and all forms of chromatic aberration are minimised, though the aerial image is affected by large amounts of astigmatism and Petzval curvature. This is an advantage as they are of opposite sign to such forms of aberrations introduced by a simple eyepiece that comprises of one or more lenses, and hence help in providing overall aberration compensation with minimum cost.

The light reflected from the concave mirror impinges again on the semi-reflecting beamsplitter, where part of it is reflected at near 900 to its original path. The light that is not reflected is unwanted. The reflected image is projected via a second plane mirror into the eyepiece, which is formed of one or more lenses that collimate and project the image into the eye.

The invention incorporates two semi-reflecting beamsplitters, that are angled at approximately 900 and vertically translated with respect to each other. Light projected into one eye reflects off the top beam splitter according to the above description, and light projected into the other eye reflects off the bottom beamsplitter. Both optical paths have the same optical design. The vertically offset beam splitters have the advantage ofminimising the number oftimes that the images are subject to semi-reflection or semi-transmission, so that the final image brightness is improved. All the other optical components are usually kept symmetrical to a common optical axis, though this is not essential.The beamsplitters therefore project the top half of the viable light paths into one eyepiece, and the bottom halfinto the other, and vignette the beam accordingly.

Each eyepiece projects the image of the object over an exit pupil, that is itself a real image ofthe concave mirror and the beamsplitters. Therefore each exit pupil, which determines the size ofthe projected beam at each eye is halved in size by the lateral offset of the beamsplitters. The top half of the exit pupil is projected into one eye, and the bottom half into the other. The axis connecting the centres of the two observer's eyes is kept horizontal by rotating the whole binocular magnifier to compensate the vertical difference, and rotating the object in the opposite direction to keep the displayed image aligned to the horizontal axis.

For the purpose of easing the calculation ofray paths through the invention, and for defming the position of optical components, the optical axis is defined as a ray that leaves the exit pupil in a direction that is normal to said exit pupil, and is traced in the reverse direction to the projection of light backwards through the magnifier by normal refraction and reflection. Two such optical axes therefore exist. These ray paths are a mathematical model rather than actually occurring in practice, as the apertures ofthe beam splitters vignette and obstruct them. Because the beam splitters are laterally offset from the optical axes, the shape ofthe projected exit pupil will not be symmetrical about the optical axes.Each optical axis is mathematically calculated to define a reference axis that defines the position and orientation of the optical components.

When the semi-reflecting beamsplitters transmit light, they act as components that are not rotationally symmetrical about the optical axis. For this reason, they introduce non-rotationally symmetrical aberrations. In a typical design, these types of aberrations will be kept small by incorporating relatively thin beamsplitters with low refractive indices. Typically, crown glass beam splitters of less than 3 millimetres thickness are adequate. However, the non-rotational symmetry also deviates the transmitted light paths sideways. Light projected from the centre ofthe object through beamsplitters that are aligned horizontally will be deviated horizontally, but light entering the left hand eyepiece will be deviated in the opposite direction to light entering the right hand eyepiece.The result is that the light paths projected into each eye from the centre ofthe object will be misaligned to each other, unless this error is compensated. One compensation method is to rotate each eyepiece and plane mirror about the intersection point of the optical axis with the said plane mirror, such that the field of view projected into each eye is deviated by an equal and opposite angle to that projected into the opposite eye. This creates an angular difference between the two images that compensates for the angular error introduced by the beamsplitters.

An alternative compensation is achieved by rotating the beamsplitters slightly away from their nominal positions. For example, the optical axis may be nominally reflected horizontally at 900 to its previous direction. If each beamsplitter is rotated about a vertical axis that intersects with the optical axis, by about 0.50 to 1.00, then the optical axis will now be reflected in a slightly different horizontal angle that differs in direction as it is projected through each eyepiece. The said difference is typically enough to compensate for the deviation of the light paths introduced on transmission through the beamsplitters.

In this invention, the orientation of one axis ofthe image is reversed because ofthe combined effect of the semi-reflecting beamsplitters and plane mirrors. In addition, the whole image is inverted by the action of forming an intermediate image. This can be compensated by adjusting the image displayed on the object. For example, if the object is a Liquid Crystal Display, then it may be turned the other way around from normal, so that it's back now faces to the front, to reverse the image in one axis only. The orientation of the whole image can be reversed by rotating the Liquid Crystal Display by 1 80 about the optical axis. Likewise, a Cathode Ray Tube is an alternative object, that may have one of its internal connections reversed to reverse one axis of the image, and may be rotated about the optical axis.One axis of the image may also be reversed by optical means by placing an additional plane mirror in the optical path adjacent to the object, or by placing an additional plane mirror in each optical path between the eyepieces and the beamsplitters.

The light transmission of the invention is normally limited to below 25% of the object brightness as the light has to transmit through and reflect off the same beamsplitter in each path. In order to maximise the transmission, both the reflection and transmission of each beamsplitter must be as close to 50% as possible.

However, a useful class of object, namely liquid crystal displays, emit linearly polarised light, which can be used to increase the light transmission ofthe invention.

This improvement is implemented by incorporating beamsplitters into the configuration described above in which the beamsplitters are tilted to reflect the image horizontally, and have a polarising coating, which reflects linearly light polarised with its electric field aligned vertically more than light with its electric field aligned horizontally. The beamsplitter's transmission is likewise greater for light with its electric field aligned horizontally. The polarisers within the liquid crystal display must be orientated to project linearly polarised light with its electric field aligned horizontally, so that the transmission of the image through the beam splitters to the concave mirror is maximised. A quarter wave plate is placed adjacent to the concave mirror with its polarisation axis aligned at 450 to the horizontal. Linearly polarised electric fields aligned in either the vertical or horizontal direction are retarded in phase by the action of the quarter wave plate, by one quarter of a wavelength of light relative to electric fields aligned in the orthognal direction, or by an odd number of quarter wavelengths.

On reflection from the concave mirror the light passes a second time through the quarter wave plate whose additional effect is to double the retardation to one half a wave or an odd multiple ofhalfwaves. The result is that. on intersecting the beamsplitter for the second time, the direction in which the light is linearly polarised has been rotated by 90 , and is now orientated to maximise the reflection off the beam splitter coating. The said beamsplitter coating would effectively increase the light transmission even if were of the simplest form, that is to say a single layer of dielectric material having a high refractive index, such as an oxide of Titanium.

More efficient coatings can be made by typically evaporating five layers of alternate Titanium and Silicon oxides.

It is not essential that the concave mirror is limited to only one component. The simplest design is a spherical surface that reflects the light off the face of the mirror that is closest to the object, that is to say the mirror only has one optical surface. The aberration compensation may be improved by making the said surface aspheric, or by adding more optical surfaces. The mirror could have a front refracting surface nearest to the object that transmits the light through to the rear surface, that reflects the light back through the front surface. One or more lenses could also be placed between the mirror and the object. Despite this additional improvement, the simplest form has an advantage over conventional lens designs in that it reflects and refocuses the image with good aberration compensation from only one optical surface, and that optical surface has a simple form.

Manufacturing costs can be kept low by moulding the concave mirror and eyepiece lenses out of a suitable plastic material, such as Acrylic. The beam splitters and plane mirrors can be cut from glass made from the float process, which produces surfaces of sufficient quality to be used without further optical processing. The plane mirrors and concave mirror can be coated with simple and cheap mirror coatings. The beamsplitters can be coated with a simple and cheap single layer of a metallic or dielectric material. Because only a few optical components are necessary to make the design work, no antireflection coatings are necessary. The optical components can be accurately positioned with respect to each other by a single mechanical frame can be cheaply moulded out of a suitable plastic to tight tolerances.

It is not necessary to the invention that the exit pupils are large enough to suit a number of observers with a wide range of interpupillary eye separations. The field of view can usefully be increased by designing more complicated eyepieces that project a smaller exit pupil. In this case the separation between the exit pupils will have to be adjusted to match the separation ofthe observer's eye pupils. The design of the invention can be adjusted to meet this requirement by ensuring that the beam splitter and plane mirror in each path are exactly parallel. The beamsplitter and plane mirror can therefore be rotated without rotating the projected image.The mechanical adjustment of the separation of the exit pupils is achieved by moving sub-assemblies incorporating one eyepiece, its adjacent plane mirror, and the beamsplitter that reflects light into The adjustment is facilitated by rotating either one or two sub-assemblies about the optical axis that intersects the centre of the object at normal incidence to the plane containing the object. This adjustment requires that the size ofthe beamsplitters is reduced, so that they do not substantially vignette the beam that is projected into the other optical path as they are rotated.

Each eyepiece assembly could be a single lens, with or without aspheric surfaces.

Better aberration compensation will result from using more complicated assemblies.

ifit is desired to adjust the focus ofthe projected images, then the invention could be implemented with sub-assemblies incorporating the eyepieces that are moved parallel to the optical axis to vary the focus. It is not necessary that all the eyepiece lenses have to be moved to adjust the magnifier, as the movement of only some of the eyepiece lenses may be sufficient. The object could also be moved parallel to the optical axis to achieve a similar effect.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows an angled view of one embodiment from the rear; and Figure 2 shows a horizontal section through the optical design of the same embodiment with ray paths; and Figure 3 shows the exit pupil shape and size projected through the same embodiment from various points on the object; and Figure 4 shows a horizontal section through the optical design of the same embodiment with selected ray paths showing how the horizontal orientation and alignment of both projected images is the same; and Figure 5 shows an angled view of an alternative embodiment from the front; and Figure 6 shows a horizontal section through the said alternative embodiment with ray paths; and Figure 7 shows a horizontal section through the optical design of the said alternative embodiment with selected ray paths showing how the horizontal orientation and alignment of both projected images is the same; and Figure 8 shows a horizontal section through a embodiment showing where polarisation selective components can be positioned to improve the light transmission; and Figure 9 shows a horizontal section of an example of the optical design whose reference numbers are referred to in Table 1 to identify the optical surfaces.

The first embodiment shown in Figures 1 and 2 relates to having the object closer to the observer than the concave mirror, whilst the second embodiment has the object farther from the observer than the concave mirror. The description in the text below is the same for both embodiments, and therefore the reference numbers in Figures 1, and 2 are the same as in Figures 5 and 6.

With reference to Figures 1 and 2, and 5 and 6, the light from an object 1 that is projected into one of two eyes traverses a semi-reflecting beam splitter 2 and impinges on a spherical or aspherical concave mirror 3 that reflects and focuses an image of the object. The magnified aerial image is formed near to point 4 on the optical axis 9 (point 4 is only shown in one halfofthe optical system for clarity). The light reflected from the concave mirror 3 intersects again on the semi-reflecting beam splitter 2, where part of it is reflected at near 900 to its original path. The reflected image is projected by a second plane mirror 10 into the eyepiece sub assembly 5, which is formed oftwo identical lenses 6. These are shown with part cut away for clarity. The eyepiece sub-assembly collimates and projects the aerial image into the eye forming the exit pupil 7 as a real image ofthe combined shape of the concave mirror and beam splitter. The second beam splitter 8 is directly underneath the first beamsplitter 2 and angled at 90 to it, to reflect the optical axis 9 in the alternative direction.

Should the observer perceive light traversing the binocular magnifier along unwanted paths, such that the resultant images are misaligned with the other images or out of focus, then thin and flat light baffles can be placed adjacent to the beam splitters 2 and 8 to block these unwanted rays. Light that is reflected into the top ofthe wrong eyepiece by beamsplitter 2 can be blocked by placing a thin baffle that is aligned normal to the object and vignettes the top half of the optical path.

Light that is correctly reflected by beamsplitter 8 will then pass underneath the said baffle. Likewise, a baffle can be placed near beam splitter 8 to block unwanted reflections into the bottom optical path. Light that is transmitted through one beamsplitter and reflected by the other can be blocked by a baffle that is aligned horizontally and placed between the two beamsplitters 2 and 8.

Referring to Figures 2 and 6, the horizontal widths of the plane mirrors, the beamsplitters and concave mirror are chosen to project the limiting rays 11 shown without vignetting.

Figure 3 shows the resultant size ofthe projected exit pupil for the first embodiment.

In Figure 3, a typical object 21 is shown with 5 points denoted 16, 17, 18, 19 and 20.

The size of the exit pupil is calculated by tracing rays from the exit pupil to each point in turn, and calculating which rays are vignetted by the optical components, and which rays are transmitted or reflected. The shaded areas enclosed by the lines 25, 26, 27, 28 and 29 in Figure 3 show the calculated extent ofthe exit pupils that are projected from points on the object 16, 17, 18, 19 and 20 respectively. That is to say, point 16 projects the exit pupil 25, point 17 the exit pupil 26, and so on.

The circle 24 represents a nominal exit pupil, that is 20 millimetres diameter for the design illustrated, and which is centred on the optical axis 9. The optical axis 9 intersects the exit pupil at point 22, whereas point 23 is the estimated position for the centre of the eyeball to be placed for best viewing of the projected image. Points 22 and 23 are in the same positions in all the diagrams shown.

That both the images seen by each eye are orientated to be parallel is illustrated by referring to Figure 4. In this embodiment, the beamsplitters 2 and 8 are slightly rotated to correct the horizontal offset induced by the refraction through the said beam splitters. The optical axes 9 are traced in parallel directions from the exit pupil 7 backwards to the object, and are reflected off the beamsplitters 2 and 8 in slightly different paths 9a and 9b. These paths 9a and 9b are reflected back through the beam splitters 2 and 8 by the concave mirror 3 onto the same point in the object 24. In this embodiment, the optical axes 9 are parallel in both halves ofthe optical system, from the exit pupil 7 to the plane mirror 10, and from the plane mirror 10 to the beamsplitter 2 or 8.The horizontal rays 12 enter each eye in parallel directions, and it can be seen that they leave the same point on the object. It is obvious that the vertical orientation of each image is identical. Likewise, the images projected from the second embodiment are also parallel, as seen by referring to Figure 7. The horizontal rays 13 also leave the same point on the object and enter the eyes in parallel directions.

Polarising components can be added to improve transmission according to Figure 8.

The Liquid Crystal Display incorporates a linear polariser 14, because of its function. This must polarise the light so that the electric field is aligned horizontally.

The light is transmitted through the polarising beamsplitter 2 to a quarter wave plate 15, which is placed in front of the concave mirror 3. The quarter wave plate 15 rotates the plane of polarisation by 900 before it is reflected back to the beamsplitter 2. The new polarisation is selectively reflected by the beamsplitter2. The beamsplitter 2 coating can be fabricated cheaply from a single layer of an oxide of Titanium.

The optical construction of one half of the binocular magnifier for the first embodiment is given in the following Table 1, which refers to Figure 9. This gives the coordinates of the positions of the poles, or centres, of each optical surface in turn relative to an origin using rectilinear (x,y,z) coordinates. The origin is defined as a point on the exit pupil, and the x-axis is defined as being normal to the said exit pupil.Table 1 consists of9 columns, namely in order: the surface number shown in Figure 10; the optical surface radius of curvature (positive numbers being convex, negative numbers concave); the optical surface aperture size (with the letter 'd' referring to a diameter, or alternatively if rectangular, 'h' referring to the horizontal dimension, and 'v' to the vertical dimension); the x coordinate of the pole of the optical surface relative to the origin; the y coordinate of the pole of the optical surface relative to the origin; the z coordinate of the pole of the optical surface relative to the origin; the angle that the optical surface makes to the x-axis of the origin in degrees; the material defining the space between two optical surfaces; the refractive index ofsaidmaterial at the 587.6 nanometer wavelength ofthe Sodium d line. The last two columns refer to the space following the surface number specified, i. e. the material specified for surface S4 is that included between surfaces S4 and S5.

Surfaces S3 and S4 are identical and aspheric. Table 2 gives the aspheric coefficients that define the surface shape by the following equation. The sag of the surface is defined by the distance to any point on the surface from a plane that is normal to the optical axis and includes the pole of the optical surface. The sag is measured in a direction normal to the said plane. The pole is defined as the point where the optical axis intersects the optical surface.If the distance of any point on the optical surface to the optical axis, in a direction normal to the said optical axis, is defined as 'R'; and the sag ofthe spherical surface defined by the radius specified in Table 1 is 's'; then the sag ofthe actual asphere is given by the equation: 4 6 sag=s+a4xR4+a6xR6 where: R4 means R raised to the power 4, R6 means R raised to the power 6 a4 and a6 are the aspheric coefficients positive coefficients reduce the thickness ofthe lens Table 1. Data Describing a Binocular Magnifier Design.

Surface Radius Aperture x Y z Angle Material Refractive Index S1 ~ Infinity 20d o 0 0 90 Air 1 52 +73.399 42d 30 0 o 90 Acrylic 1.490082 S3 +33.651 42d 40 0 0 90 Air ] S4 +33.651 42d 41 0 0 90 Acrylic 1.490082 S5 +73.399 42d 51 0 0 90 Air 1 S6 Infinity 32h by 91 0 0 45 Air 1 32v S7 Infinity 56h by 95.98 -36.92 15 44.6 Air 1 30v S8 -70 722 52h by 121 -32 0 90 Air 1 70v S9 Infinity 56h by 95.98 -36.92 15 44.6 Crown 1.522489 30v Glass S10 Infinity 56h by 94 93 -37.98 15 44.6 Air 30v S I 1 Infinity 1 5h by 59.9 -3' O 90 Air llv Table 3 gives the object size and the field of view projected from the design given in Table 1.

Table 2. Aspheric Coefficients ofthe Surfaces S3 and S4 Defined in Table 1.

a4 Coefficient (units: millimetres to the a6 Coefficient (units: millimetres to the power -3) power5) -4.901 794e-O6 -3.678384e-09 Table 3. The Field of View ofthe Binocular Magnifier Design Given in Table 1.

Horizontal field ofview (in degrees) Vertical field ofview (in degrees) 145.2 33.5

Claims

Claims 1. A binocular magnifier comprising a concave mirror that reflects and focuses an image of an object near to two eyepieces, the light being projected onto said concave mirror through two semi-reflecting mirrors, and from the concave mirror by reflection off said beamsplitters. Two plane mirrors are incorporated to reflect the images into two eyepieces that comprise one or more lenses. The said beamsplitters are angled at approximately 900 to divide and reflect the optical paths in opposite directions, and are translated vertically so that light traversing the top beam splitter is reflected into one optical path, and light traversing the bottom beamsplitter is reflected into the bottom optical path.

2. A binocular magnifier according to claim 1 that incorporates lenses elsewhere in the optical path than is specified in claim 1.

3. A binocular magnifier according to claims 1 or 2 that incorporates plane mirrors elsewhere in the optical path than is specified in claims 1 or2.

4. A binocular magnifier as claimed in any preceding claim that has the angle of the beamsplitters adjusted to compensate for optical misalignments between the two projected images that are caused by refraction through the said beamsplitters.

5. A binocular magnifier as claimed in any preceding claim that has the angle of the plane mirrors and eyepieces adjusted to compensate for optical misalignments between the two projected images that are caused by refraction through the beamsplitters.

6. A binocular magnifier as claimed in any preceding claim which incorporates baffles in the subassembly that mounts both the beamsplitters and the concave mirror, the said baffles being used to block or absorb unwanted light.

7. A binocular magnifier as claimed in any preceding claim that incorporates beamsplitters that do not have plane optical surfaces that are parallel.

8. A binocular magnifier as claimed in any preceding claim that incorporates beam splitters that are made from a combination of prisms that reflect the optical axis at substantially 900 to its original path.

9. A binocular magnifier as claimed in any preceding claim that incorporates a methodofadjustment for varying the separation ofthe exit pupils.

10. A binocular magnifier as claimed in any preceding claim that projects light from a linearly polarised object, and that incorporates a beamsplitter that reflects one polarisation oflight more than another, and a polarisation retarding optic placed between the beam splitters and the concave mirror. The said polarisation retarding optic being used to enhance the intensity of light transmitted and reflected from the beamsplitters.

11. A binocular magnifier as claimed in any preceding claim, where the said concave mirror may be either spherical or aspherical.

12. A binocular magnifier as claimed in any preceding claim, where the said concave mirror is replaced with a combination of lenses and one mirror: or where the said concave mirror has one refracting surface and one reflecting surface.

13. A binocular magnifier as claimed in any preceding claim where the two optical paths are divided by the two beamsplitters such that their optical axes are not horizontal, or not aligned in parallel directions.

14. A binocular magnifier as claimed in any preceding claim 1 where the said plane mirrors or the said beam splitters are not plane, but curved.

15. A binocular magnifier as claimed in any preceding claim where the eyepiece lenses are deliberately made to be non-rotationally symmetrical about the optical axis.

16. A binocular magnifier as claimed in any preceding claim where any of the eyepiece lenses or the object are moved in a direction parallel to the optical axis to adjust the focus ofthe projected images.

17. A binocular magnifier as claimed in any preceding claim that incorporates optical components that have been moulded.

18. A binocular magnifier as claimed in any preceding claim incorporating mirrors or beamsplitters fabricated from float glass.

19. A binocular magnifier as claimed in any preceding claim incorporating a moulded frame onto which the optical components are mounted, such that the positions ofthe optical components are accurately controlled by the frame.

Amendments to the claims have been filed as follows 1. A binocular viewing apparatus comprising a display screen, an offset beamsplitter having left and right hand semi-reflecting mirrors being ritually inclined and offset respectively above and below an optical axis, a concave mirror generally coaxial with said optical axis and disposed forwardly of the display screen, left and right hand fold mirrors, and left and right exit pupils disposed forwardly or rearwardly of the concave mirror, in which the offset beamsplitter is mounted intermediate said display screen and said concave mirror and said fold mirrors are each disposed generally ahead of the corresponding exit pupil and are further disposed laterally either side of the offset beamsplitter with the arrangement prwidkkg that an initial image is displayed on the display screen, the initial image is projected forwardly through the offset beamsplitter to be incident on the concave mirror which reflects and focuses said image received, the reflected image is incident on reflective faces of the semi-reflecting mirrors which divides the reflected image into two opkical paths, the first optical path is incident on the left hand fold mirror which reflects the first path toward the left exit pupil and the second optical path is incident on the right hand fold mirror which reflects the second path toward the right hand exit pupil.
2. Apparatus in accordance with claim 1, further comprising magnifying optics adapted to magnify an image received fran the display screen and/or to collimate an aerial image formed by the concave mirror.
3. Apparatus in accordance with claim 2, in which said magnifying optics comprises a first lens or lens array disposed intemediate said left hand fold mirror and said left exit pupil and a second lens or lens array disposed intermEdiate said right hand fold mirror and said right exit pupil.
4. Apparatus in accordance with claim 3, in which one or more of the lens of each of the lens arrays are non-rotationally symnetrical about their respective optical axes.
5. Apparatus in accordance with claim 3 or claim 4, in which the lens arrays are moveable along their respective optical axes to adjust focus of received images.
6. Apparatus in accordance with any one of the preceding claims, in which said sani-reflecting mirrors are substantially mutually perpendicular.
7. Apparatus in accordance with any one of the preceding claims, in which said semi-reflecting mirrors have substantially upright planar reflective surfaces.
8. Apparatus in asstDdance with any one of the preceding claims, in which the display screen is a ICD video display mounted coaxially with the concave mirror.
9. Apparatus in accordance with any one of the preceding claims, in which said right and left hand fold mirrors are planar mirrors.
10. Apparatus in -'-- ce with any one of the preceding claims, in which one or more further fold mirrors are disposed intermediate said left hand fold mirror and said left exit pupil and one or more still further fold mirrors are disposed internidiate said right hand fold mirror and said right exit pupil.
11. Apparatus in accordance with any one of the preceding claims, in which angular adjustment is provided for either or both said semi-reflecting mirrors and/or at least one of said fold mirrors to compensate optical misalignments that are caused by refraction through the semi-reflecting mirrors of said beamsplitter.
12. Apparatus in accordance with any one of the preceding claims, in which median planes of external surfaces of said semi-reflecting mirrors are non-parallel.
13. Apparatus in accordance with any one of the preceding claims, further comprising baffles to block or absorb unwanted light.
14. Apparatus in accxzdbJse with any one of the preceding claims, in which said left and right hand semi-reflecting mirrors are each proviòsd by a respective prism.
15. Apparatus in acscrdance with any one of the preceding claims, further comprising a device permitting interoccular adjustment of the lateral spacing of the left and right exit pupil.
16. Apparatus in accordance with any one of the preceding claim, in which the inital image is of a linearly polarised object, the beamsplitter is adapted to reflect one polarisation of light more than another and a polarisation retarding object is disposed interM'diate the beamsplitter and the concave mirror with the arrangement praviding that the polarisation retarding object facilitates enhancement of the light intensity transmitted and reflected by the beamsplitter.
17. Apparatus in accordance with any one of the preceding claims, in which the concave mirror is spherical or aspherical.
18. Apparatus in accordance with any one of the preceding claims, in which the concave mirror has one refracting surface and one reflecting surface.
19. Apparatus in accordance with any one of claims 1 to 16, in which the concave mirror is provided by a lens/mirror array.
20. Apparatus in accordance with any one of the preceding claims, further comprising a moulded frame an to which the screen, beamsplitter, concave mirror, fold mirrors and magnifying optics are mounted so as to be accurately relatively positioned.
21. A binocular viewing apparatus in accordance with any one of the preceding claims, when forking a part of a head mounted display appartus usable in conjunction with virtual reality software applications.
22. A binocular viewing apparatus substantially as hereinbefore described and as illustrated in Figure 1 to 4 and Figure 9 or Figures 5 to 8 of the aocompanying drawings.