GB2205416A

GB2205416A - Stereoscopic viewer having a pair of viewing lenses

Info

Publication number: GB2205416A
Application number: GB08811818A
Authority: GB
Inventors: Anthony Tak-Sau Lo
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-05-22
Filing date: 1988-05-19
Publication date: 1988-12-07
Also published as: IT8821265V0; DE8806831U1; GB8811818D0; FR2615633A1; CN2044735U; IT214005Z2; GB8712153D0; JPS644415U

Abstract

A viewer 10 for a stereoscopic pair of images where the stereoscopic pair 22 is mounted behind a pair of viewing lenses, preferably fresnel lenses, 12, 14 at a spacing less than the focal length so that a magnified virtual image is seen. The lenses 12, 14 are at least as large in width and height as the image of the stereoscopic pair 22. In this way long or short sighted persons can view the images in focus by holding the viewer at an appropriate distance from the eyes. The width-wise spacing between the optical axis of the lenses is such that the rays from the left and right hand images emerging from the lenses at the join between the two lenses are substantially parallel. In this way the user's eyes do not have to diverge when looking towards the lens to view the stereoscopic images and the lenses 12, 14 provide the equivalent of an optical wedge to bend the images as seen by the user in an outwardly diverging sense. The viewer may comprise a rangefinder (Figs. 8-10). <IMAGE>

Description

IMPROVEMENTS IN VIEWING STEREOSCOPIC IMAGES This invention relates to the viewing of stereoscopic images and in one aspect it relates to an improved stereoscopic viewer to enable users to view stereoscopic pairs of images.

It is well known that when two photographs are taken from slightly different angles and then appropriately viewed separately by the left and right eyes of a person, a single combined view with the effect of three-dimensional depth is given. Such pairs of photographs are known as a stereoscopic pair.

There have been many proposals for cameras which are capable of taking a stereoscopic pair and viewers for viewing the resulting stereoscopic pair.

One popular viewer has two small magnifying lenses acting as eye pieces and a stereoscopic pair is mounted at a suitable distance from the lenses and viewed through the lenses. In practice, it is very much like looking through a pair of binoculars and has the same disadvantages: 1) In order to view the whole image, it is necessary to put the eyes very close to the eye pieces. Spectacles users will sometimes find this difficult to do, besides it has been known that plastics spectacle lenses can be scratched through rubbing against eye piece mounts.

2) Not everyone1 5 vision is perfect and sometimes even spectacles cannot fully correct the faults. To read comfortably, a short sighted person will tend to hold the print closer to the eyes and vise versa for a long sighted person.

With a conventional stereo viewer, it is not possible to hold the device closer or further from the eyes. One must always hold it right up to the eyes. To overcome this problem, the better viewers have a focus adjustment which moves the eye pieces along the lens axis which in effect moves the virtual images closer or further from the eyes.

3) The pitch of our eyes also varies between approximately 55mm to 70mm and better binoculars and stereo viewers also have a pitch adjustment so that the optical axes of the eye pieces can be adjusted to coincide with those of the eyes.

Some people are not competent at adjusting these eye pieces properly.

4) Eyes are capable of compensating for minor errors in adjustment of the eye pieces, however, this causes eyes strains and sometimes results in a headache, so some people dislike looking through devices similar to binoculars.

It is therefore an object to the present invention to provide a viewer for stereoscopic pairs of images in which these disadvantages are overcome or mitigated.

According to the invention, the viewer comprises a pair of lenses through which the user views a stereoscopic pair of images, means for providing a stereoscopic pair behind the lenses in such a way that the distance between the lenses and the stereoscopic pair is less than the focal length of the lenses so that a magnified virtual image is seen, each lens being at least as large in width and height dimensions as the corresponding width and height dimensions of each image of the stereoscopic pair so as to enable users ranging from long-sighted persons to short-sighted persons holding the viewer at an appropriate distance from the eyes such that the images are in focus for that person to view the whole image, the width-wise spacing between the optical axes of the lenses being such that the rays from the left and right hand images emerging from the lenses at the point between the two lenses are substantially parallel so that the user's eyes do not have to diverge when looking towards the lenses and the lenses provide the equivalent of optical wedges to bend the images seen by the user in an outwardly diverging sense in a direction forward of the lenses as a continuation of the direction from the stereoscopic pair to the lenses.

I have found therefore that by using this arrangement, one can avoid having to have actual mechanical adjustment of the focal length and the width-wise spacing of the lenses and obviously this greatly simplifies the construction of the viewer. The user merely has to hold the viewer at a distance from his eyes to suit his own circumstances such that he brings the images into focus and the desired three-dimensional effect will be achieved.

Also, the lenses will provide a large enough aperture such that all users will be able to view the stereoscopic pair fully.

Because the lens will be quite large, fresnel lens will desirably be used to reduce bulk and weight.

The stereoscopic pair will normally be a pair of photographic or printed images obtained by photographing a scene with a stereo camera. Then the viewer is provided with means to mount the pair.

In one simple embodiment of the invention the lenses are provided with range finding means. Then the user can simply hold the lens in front of the stereoscopic pair.

The invention will now be illustrated, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an example of a viewer according to the invention; Figures 2 to 7 are diagrams showing the design of the viewer; Figures 8 to 10 are diagrams showing the design of a modified system which incorporates a range finder; and Figure 11 is a diagram of the resulting lens system.

Referring to the drawings, the viewer 10 comprises a front face 11 in which are supported a pair of fresnel lenses 12 and 14. These abut about an upright from line 15. At the rear is provided an upright support 20 for a stereoscopic pair of images 22 provided as a single photograph with two separate images positioned upright and parallel to the lenses 12 and 14. The images 22 are held upright against the support 20 by a pair of forwardly spaced arms 24 which provide slots 25 into which the edges of the images 22 are slotted.

The front face and rear support are held spaced apart by a trapezium-shaped base 26.

In designing the viewer 10, the distance U of the stereoscopic pair of images 22 to the lens 14 and 16 has to be determined. This can be achieved using the well known formula: U V F where U is the distance from the lens to the stereoscopic pair, V is the resulting image distance from the lens, and F is the focal length of the lens.

If one chooses a virtual image of, say, 500mm behind the lens to ensure that U is less than F, and chooses a lens of focal length of say 190mm, then from the above formula U will be 138mm.

The resulting viewer 10 is shown diagrammatically in side view in Figure 2. As can be seen, the virtual image is behind the object, i.e. the stereoscopic pair, and is enlarged relative the individual image of the pair.

Next, to determine the lens size, image size and maximum viewing distance relationship, one has to consider magnification and with reference, for example, to page 140 of the book "Optics" by Francis Weston Sears: Magnification = 250 .

when the user views from F 250mm in front of the stereoscopic pair, i.e. 112mm in front of the lens. In this case, therefore, if the print height h is 90mm, then the magnified apparent print size will be 90 x 250mm 90 x 190 (F being 190mm), i.e. 118mm.

In case of the viewer 10 and referring to Figure 3, the image distance has been assumed to be 500mm from the lens, and so the actual virtual image size is 289mm as can be determined mathematically by ratios, i.e.: image size = 118 x 500 +112 = 289mm.

250 Again, and referring to Figure 4, by using the ratio of similar triangles, if the height H of the lens is 130mm, then if our viewing lens size is 130mm, then the eyes can be X mm away and still see the whole image. X can be calculated as follows: X + 500 - X 289 130 from which one can deduce that X = 408mm.

In other words, the viewer can be held from right up to the eyes or as far as 408mm away and the eyes will still be able to see the whole image.

The lenses 12 and 14 should be cut to a rectangular shape of the same format, i.e. proportion of height to width as the prints of the stereoscopic pair and mounted right next to each other with no gaps between the two lenses.

It should also be noted that if the lens size in the width and height dimensions is equal to or is larger than the image size, then it is possible to see the whole image at any distance up to infinity.

Next, the eye spacing has to be considered from the plan view of the viewer. Thus it is necessary to present the two images to the brain as though these light rays are coming from a single three dimensional source.

Referring to Figure 5, if one assumes that the pitch of the stereo pair is 65mm, that is to say the width-wise spacing of identical points at the centre of the two images, and that one wishes the rays to emerge from the lenses parallel at the region of the joint between the two lenses, then: tan e = 32.5 = 0.236 138 and # = = 13.30.

Assuming that the refractive index of the lenses is 1.5, than by Snells Law Sin Sino( = 1.5, where α is the angle to the normal of the emerging ray and p is the angle to the normal of the incident ray. Also p 9 = p = 13.30. by experiment using a simple computer program and measuring the value of α in steps of 1 , it can be shown that an angle α = 24 will meet this requirement, that is to say the prism angle of each lens at the edges where they abut should be 240.

It can be shown that a lens will act as a prism if it is used off its optical axis. Thus, since the lenses 12 and 14 in the viewer 10 have to give the effect of a 240 prism, and since it is also known from page 91 of the above referred to book "Optics", that for a thin lens of refractive index 1.5, the radius of curvature is half the focal length the approximate radius of curvature of the lenses 12 and 14 are: 190 - T Referring to Figure 6, if x is the displacement of the centre of curvature to the edge of the lens, Sin 240 = x 25 so x = 38.6mm.

Therefore the two side by side lenses 12 and 14, should have their centres set 38.6 x 2 apart, or in other words 77.2mm apart.

With the arrangement, the result is as shown in Figure 7. Thus when viewed anywhere along the region 40, the magnified virtual images of distant objects will always appear as one image and the pair of lenses will seem to act as a single continuous lens. There will therefore be no risk of double vision, and an observer at a long distance (say 1 metre) always will see a whole image of the print half of which is formed by the left print and the other half the right one. This has advantages therefore in providing an "eye catching" image which can be useful for advertising purposes and thus when the observer moves closer, he will still see the 3-dimensional effect.

In the situation shown in Figure 7 it will be noted that at the joint between the two lenses, the left hand and right hand images will appear as parallel rays.

The other rays diverge progressively and so one the user's eyes do not have to diverge when looking towards the lenses. Also, provided the width-wise spacing between the optical axes of the lens is greater than the minimum normal spacing between a person's pair of eyes, no one's eye will ever need to diverge to look at the viewer. Here in this example the spacing is 77.2 mm and so this is indeed greater than the typical minimum human eye spacing which is 70mm.

It is also a feature of the viewer according to the invention that no unwanted mono images will normally be seen on either side of the stereo image if one views within a cone based on the join between the lenses and having an angle Q as above as is often the case with conventional viewers. Therefore it is not necessary to provide a mask which greatly simplifies the construction of the viewer and in addition it then becomes readily possible for the lenses 12 and 14 and the rear support 20 to be hinged flat with the base 26 when the viewer is not in use.

In a modified embodiment, only the lens 12 and 14 need be provided but then it is useful to provide a range finding device 50 which can be useful to the user to determine that he is holding the stereoscopic pair 22 at about the correct distance from the lenses 12 and 14. Such an arrangement is shown in Figure 11. The range finding device 50 can be positioned beneath the lenses 12 and 14 and as will be explained can be formed by portion of fresnel lenses. Indeed these lenses and the lenses 12 and 14 can all be formed as a single plastics moulding.

The operation of the range finder 50 will now be explained. Referring initially to Figures 8 and 9, if one considers a point on a print, when the optical prism is next to the print, the image is nearly in the same position as the point on the print. As the optical prism is moved away from the print, though the angle of deflection is still the same (as shown in Figure 9) but the apparent image has now moved towards the right hand side or the smaller side of the prism. In other words the image deflection x is governed by tan j x y.

where g = (angle of deflection through the prism) y = distance between the print and the prism Since J is fixed by the refractive index of the prism material and the included angle of the prism, value of x is simply governed by y.

If a pair of images such as a pair of stereo photos are viewed through a pair of prisms one on top of the other with the thin end of one next to the thick end of the other as shown in Figure 10, as these prisms are moved perpendicularly away from the print, the right hand image viewed through prism A -and the left hand image viewed through prism B will appear to move towards each other, meet and then pass each other.

If a lens panel is fitted with these prisms and suitable choice of refractive index and prism angle, these images can be arranged to meet when the viewing lens is at the correct distance from the print.

In practice because one wants to have a central point on each image coincide on the centre line of the viewer, the deflection requirement is the same as for the edge of the fresnel lenses 12 and 14, provided the refractive indices are the same. Therefore the prism angle should also be 240.

With such an arrangement the frame normally required to keep the lenses and stereo pair and the correct position can now be dispensed with and instead all one needs is the lenses 12 and 14 and the range finder 50 which will be held in one hand and the stereo pair which are held in the other hand of the user.

The prisms A and B shown in Figure 10 are desirably formed as parts of fresnel lenses as is shown in Figure 11.

Claims

C L A I M S:

1. A viewer comprising a pair of lenses through which the user views a stereoscopic pair of images, means for providing a stereoscopic pair behind the lenses in such a way that the distance between the lenses and the stereoscopic pair is less than the focal length of the lenses so that a magnified virtual image is seen, each lens being at least as large in width and height dimensions as the corresponding widthXand height dimensions of each image of the stereoscopic pair so as to enable users ranging from long-sighted persons to short-sighted persons holding the viewer at an appropriate distance from the eyes such that the images are in focus for that person to view the whole image, the width-wise spacing between the optical axes of the lenses being such that the rays from the left and right hand images emerging from the lenses at the join to between the two lenses are substantially parallel so that the user's eyes do not have to diverge when looking towards the lenses and the lenses provide the equivalent of optical wedges to bend the images seen by the user in an outwardly diverging sense in a direction forward of the lenses as a continuation of the direction from the stereoscopic pair to the lenses.

2. A viewer as claimed in Claim 1 in which the lenses are fresnel lens.

3. A viewer as claimed in either preceding claim in which the stereoscopic pair is a photographic pair and means are provided to mount the pair a predetermined spacing behind the lenses.

4. A viewer as claimed in Claim 1 or Claim 2 which incorporates range finding means to enable the user to position the stereoscopic pair the desired spacing behind the lenses.

5. A viewer as claimed in Claim 4 in which the range finding means comprise a pair of crossed prisms arranged so that central points of the left and right hand images appear to coincide when at the desired spacing.

6. A viewer as claimed in Claim 5 in which the prisms are integrally moulded together with the lenses.

7. A viewer substantially as herein described with reference to the accompanying drawings.