WO1988004406A1

WO1988004406A1 - Photogrammetric apparatus

Info

Publication number: WO1988004406A1
Application number: PCT/AU1987/000416
Authority: WO
Inventors: John Kingsley Newman
Original assignee: Viici Pty. Ltd.
Priority date: 1986-12-08
Filing date: 1987-12-08
Publication date: 1988-06-16

Abstract

A photogrammetric optical apparatus for obtaining information from a stereo pair of images (1). Light rays from images (1) are directed to a region (23). Reference mark light ray generating means (13) provide reference mark light rays which are also directed to region (23). All of the light rays are collimated in region (23). A telescope (61, 63) is provided to receive light rays whose direction are changed orthogonally by combined prisms and beam splitters (76) which are within region (23). The arrangement is such that the light rays from reference mark generating means (13) is factory adjusted for collimated light output and this forces all light rays in region (23) to be collimated regardless of normal operator focus adjustments. The arrangement permits a compact and lightweight but very stable precision system to be produced.

Description

PHOTOGRAMMETRIC APPARATUS Field of the Invention

This invention relates to photog 'ammetric apparatus for use in determining 3-dimensional information which is derived from viewing a stereo-pair of photographs of an original target. The invention has particular, although not exclusive, application in providing surveying plots of terrain.

Description of the Prior Art It is well known that the most rapid method of surveying large areas is to employ aerial photography. If a number of aerial photographs are taken of the terrain vertically below and along the flight path of an aircraft, an approximate mosaic map may be made up of the terrain photographed by joining the photographs together. However, if the scale, the orientation of the map and the true positions of at least three points of detail identified on both the map and the photos are known, then the true position of any other points may be deduced and a very accurate map of the terrain constructed employing the science of photogrammetry. When the terrain is photographed in this manner by two cameras arranged at spaced apart positions, a pair of photographs, referred to as a stereo-pair may be used to form an apparent 3-dimensional view of the terrain. The science has developed since the 1920s, particularly with regard to the production of high quality topographical maps using aerial photographs taken with large special purpose cameras. More recently there has been a growing demand for work with smaller cameras and the application of the science to 3-dimensional measurements throughout science, medicine and industry^'as well as in aerial survey and mapping.

Apparatus which is currently being used for this purpose falls within one of two general groups. The first group comprises mechanical and optical analogue machines which are operated by setting a stereo-pair of photographs in correct orientation relative to one another and by arranging a viewing system which includes mechanical and optical means with co-operates with the photographs to produce a drawing or map of the terrain. The second group includes machines known as analytical machines which measure the co-ordinates of selected corresponding points on each photograph of the stereo-pair and compute from the derived observations the true position of each point. The analytical machines can be broken down into two sub-groups consisting of simple systems and precision systems. The simple systems operate by a reference mark being manually positioned on a selected point on each photograph in the stereo-pair. Measurements are then made relative to a datum followed by geometric calculations leading to the actual geographic location of the point being determined. The precision systems incorporate computer automated systems to aid in the scanning and measuring processes and set up a model in a host computer to represent relations between points in the terrain or object that is being studied, and images on the photographs. The application of numerical analysis to the model then results in the solution of problems relating to lens and film distortion, camera orientation, and calibration of the total system, followed by an analytical solution and transformation from photo co-ordinates to the desired ground or object co-ordinates.

__» Virtually all the known analytical machines to date have been very expensive, large and requiring specialist operators. A few relatively low cost more compact machines have begun to appear on the market, designed for the more general user as well as the specialist, but with precision and optical resolution below that desired for many tasks.

Traditionally an operator has viewed the stereo image through eyepiece devices similar or identical with viewing means provided with binocular microscopes, but there is a trend towards alternative image viewing means with various screen displays and projection systems replacing the binocular viewing devices. It would therefore be advantageous if a single system could be provided with alternative displays, e.g. enabling an operator to remove the binocular viewing arrangement and replace it with an overhead projection system or incorporate other optical devices therewith. Very recently there has been a demand for electronic camera devices to be attached to the apparatus for capturing and storing in digital form details of the stereoscopic image.

The stereo photo-pairs have traditionally been

2 "large format" aerial photographs, typically 230mm and a precision in the order of microns has been required.

This has only been achieved in prior art systems which are physically large, massive and very expensive. Recent low cost small format- versions of the known apparatus have not been sufficiently rigid to withstand reasonable external strains induced, by vibrations, temperature drift or operator interaction without shifting the measuring datum and altering calibration.

Prior art photogrammetric apparatus employs an optical reference marking means in the form of two small and very regular illuminated or opaque marks, one of which appears in the field of view of each eye of an operator. In use, the marks appear fused as a single mark which floats in 3-dimensions with respect to the image formed by the stereo pair. Thus, by moving each of the photos in a plane, defined by x and y axes, the fused floating mark will seem to move relative to the 3-dimensional image. By recording the amount of relative movement in the x and y direction of each photo and differential movements between them and applying appropriate transformation equations the host computer in an analytical system is then able to determine the x, y and the z co-ordinates of the points and features of interest in a continuous manner. The output of this process can then be applied to the simultaneous plotting of topography and contours, or can be stored for further analysis and off-line plotting. Figure 1 illustrates the process in very general form. Statement of the Invention It is an object of the present invention to attempt to overcome problems relating to the prior art apparatus. According to the present invention there is provided a photogrammetric optical apparatus for obtaining information from a stereo-pair of optical images, said apparatus having means for supporting a first optical image source and means for supporting a second optical image source in spaced apart relation, means for independently moving the x, y co-ordinates of at least one image relative to the x, y co-ordinates of the other and means for obtaining optical images from each optical image source, and information obtaining means for obtaining information from each optical image, respective reference mark light ray generating means for each of said optical images, characterised in that all of the optical light rays to said information obtaining means are collected in a single region where they are all collimated.

It is particularly preferred that each of the image sources comprise photographic plates (photographic transparencies) and that both plates be mounted for independent x, y movement in a common plane. Desirably the apparatus can be used in conjunction with appropriate computer means and software to produce co-ordinates in digital form that may be stored, and for example, further processed at a later stage or used to generate plots. To achieve this purpose photo carriers which support each of the plates may be provided with stepping or linear positioning motors controlled by the computer and from which x, y positional data is obtained.

Various attachments such as rear projection or overhead projection apparatus can be attached. A typical example of this is disclosed in U.S. Patent Specification 4,057,336. The advantage of such an arrangement is that an operator may be able to observe the 3-dimensional view on a screen, without having to resort to relatively tedious viewing in the eye pieces of a binocular microscope.

Brief Description of the Apparatus In order that the present invention can be more easily ascertained, preferred embodiments of analytical photogrammetric apparatus for use with computers and used for generating plots , will now be described with reference to the accompanying drawings, wherein:

Figure 2 is a vertical cross-section view through one such apparatus where the photographic stereo-pair of plates are mounted in vertical and parallel spaced apart planes. The apparatus shown in Figure 2 is a diagrammetric view only;

Figure 3 is a top perspective view of a particularly preferred embodiment where the photographic stereo-pair of plates are mounted in a common horizontal plane but at spaced apart locations;

Figure 4 is a side elevational cross section along the x centre-line of the apparatus shown in Figure 3;

Figure 5 is a "bottom-up" plan view of the main platform, showing the placement of critical components; Figure 6 is an exploded isometric view of the left hand beam splitter prism assembly;

Figure 7 is a side view of the apparatus shown in Figure 3 but wherein a hinged lid portion and binocular eyepieces have been removed? and

Figure 8 is a mid section in the y direction of the main platform of Figures 3, 4 and 5 in Figure 3, showing one of the attachments . Referring firstly to Figure 2, it can be seen, that one embodiment of the invention, in diagrammatical form, comprises mounting of photographic plates 1 of a stereo-pair in spaced apart and parallel vertical planes. The photographic plates may range from 230mm 2 down to

70mm 2 and 35mm2. Left and right sections are identical.

Precision motor driving means (not shown) is provided to move each of the photographic plates 1 in x, y directions relative to the plan image presented by each photograph. In other words, the photographic plates remain in fixed spaced apart plannar positions as shown but can be moved in and out and up and down relative to the page. Respective left and right hand light sources 3 are provided. In this typical implementation each has its own reflector 5 and a condensing lens system 7 which provides uniform illumination of the photographic plates 1. Light transmitted through the photographic plates 1 passes to an objective lens system 9 where it is collimated as it passes therethrough. Each of the lenses 9 are on a respective rack and pinion mounting bracket (not shown) and are positionally adjustable to focus the image of the photographic plate at infinity and to thereby provide the system optically therepast with collimated light. Both lenses 9 are identical and are highly corrected flat field, large aperture, high resolution lenses as typically required for the objective lens in modern high quality stereo microscopes. One source of these lenses is from an SMI binocular microscope sold by Industrial and Scientific Supply Company Pty. Ltd. of 329 Concorde Road, Concord West, New South Wales, Australia. Collimated light which passes through objective lenses 9 enters beam splitting cubes 11 and is reflected orthogonally upwards by the beam splitter mirror surfaces therein into the viewing optics (not shown) . Because the light is collimated, i.e focused to an image at infinity, it may subsequently be magnified, and provided with an operator selectable range of magnifications by interposing a system of telescope pairs between the exit of the beam splitting cubes 11 and the operators eyepieces as in modern practice with binocular microscopes. Galilean telescope optics are particularly advantageous for this purpose because of their inherently compact design. A turret with four pairs of Galilean telescopes is available as a subsystem of the SMI binocular microscope referred to above. Together with lOx focusing eyepieces from the same source, a practical viewing system with a range of 6x to 50x magnification may be assembled and positioned above the beam splitting cubes 11, thereby providing the operator with a 3-dimensional image of the photographic plates.

Reference image light ray generating means 13 is provided for each of the respective photographic plates 1. Typically each reference image light ray generating means 13 comprises an ultra bright Light Emitting Diode (LED) such as the Hewlett Packard HLMP

3700 series with red, amber or green colour available to suit operator preference. Light emitted from the reference image light ray generating means 13 illuminates an aperture plate 15. The aperture plate 15 has a precision aperture therein, typically in the range of 10 to 50 microns in diameter, and is easily changed to give the operator a choice of aperture.

Suitable aperture plates 15 are available from Quentron Optics Pty. Ltd. of 75A Angus Street, Adelaide, Australia. Light which passes through aperture plate 15 then passes to objective lens 12 which is preferably the same focal length as lens 7, but has smaller aperture and simpler design suited to the minute field monochromatic light sources from the aperture. Suitable lenses are stocked on the shelves of the major optical supply houses and can be manufactured in most competent optical workshops.

Lens 17 sees the aperture in the aperture plate 15 as a sharply defined source of light and focuses the image at infinity. In other words, the aperture is placed at the principle focus of lens 17 and the light which is passed through the aperture is collimated thereby. The collimated light from lens 17 is then directed orthogonally by a prism 19 to beam splitter 21. The beam splitter 21 mirror surface passes a proportion, typically 40 per cent, of the collimated rays from the aperture plate 15 where it mixes with the photo image rays, also collimated, which are partially reflected from the same mirror surface. Both collimated images are thereby projected upwards into the entrance apertures of the previously described viewing optics.

Thus, it can be seen that the light rays of the image of the plates 1 are passed through lens 9 where they are collimated. Additionally, an image of the aperture in aperture plate 15, as a bright and sharply defined source of light, passes through lens 17 where it is collimated. Thus, there is a region 23 where all light rays are collimated from all optical sources. The beam splitters 21 and prisms 19 are provided in the region 23.

In use, an observer looking through the stereo viewing optics mounted above the beam splitter 11 will see a stereoscopic image corresponding to the two areas of the photographic plates 1 positioned adjacent to the illuminating light sources 3. This of course assumes that the two plates 1 are oriented so that the images overlap and are in alignment for this purpose. The viewer will also see a single reference mark image, a sharply defined disc of light when magnified by the viewing optics, in the field of view which can be used in the manner previously described.

The reference mark projection systems desirably 5 should be factory mounted and non user adjusted to mark or point to detail on the optical axis and the centre of the observable field of the photogrammetric apparatus . Though all measurements are made with reference to this detail in the centre of the field it is highly desirable 0 for the operator to be provided with as wide a field of view as possible to support the stereo interpretation and look-ahead process essential for high productivity in mapping and other applications of photogrammetry. This requires objectives lenses 9 to be wide aperture flat 5 field lenses which avoid problems of vignetting or peripheral image loss despite the interposition of the beam splitters and allowance of space for focus ^■ adjustment.

Figures 3 through 7 show a particularly 0 preferred embodiment of the apparatus where the photographic plates 1 are mounted in a common horizontally extending plane. In this connection, the apparatus features a stable metal platform 25 to which are attached all the critical optical and mechanical 5 assembles. Moreover, this platform 25 is formed from aluminium by extrusion from a custom designed die and incorporates strengthening ribs 27 and 29 (see Figures 5 and 7) which also serve the purpose of providing rigid attachment points and enclosures for the mechanical and 0 optical sub-assembles. Though typically extruded in a 4 mm section and therefore very light in weight, this platform is extremely rigid and ensures that the critical mechanical and optical sub-assemblies are unaffected by external forces and strains. The platform is loosely 5 mounted (not shown) on frame 31 so that it "floats" and is further insulated from external strains and vibration. The platform 25 has a dust cover 33 hinged to frame 31 by hinges 35. The dust cover 33 incorporates photo illumination sub-assemblies 37 which incorporate miniature high colour temperature fluorescent lamps (not shown) and a light mixing box (not shown) to provide uniform illumination via apertures 39 which align over the currently viewed portion of the photographic plates 1 when the dust cover 33 is closed on the platform 25. The light mixing boxes follow modern colour enlarger illumination technology and for that reason have not been detailed. Intensity of illumination can easily be varied to suit demand by a variety of means including neutral density filters and mechanical flaps.

Exchangeable photo plate holders 41 are designed to accept commercially available Gepe 70 mm and 35 mm film holders with anti-Newton-rings treated glass covers. Larger formats require custom designed holders 41.

Photo carriers 43 are transported in the y direction by linear motion systems 45 driven by stepping motors 47, and the combination of photo carriers 43 and y linear motion systems 45 are transported in the x direction by linear motion systems 49 and stepping motors 51 (not clearly shown in Figure 5). Each complete assembly of left hand or right hand photo carrier 43 plus x and y motion systems is attached by two bolts (not shown) to strengthening rib 27 of the stable platform 25. The linear motion systems 45 and 49 employ recirculating ballscrews 53 and 55 and linear motion slides 57 typically supplied by NSK or THK of Japan (see Figure 7). Suitable x and y stepping motors 47 and 51 are type MS-400 from Warner Electric Australia, 16 Prince William Drive, Seven Hills, 2147, New South Wales, Australia. The ballscrews 53, 55, linear motion slides 57 and stepper motors 47 and 51 are standard components of floppy and hard disk drives in computers. A combination of SM-400 motors providing 800 half steps per revolution and 1.0 mm pitch ballscrews from THK will provide 1.25 micron resolution in the movement of photographic plates 34. Referring now particularly to Figure 4 where the apparatus is shown in side elevation without the dust cover 33, it can be seen that each of the photographic plates 1 are illuminated and the light which passes through apertures 39 therein is directed onto inclined mirrors 59. The light which falls on mirrors 59 is transmitted to be collimated by respective adjustable focus objective lenses 9. The adjustable focusing means has not been shown but comprises rack and pinion means. The lenses 9 are of the type referred to in the previous embodiment and are typically F=100 mm wide aperture flat field fully corrected lens systems available with the SMI microscope previously referred to, for the small format 35 mm to 70 mm version. The preferred horizontal planar configuration of the photo carriers 43 forces a change to ^* longer focal length objectives to accommodate larger formats. These are again available as standard SMI objectives with 0.5x auxiliary lenses extending focal lengths to 200 mm. The mirrors 59 are identical and are ground flat to a fraction of the wavelength of visible light to match the high resolution optics of the binocular microscope components.

Figure 4 shows reference image light ray generating means 13 in the form of LEDs 13 of the same type referred to in the previous embodiment. The light therefrom passes through an aperture plate 15 of the same type referred to in the previous embodiment and objective lens 17 of focal length equal or within 10 per cent of that of objective 9 to prism 19.

The combined photographic and reference mark images, still collimated, then pass into an operator selected pair of four Galilean telescopes 1 and 2, within turret 65 (as described for the previous embodiment) . Figure 4 shows a telescope lens pair in magnifying configuration. Two magnifying and two reducing telescopes, making the four in total, are assembled in the turret 65 which provides a range of operator selectable magnifications varying from 6x to 50x when combined with standard lOx magnifying eyepieces 67. The eyepieces 67 are not shown in Figure 4 but eyes 69 are sketched in to indicate that a stereo image can be obtained at the top of the turret 65 by eyes 69 focused to infinity, or manipulated in various viewing systems including eyepieces and screen projectors. The turret 65 and eyepieces 67 can, in fact, be removed and a good stereo view still obtained, providing the eye base matches the separation between apertures 71 in the platform 25.

Because the images derived from the photographic plates 1 and the aperture plates 15 are collimated, and the associated optical sub-assemblies are maintained in very rigid angular relationship with each other, the positioning of the telescope turret 65 and associated viewing devices is non critical . Movements and deflections of the turret 65 and eyepieces 67, for instance, do not affect the calibration and photogrammetric "orientation" of the apparatus.

Figure 5 shows a "bottom-up" plan of the underside of the stable platform 25 with critical components attached. A CCD (Charge Coupled Device) digitising camera 73 option is shown in its high resolution version designed to extract detail regarding small target areas on the optical axis. A proportion of the collimated rays from the photographic plates 1 pass through, rather than being reflected by, mirror surfaces in beam splitter cubes 75 of combined beam splitter and prisms 76 and are reflected orthogonally by right angle prisms 77 and again by double front surface mirror 79 into the front aperture of magnifying Galilean telescopes 81, 83 and thence into a standard industrial camera lens 85 mounted on CCD camera 73.

This high resolution version of the digitising camera assembly fits into the platform 25 without compromising the optics of the other sub-systems or increasing the external bulk of the apparatus . It requires optical elements 76, 79, 81 and 83 which may be fabricated economically to the required precision in a good optical workshop, plus a miniature CCD camera body such as the Hitachi KP-232 and a standard F=50 mm lens such as the Cosmicar C25010, these latter elements fitting in the outline shown. With a Cosmicar B7514C 2x range extender doubling the camera lens focal length to 100 mm and a achievable magnification of 2Ox in telescope assembly 81, 83, a resolution of around 0.7 micron per pixel is achieved with a typical CCD array of 500 x 500 pixels. This is more than adequate for the most precise photogrammetry with very high resolution film and cameras, and can be reduced to match a wide range of requirements by simply changing to readily available shorter focal length lenses. As the light leaving the Galilean telescope is still collimated,the camera lenses need only be focused to infinity. The camera and lens position along the line of the telescope is therefore totally no critical with respect to this focus.

Figure 6 shows an isometric view of a left hand combined beam splitter prism assembly 78 - the right hand version being a mirror image - exploded into component parts, which provides an optical interface between the various sub-systems. Beam splitter cube 75 is provided with a mirror interface 87 which is partially reflecting and partially transmitting. Typically in the ratio 40 per cent to 40 per cent with 10 per cent lost, for light entering faces adhe and ehgf. Right and left prisms 89 and 91 are attached by optical cement to faces ehgf and bcgf of the cube 75. All entrance and exit faces of the assembly have anti reflection coatings and these faces plus the beam splitter mirror interface 87 are manufactured with surfaces flat to a small fraction of the wavelength of visible light to retain image resolution and match the quality of the binocular microscope lens components. These components and their assembly into left hand and right hand systems can be obtained as custom designed items from any of the major optical warehouses, or manufactured economically in a quality optical workshop.

Referring now to Figures 4 and 5, and to 6, it can be seen that there are two distinct and separate optical paths and two combined light paths for each of the left hand and right hand assemblies of components on the platform 25.

There is a first path 93 defined by light rays directly from the photographic plates 1. Path 93 enters face adhe of the combined beam splitter prism assembly 76. There is a second light path 95 from the reference image generating means 13 which enters face ghij of the combined beam splitter prism assembly 76. A first combined light path 97 mixes photo images reflected from mirror interface bche with reference mark images transmitted through the same interface, and conveys them to the viewing system via Galilean telescopes 61, 63. A second combined light path 97 mixes photo images transmitted through mirror interface bche with reference mark images reflected from the same interface, and conveys them to the CCD cameras 73 via Galilean telescopes 81, 83.

The "bottom-up" plan of the platform 25 in Figure 5, and the cross section view of Figure 7 show the position of mounting of all critical optical sub-systems in channels formed by the strengthening ribs 27, 29 of the platform. The preferred method for fabricating this platform is by custom extrusion in aluminium to obtain a light weight but extremely stiff structure with clean surfaces which do not require any surface finishing other than powder coating or anodising. This platform 25 is then loosely attached to frame 31 as referred to previously, so that it floats and is not subject to externally induced strains.

Thus, the total optical assembly is a very rigid but light weight structure and is not subject to the flexing problems encountered in some of the recent low cost and compact prior art apparatus. It is inherently a low cost, easily assembled system with the potential to match the precision and stability of the most expensive and massive prior art apparatus. Steps are taken in the factory assembly process, to be described below, to ensure that the photographic plates 1 and the reference mark apertures in aperture plates 15 are always adjusted to the principal focus of objective lenses 17. This ensures that all 'the light rays in the region designated 23 in Figures 4 and 5 are collimated. This collimation is carried on to the regions beyond the Galilean telescopes 61, 63 and 81, 83 which are designed, as astronomical telescopes, to accept, process and transmit collimated light. Any prisms, beam splitters or other optical devices which introduce substantial thicknesses of glass in an optical system should ideally, as in this apparatus, be inserted in regions where the light is collimated where they will introduce little, if any, degradation in optical performance, providing that the prism surfaces and interfaces are fashioned flat, within the appropriate fraction of wavelength of light.

If these prisms and beam splitters were placed in front of objectives 9 and 17 they would introduce serious abberations and degrade the performance of the system which has been designed to operate with wide aperture and with resolution to match the finest grain modern photographic film.

It should be noted that it would be possible to employ various techniques to compensate or reduce the effect of any abberations caused by placement of beam splitters and/or prisms outside of region 23 where the light rays are collimated but such would require a redesign of the various lens system for each configuration of prisms and beam splitters to maintain ultimate resolution and therefore has inherent problems.

In addition, it should be noted that the individual optical systems in this embodiment are isolated by the regions of collimated light. Thus, each of the separate light paths can be adjusted and focused for a specific job or disassembled and reassembled without requiring readjustment of the remaining portions of the system. This relates to the adjustment of the left hand and right hand photographic plate image light rays 93, the left hand and right hand reference image light rays 95 from generating means 13, each of the combined light rays 97 to the cameras 73, and telescopes 81, 83, and the turret 65 microscope or other viewing means positioned over apertures 71 in the platform. The reference mark generating system is employed as the means for ensuring that the apparatus is maintained with all light rays in region 23 collimated. This is by mounting the reference mark generating systems 13, 15, 17 and 19 in a self contained assembly 99 which can be factory adjusted for collimated light output, such adjustment being protected from later intervention by operators of the total apparatus. The critical adjustment requires the aperture plate 15 to be set at the principal focus of the objective lens 17. As these aperture plates 15 are provided in the form of ultra thin discs, the assembly 99 can still be provided with means for operator changing of apertures without affecting the collimation.

Adjustment to suit individual operator eye characteristics is then by way of conventional focusing eyepieces, first bringing the reference ("floating") marks into sharp focus , and then bringing the photographic plates 1 into focus by rack and pinion means (not shown) for adjustment of the main objective lenses 9. Once the main objective lenses 9 are adjusted for a given photographic plate format, the only adjustment required should be with focusing eyepieces. If the reference marks are sharply focused the apparatus must be correctly collimated. Figure 7 shows a side view of the platform 25 with the microscope turret 65 assembly and eyepiece assembly removed.

The apparatus is typically connected with a personal computer such as an IBM P.C. or clone which acts as host computer for analytical computing processes, human and peripheral interfaces, and control of the stepping motors 47 and 51. Microelectronics for driving the stepper motors 47 and 51 are incorporated in custom designed boards fitted to expansion slots in the P.C. The design of these boards is considered to be non inventive and capable of construction by any competent electronics engineer familiar with computer and instrument interfaces.

Keyboard, mouse and/or digitising tablet (not shown) are required for operator interface via the P.C. to the apparatus. In its preferred embodiment the stable platform 25 with its mechanical and optical sub-assemblies is very shallow and allows the apparatus to be raised on legs 101 in a manner which permits keyboard, digitising pad or mouse working area to be accommodated underneath the apparatus. This provides for a desktop working area or footprint which is very substantially less than prior art large or small systems. Figure 8 shows a rear projection screen system replacing binocular eyepieces on top of the microscope turret 65 to facilitate viewing of the images from the photographic plates 1 on a single screen. There are several known means for producing a screen display. Figure 8 assumes the use of polarised light as disclosed in U.S. Patent 4,057,336. Polarising filters 103 are placed immediately above the telescope apertures. Thus, the light rays from each of the respective photographic plates 1 is appropriately polarised. A operator would then wear appropriate polarising spectacles. Light rays from the left hand side of Figure 8 pass through prism 105 where they are orthogonally projected onto combined prism and beam splitter 107. Light from the right hand side of Figure 8 is passed through the prism and beam splitter 105 and combined with the light beams that pass from prism 105. Thus, the two images are combined but are polarized differently. The combined light beams are then orthogonally projected from the prism and beam splitter 105 through projection lens system 109 onto a first mirror 111 and then onto a second mirror 113 and ten onto a screen 115.

The telescope turret 65 may be lifted and image rotation devices such as dove prisms or various mirror assemblies inserted beneath without any degradation in performance other than a possible reduction in the wide field available with the unmodified apparatus. Such devices may be manually or computer controlled to eliminate the relatively crude "Kappa" slide rotation adjustment with otherwise is conventionally provided at the plate carrier. Automatic Kappa rotation is now being introduced in some state-of-the-art prior art systems and is essential to exploit many of the potential applications in close range photogrammetry with converging photography, for instance in medicine and ophthalmolog .

Claims

CLAIMS :

1. A photogrammetric optical apparatus for obtaining information from a stereo-pair of optical images, said apparatus having means for supporting a first optical image source and means for supporting a second optical image source in spaced apart relation, means for independently moving the x, y co-ordinates of at least one image relative to the x, y co-ordinates of the other and means for obtaining optical images from each optical image source, and information obtaining means for obtaining information from each optical image, respective reference mark light ray generating means for each of said optical images, characterized in that all of the optical light rays to said information obtaining means are collected in a single region where they are all collimated.

2. Apparatus as claimed in Claim 1, wherein optical means are provided in said region to change the direction of collimated light to said information __* obtaining means .

3. Apparatus as claimed in Claim 2, wherein said^' optical means comprises beam splitter and prism means.

4. Apparatus as claimed in Claim 3, wherein said beam splitter and prism means comprises separate beam splitters and prisms for said first optical image and said second optical image.

5. Apparatus as claimed in Claim 1, wherein said reference mark light ray generating means comprises two light ray generators, one being for said first optical image and the other being for said second optical image.

6. Apparatus as claimed in claim 5, wherein there is a combined beam splitter and prism for the light rays for said first optical image and from the light ray generator for that image whereby the light rays can be combined and wherein there is a similar combined beam splitter and prism for the light rays from said second optical image and from the light ray generator for that image.

7. Apparatus as claimed in Claim 1, including telescope means for receiving collimated light rays from said region prior to being received by said information obtaining means.

8. Apparatus as claimed in Claim 1, including objective lens means fixedly focused relative to said light ray generating means, so that light rays from said reference mark generating means will, in use, be preset to be collimated in said region and so that when said information obtaining means sees a focused reference mark, it will be focused at infinity.

9. Apparatus as claimed in Claim 1, including two camera means for receiving collimated light from said regio .

10. Apparatus as claimed in Claim 1, wherein said first image and said second image are-derived from plannar photographic transparencies, and wherein said means for supporting the images supports them in a common plane.

11. Apparatus as claimed in Claim 10, wherein said common plane is in a substantially horizontal plane and wherein said information obtaining means receives light rays eminating in a generally orthogonal direction to said common plane from said region, there being mirror means for directing light rays passing orthogonally from said first optical image and said second optical image in a substantially horizontal direction to said region.

12. Apparatus as claimed in Claim 11, comprising a rigid platform supporting said means for supporting said first optical image and said second optical image so said first optical image will be to one side of said region and said second optical image will be on the opposite side of said region.

13. Apparatus as claimed in Claim 12, including adjustable focus, objective lens means underneath said table for permitting adjustment of the focus of the light rays from said first and said second optical images to be collimated in said region.

14. Apparatus as claimed in Claim 12 or Claim 13, including objective lens means fixedly focused relative to said reference mark generating means, so that light rays from said reference mark generating means will , in use, be preset to be collimated in said region and so that when said information obtaining means sees a focused reference mark it will be focused at infinity.

15. Apparatus as claimed in Claim 14, wherein said reference mark generating means comprises two LED devices, one being for providing a reference mark for said one optical image and the other being for providing a reference mark for said second optical image.

16. Apparatus as claimed in Claim 15, wherein light rays from said reference mark generating means are directed substantially in a horizontal direction to said region.

17. Apparatus as claimed in Claim 12, including a cover for said platform, said cover being closeable onto said platform, there being illuminating means in said cover which is aligned relative to said first and said second optical images when it is closed onto said platform whereby to provide light rays from said first and said second optical images.

18. Apparatus as claimed in Claim 16, including beam splitters and prisms in said region to direct light rays from said first optical image and said second optical image and said reference mark generating means orthogonally to a plane of said platform.

19. Apparatus as claimed in Claim 18, where said beam splitters and prisms combine light rays from said first optical image with light rays from said reference mark generating means, and combine light rays from said second light ray generating means with light rays from said reference mark generating means.

20. Apparatus as claimed in Claim 18, including light ray rotation means for rotating light rays received from said region prior to being received by said information obtaining means.