CAMERA WITH IMAGE REDUCTION RELAY SYSTEM

Abstract: A focal reducing lens system (5) reduces the image diameter of a conventional photographic objective lens system (2), so as to increase the relative aperture and hence the speed of the objective lens system (2). A high resolution image detector, such as a CCD sensor, may be placed at the focal plane (6) of the focal reducing lens system (5). The focal reducing lens system (5) can be based on a modified Hawkins and Linfoot system including a meniscus lens (8) and a spherical mirror (7). It may alternatively be based on Schmidt, Bouwers or Baker-Nunn lens systems.

Description: CAMERA WITH IMAGE REDUCTION RELAY SYSTEM Field Of The Invention The present invention relates to high speed, high resolution camera systems. In particular, this invention relates to a high speed, high resolution camera system which employs a conventional photographic camera subsystem and which is used with high resolution, solid-state image detectors such as CCDs, or the like. Background Of The Invention Photographic camera systems are well known with many variations in existence. These camera systems are generally designed to produce an image of reasonably high resolution onto a photographic film, and to have as high a relative aperture, or smallest f-number value, as possible. Of course, there is a trade off between resolution and relative aperture. The optimum practical solution is achieved by considering the properties of the photographic film to be used with the camera. Of particular interest is the resolution of commonly used photographic film The resolution is limited by scattering within the film emulsion itself, and is typically approximately 20 microns. Simply put, the designers of these camera systems trade off resolution for relative aperture, whilst minimising cost, until the resolution of the image produced approaches the resolution of the film being used. Photographic camera systems are available which are relatively cheap and of high quality. In particular, many relatively cheap systems are available in the industry standard 35 millimetre photographic film format. Also of interest is the type of film used. These vary in speed - the speed depending largely on the size of, for example, the silver halide crystals in the emulsion. Unfortunately, there tends to be a trade off between speed and resolution in film also. Some attempts to provide high-speed camera systems, whilst maintaining relatively high resolution, approach the problem by using large diameter lens systems with correspondingly large "entrance pupils". The large entrance pupil results in a high speed, or relative aperture, as illustrated by the following relation: (1 ) Relative aperture = D/f, where D is the diameter of the entrance pupil and f is the focal length. Where larger values of the relative aperture (D/f) correspond to smaller values of the f-number and hence higher speeds. The higher the speed of the optical system, the more energy gathered and the brighter the image. These systems suffer the disadvantages of cost and weight of the large diameter lens systems. Also, large lens systems often suffer constructional defects which, in practice, limit the resolution of images produced. In the case of any system using photographic film, the resolution of an image is ultimately limited by the resolution and exposure requirements of the film used. Solid state electronic image detectors such as CCDs, or other semiconductor detectors, generally have higher resolutions than photographic films. Typically, the resolution of a CCD detector will be approximately 5 to 10 microns compared to approximately 20 microns for photographic film. Also, solid state detectors generally have a smaller area than the 35 millimetre film. There is clearly an advantage in providing a camera system for use with a CCD, or similar, which provides an image of the resolution of a CCD, or similar. This will provide a faster system which is possibly of higher resolution than conventional 35 millimetre film. Conventional video camera systems attempt to do this by using lens systems that produce an image of the resolution of a CCD, or similar. However, to provide such a relatively tightly focused, low aberration image requires relatively many lens elements to be used. This adds to the cost and reduces the speed of the system. Accordingly, it is an object of the present invention to provide a high relative aperture camera system which employs a photographic camera subsystem and produces a relatively high resolution image, or to at least provide the public with a useful choice. It is a further object of the present invention to provide a high relative aperture camera system optimised for use with solid-state image detectors and which employs a photographic camera subsystem, or to at least provide the public with a useful choice. Other objects of the present invention will become apparent to those skilled in the art from the following description. Summary Of The Invention According to the present invention, there is provided a camera system comprising: a photographic camera subsystem having a characteristic first aperture and a focus at a first focal surface; a focal reducing lens relay subsystem having a characteristic second aperture and a focus at a second focal surface having a characteristic image diameter; wherein the focal reducing lens relay subsystem is arranged to reduce the image diameter of the photographic camera subsystem; and - wherein the subsystems are arranged such that the relative aperture of the camera system is greater than the relative aperture of the photographic camera subsystem considered alone. Preferably the camera system is arranged such that substantially the entire first aperture is mapped onto the second aperture. In one embodiment, this is effected by including a field lens subsystem in the camera system. Preferably the camera system is adapted to be capable of incorporating an image detector. Preferably the camera system is adapted to be capable of incorporating an electronic image detector at the second focal surface, the electronic detector having a characteristic pixel resolution and a characteristic effective detector area. Preferably the camera system is adapted to be capable of incorporating an electronic image detector in the form of a semiconductor image detector. The detector may be a charge coupled device array. Preferably the camera system is arranged so that the first focal surface diameter is mapped onto the effective detector area of an image detector provided. Preferably the focal reducing lens relay subsystem comprises a Hawkins and Linfoot arrangement modified to act as a focal reducing subsystem. Preferably the Hawkins and Linfoot arrangement includes an aberration compensating means to compensate for aberrations introduced by the other parts of the Hawkins and Linfoot arrangement. Preferably the aberration compensating means comprises a meniscus lens. Preferably the Hawkins and Linfoot arrangement includes a chromatic distortion compensating means to compensate for chromatic distortion introduced by other parts of the Hawkins and Linfoot subsystem. Alternatively, the chromatic distortion compensating means may be situated elsewhere in the optical system as would be evident to those skilled in the art. Preferably the chromatic distortion compensating means comprises a lens doublet. Alternatively, the chromatic distortion compensating means may comprise a lens triplet. Alternatively, to a Hawkins and Linfoot arrangement, the focal reducing, lens relay subsystem may comprise any one of the following arrangements particularly adapted to act as a focal reducing system: Schmidt Bouwers Baker-Nunn Others known to those skilled in the art. Preferably the camera system includes an adjustable aperture thereby allowing adjustment of the input aperture of the camera system. Preferably the focal reducing, lens relay subsystem is arranged so that the image diameter of the photographic camera subsystem is reduced as much as possible before the resolution of the image at the second focal surface falls below that of the pixel resolution of the electronic detector provided. This is where aberrations introduced by the focal reducing lens relay subsystem cause an image formed by the camera system of a point source to be substantially the size of a pixel of an electronic image detector provided. Other aspects of the invention will become apparent to those skilled in the art from the following description. Brief Description Of The Drawings Figure 1 : Schematically shows the configuration of an embodiment of the present invention. Figure 2: Schematically shows the configuration of the original Hawkins and Linfoot camera arrangement, intended for imaging an object at infinity. Figure 3: Schematically shows the configuration of a modified Hawkins and Linfoot camera, intended for finite conjugate imaging, and with an external focus. Detailed Description Of The Invention Figure 1 shows a camera system 1 according to an embodiment of the present invention. The camera system 1 includes a standard photographic camera lens system 2 having a characteristic aperture 3 and a focus at the focal surface 4. In this case, the focal surface is a plane, although it is also possible for this to be part of a sphere, parabola or some other type of surface. The useable image projected onto the focal surface will typically have an industry standard useable diameter of 45 millimetres which is suitable for industry standard 35 millimetre camera format for which good quality, relatively cheap camera lens systems are readily available. Of course, other camera formats may alternatively be employed. The resolution of the image at the focal surface of the photographic camera subsystem will typically be approximately 20 microns to match the resolution of typically used film. The speed, or inverse of the relative aperture or f-number, of the photographic camera subsystem is denoted by f/D, where f is the focal length of the photographic camera subsystem and D is the diameter of the entrance pupil, which is determined by the diameter of the characteristic aperture 3. Increasing the relative aperture results in a reduction of the f- number which, according to convention, thereby increases the speed of the optical system. For example an f/1 .4 lens system is said to be twice as fast as an f/2 lens (the area of the entrance pupil being proportional to the square of the pupil diameter). The speed of the photographic camera subsystem may be determined by the relation 1 given above. Figure 1 also shows a camera subsystem in the form of a focal reducing lens relay system 5. A preferred arrangement for this focal reducing subsystem is the Hawkins and Linfoot camera modified to act as a focal reducer. Suitable modifications will be apparent to those skilled in the art. However, other suitable focal reducing subsystems may consist of any one of the arrangements known in the art including: Schmidt; Bouwers; and Baker-Nunn. Other suitable arrangements, which are known to those skilled in the art, may also be used. The preferred Hawkins and Linfoot arrangement, shown in Figure 1 , includes a spherical mirror 7 and a meniscus lens 8, and is modified from the original ( 1 945) arrangement shown in Figure 2 so as to operate with a finite object conjugate. The meniscus 8 serves to correct for and minimise aberrations introduced by the spherical mirror 7. The focal reducing subsystem may also include a chromatic distortion compensating system to compensate for chromatic distortion introduced by the meniscus 8. This system may be a lens doublet, lens triplet or any other suitable system, which are commonly known in the art. Alternatively, the chromatic distortion compensating means may be situated elsewhere in the optical system as would be evident to those skilled in the art. The camera system may also be provided with an image detector 9. The detector 9 will typically consist of an electrooptic image detector such a Charge Coupled Device (CCD) array, or some other semiconductor array, or an image intensifier tube. Any suitable electrooptic image detector known to those skilled in the art may be used. A common size for a CCD array may have a diagonal dimension of 1 1 millimetres, which corresponds to approximately 2/3 inch. However, other available "standard video" sizes range from 1 /4 inch to 1 inch. Image intensifier tubes range from 1 2 to 40 millimetres in diameter. As commonly available detectors are flat and the focal surface 6 of the focal reducing subsystem will be spherical, the image produced by the camera system will be improved if a field-curvature-correcting lens is bonded to the surface of the detector. Suitable lenses and, indeed, alternative arrangements to such a lens for compensating for the curvature of the focused image will be apparent to those skilled in the art. The camera system 1 also includes a field lens 10, which serves to match or couple the photographic camera subsystem aperture 3 to the focal reducing subsystem aperture 1 1 to reduce vignetting and also to ensure that the entire effective area of the detector 8 is utilised. Any suitable alternative to a field lens which is known in the art may be used. In use, various photographic camera subsystems may be interchanged with the camera subsystem. These will correspondingly have various characteristic apertures. Therefore, various field lenses will need to be interchanged accordingly. The focal reducing, relay lens subsystem acts to reduce the image created by the photographic camera subsystem to produce a smaller, brighter image at the focal surface 6 of the focal reducing subsystem. This reduction in the focus, or image diameter, corresponds to a larger relative aperture, or lower f- number for the camera system than the relative aperture of the photographic camera. The effect of the focal reducing lens relay subsystem can be clearly demonstrated by considering an image produced by the photographic camera subsystem, and also the camera system as a whole, of a point source of light. At the focal surface of the photographic camera subsystem, the point source typically will be imaged as a distorted, blurred spot having an average diameter of approximately 20 microns, for example. The shape and diameter of this image will be a product of the aberration and chromatic distortion of the photographic lens system. As mentioned above, these aberrations have, simply put, been allowed to increase as a trade off for relative aperture of the subsystem to the point where the approximate size of the image approaches the minimum image size capable of resolution by the photographic emulsion for which the subsystem has been designed to be used. The focused image of the point source focused at the focal surface of the photographic camera subsystem is reduced by the focal reducing subsystem onto the focal surface 6 of the focal reducing lens relay subsystem. An ideal focal reducer would reduce the image diameter in accordance with its focal reducing ability. This would, of course, produce a reduced blur spot corresponding to a higher resolution image. However, a real focal reducing, lens relay subsystem will itself introduce aberrations. These will primarily be high-order sphero-chromatic aberrations caused by the residuum of the aberration correction of the lens relay subsystem. By relation 1 given above, the relative aperture of the camera system as a whole, will increase as the focal length is further reduced. However, as mentioned above, the aberrations introduced by the focal reducing subsystem will act to increase the minimum image size. The optimum degree of focal reducing is where the size of an image of a point source approaches the minimum spot which can be resolved by the detector used with the camera system. In the case of a typical CCD, this will be approximately 5 to 10 microns. The image produced at the focal surface 6 of the focal reducing, lens relay subsystem is reduced, and correspondingly brighter, with respect to the image produced by the photographic camera subsystem alone. By this means, the camera system takes advantage of the higher resolution of the electrooptic detector to provide a faster imaging system. Such a system can additionally be advantageous to the low light imaging properties of some electrooptic image detectors available. Although a telescopic lens arrangement is shown in Figure 1 , it will be clear that any photographic camera arrangement may be used. These may include zoom, wide angle or even "fish eye" lenses. The photographic camera arrangement may include any known devices or means to focus or vary the aperture of the photographic camera subsystem. It will also be apparent to those skilled in the art that the camera system may be arranged to have a variable input aperture. Figure 3 shows an alternative focal reducing lens relay subsystem 1 1 which is, essentially a further modified Hawkins and Linfoot camera arrangement. This alternative subsystem is primarily arranged for a finite object conjugate as in Figure 1 , but also so that the focal surface 1 2 to be conveniently positioned at the rear of the camera system. This alternative arrangement replaces the focal reducing mirror 7 of the above-mentioned arrangement with a flat folding mirror 1 3 and spherical mirror 14. The action of these mirrors or further alternatives to these mirrors will be apparent to those skilled in the art. Similarly to the above-mentioned arrangement 5, the alternative arrangement 1 1 includes a chromatic distortion compensating doublet 1 5, a meniscus lens 1 6 and a field lens 1 7. The meniscus lens 1 6 has been placed in front of the chromatic distortion compensated doublet 1 5, as opposed to the arrangement of Figure 2, so that the meniscus lens 1 6 is closer to the image thereby allowing a smaller meniscus lens 1 6 to be used. This is merely a constructional detail which will be apparent to those skilled in the art. It will be clear to those skilled in the art that the focal reducing lens relay subsystem can be arranged to position the focal surface 1 2 at virtually any desired position by suitable arranging or substituting of optical elements. Commercially available, high quality telephoto photographic camera subsystems may have a relative aperture of between f/2 and f/3. A camera system in accordance with the present invention using such a telescopic lens will typically have a relative aperture of f/0.8. Therefore, it is clear that the present invention provides the advantage of an exceptionally high speed camera system. Due to the use of commercially available photographic camera subsystems, a camera system in accordance with the present invention will also be relatively cheap and/or easy to construct. A system in accordance with the present invention also lends itself to employing relatively small, lightweight photographic camera subsystems which facilitate minimisation of the size and weight of the camera system as a whole. Where in the foregoing description reference has been made to specific components or integers of the invention having known equivalents then such equivalents are herein incorporated as if individually set forth. Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be understood that modifications or improvements may be made thereto without departing from the scope of the appended claims.

Claims: Claims A camera system comprising: a photographic camera subsystem having a characteristic first aperture and a focus at a first focal surface; a focal reducing lens relay subsystem having a characteristic second aperture and a focus at a second focal surface having a characteristic image diameter; wherein the focal reducing lens relay subsystem is arranged to reduce the image diameter of the photographic camera subsystem; and wherein the subsystems are arranged such that the relative aperture of the camera system is greater than the relative aperture of the photographic camera subsystem considered alone. A camera system as claimed in claim 1 wherein the camera system is arranged such that substantially the entire first aperture is mapped onto the second aperture. 3. A camera system as claimed in claim 2 wherein the entire first aperture is mapped onto the second aperture by including a field lens subsystem in the camera system. 4. A camera system as claimed in any preceding claim wherein the camera system is adapted to be capable of incorporating an image detector. 5. A camera system as claimed in any preceding claim wherein the camera system is adapted to be capable of incorporating an electronic image detector at the second focal surface, the electronic detector having a characteristic pixel resolution and a characteristic effective detector area. 6. A camera system as claimed in any preceding claim wherein the camera system is adapted to be capable of incorporating an electronic image detector in the form of a semiconductor image detector. 7. A camera system as claimed in any preceding claim wherein the camera system is arranged so that the first focal surface diameter is mapped onto the effective detector area of an image detector provided. 8. A camera system as claimed in any preceding claim wherein the focal reducing lens relay subsystem comprises a Hawkins and Linfoot arrangement modified to act as a focal reducing subsystem. 9. A camera system as claimed in claim 8 wherein the Hawkins and Linfoot arrangement includes an aberration compensating means to compensate for aberrations introduced by the other parts of the Hawkins and Linfoot arrangement. 10. A camera system as claimed in claim 9 wherein the aberration compensating means comprises a meniscus lens. 1 1 . A camera system as claimed in any one of claims 8 to 10 wherein the Hawkins and Linfoot arrangement includes a chromatic distortion compensating means to compensate for chromatic distortion introduced by other parts of the Hawkins and Linfoot subsystem. 1 2. A camera system as claimed in claim 1 1 wherein the chromatic distortion compensating means may be situated elsewhere in the optical system as would be evident to those skilled in the art. 13. A camera system as claimed in any one of claims 9, 1 1 or 1 2 wherein the chromatic distortion compensating means comprises a lens doublet. 14. A camera system as claimed in any one of claims 9, 1 1 or 1 2 wherein the chromatic distortion compensating means may comprise a lens triplet. 1 5. A camera system as claimed in any one of claims 1 to 7 wherein the focal reducing, lens relay subsystem may comprise any one of the following arrangements particularly adapted to act as a focal reducing system: Schmidt; Bouwers; Baker-Nunn; or others known to those skilled in the art. 1 6. A camera system as claimed in any preceding claim wherein the camera system includes an adjustable aperture thereby allowing adjustment of the input aperture of the camera system. 1 7. A camera system as claimed in any preceding claim wherein the focal reducing lens relay subsystem is arranged so that the image diameter of the photographic camera subsystem is reduced as much as possible before the resolution of the image at the second focal surface falls below that of the pixel resolution of the electronic detector provided. 18. A camera system substantially as herein described and with reference to the accompanying drawings.

Inventors: BEACH ALLAN DAVID, BEACH, ALLAN, DAVID
Applicants: BEACH ALLAN DAVID, IND RES LTD, BEACH, ALLAN, DAVID, INDUSTRIAL RESEARCH LIMITED
Application year: 1998
Main classification: G03B17
IPC codes: G02B13, G02B17, G03B17, G03B19, H04N5