GB1586587A - Photographic camera - Google Patents

Description

(54) PHOTOGRAPHIC CAMERA (71) We, POLAROID CORPORATION, a corporation organised under the laws of the State of Delaware, United States of America, of 549 Technology Square, Cambridge, Massachusetts, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to photographic cameras.

Conventional snapshot cameras derive compactness principally from the use of film having a relatively small frame format and, in some instances, a collapsing bellows arrangement. For 35 millimeter cameras, the film frame is typically 24 millimeters by 36 millimeters. Since the focal length of the objective lens of the camera is usually at least as long as the diagonal of the film frame to provide a useful angular field, the lens-to-film plane distance would then be of the order of 43 millimeters (approximately 1 and 11/16 inches). (Of course, wide angle lenses for 35 millimeter cameras can have a significantly shorter focal length). In conventional cameras where the light from the lens follows a straight path in air to the film, the focal length is therefore a controlling limitation bn the front to back dimension of the camera. A feature of this construction is that there is room for only one or two optically active surfaces between the lens and the film which can reflect or scatter light. Also, the relatively small film format reduces the likelihood that stray light from outside the angular field of the lens, or other unwanted light, will reach the film.

The difficulties in achieving a camera having a compact size are greatly increased when the camera uses self-developing film, such as that marketed by the Polaroid Corporation. While in conventional photography, a relatively small negative can produce a relatively large print or a viewable image by enlargement or projection, the exposed photographic area on self-developing film must be the same as that of the actual developed print. As a result, instant cameras, i.e.

designed for use with self-developing film, generally have a significantly longer focal length than that found in compact cameras using conventional film. Also, instant cameras generally require a fairly high speed lens to provide sufficient light to expose the film at exposure times useful in hand-held cameras for general purposes. These requirements of self-developing film therefore present significant constraints on the design of a compact camera.

Early solutions to the problem of achieving compactness in an instant camera involved mechanical folding arrangements to move the objective lens in a direction perpendicular to the film plane. A folding bellows encloses the camera space between the objective lens and the film plane. More recently, cameras developed and marketed by the Polaroid Corporation utilize a reflective element in the exposure chamber to fold the optical path between the lens and the film plane. A collapsible version of such a camera which achieves a high degree of compactness is described in U.S. Patent No. 3,753,392. Another instant camera of this type which employs a reflective element is described in U.S. Patent No. 3,938,167 and U.S. Patent No. 3,940,774. This camera is noncollapsible, or rigid, and therefore has a lower cost of manufacture than the collapsible camera, but suffers from a comparatively bulky, cumbersome configuration.

U.S. Patent No. 3,818,498 discloses a compact camera designed for selfdeveloping film which employs a pair of spaced apart, mutually inclined reflective elements to generate a multiply-folded optical path. One of the elements is fully reflective, while the other is partially reflective. The partially reflective element overlies a selector element formed of mechanical light collimators held in a transparent medium and which overlies the film. While this arrangement may achieve a highly folded optical path, it nevertheless suffers from several disadvantages. First, at least a portion of the light reaching the film plane undergoes multiple reflections from the partially reflective element. At each reflection the incident light looses a significant portion of its entensity. As a result, the intensity of the light reaching the film plane is generally low and of varying values depending on the number of reflections the light has undergone. Another problem is that use of mechanical collimators in the selector element and their location direction over the film plane causes them to cast a shadow on the film, or produce granularity, or defocus.

While the light folding properties of prisms are well known, the principal uses of prisms have been in nonphotographic optical instruments such as binoculars, telescopes, periscopes rangefinders and spectrometers. Many applications rely on the ability of a prism to redirect by total internal reflection light incident on a surface adjacent a medium of a lower index of refraction at an angle greater than a critical angle. Common prisms which utilize this property are the Porro prisms (of first or second kinds) commonly employed in binoculars. Other common prism configurations such as Dove, Lehman, and Amici prisms also use total internal reflection for image inversion, field rotation or scanning. In these applications while the incident light beam is reflected one or more times, and hence is to some extent "folded," the main purpose of the prism element are not to fold the optical path to achieve compactness, but rather to redirect, laterally displace, invert, split, combine or rotate the beam or beams.

Another prism utilizing multiple internal reflections is the so-called Schmidt prism. One characteristic of the Schmidt prism is that a portion of one prism face can provide total internal reflection while another portion is transmissive. Schmidt prisms, singly and in matched pairs, have also found applications in optical instruments. A discussion of some applications can be found in applicant's article "Optical Systems for Telescopes and Binoculars" at pp. 435471 of Summary of Technical Report of Division 16, NDRC, Volume 1, Optical Instruments (Wash. D.C.

1946).

U.S. Patent No. 3,417,685 discloses a matched pair of Schmidt prisms operating as a field rotator in a microscope. In this patent, as is common with optical instruments such as telescopes and periscopes, photographic apparatus can be attached to the eyepiece to record the output image of the instrument.

Heretofore, prisms have been used in cameras principally as image directing elements in viewfinders. For example, many 35 millimeter single lens reflex cameras employ a roof pentaprism to direct light from a deviating mirror to the viewfinder eyepiece. U.S. Patent No. 3,819,255 discloses a more complex viewfinder structure employing an opposed pair of Schmidt prisms that are mutually rotatable about a fixed pivot with an air gap separating the opposed faces.

A portion of each opposed face internally reflects the incident light beam and another portion transmits or receives the light beam. It is noteworthy, however, that the light transmission through these prisms is over a relatively small portion of the opposed prism faces. Further, the light transmission to a viewfinder does not require the optical quality or transmission efficiency necessary for light transmission to photographic film. Also, unwanted or stray light, and the loss of light intensity, are not as critical in viewfinder optics as in the image-path optics of the camera.

U.S. Patent No. 2,784,645 discloses a prism element located within the exposure chamber of a camera and forming part of the optical path between the objective lens and the film plane. More specifically, this patent teaches that prism elements are disposed in a stereoscopic camera to laterally displace two light beams each originating at separate objective lenses so that they are recomposed in a side by side relationship on two halves of a single film frame. This displacement function is roughly analogous to that of roof prism pairs in binoculars. This patent also deals with numerous optical design problems generated by the prism elements, including such prism characteristics as distortion astigmatism, chromatic aberration, spherical aberration, light absorption, the weight of the prism elements, and the elimination and/or control of stray light. Another design consideration is that the use of prisms in cameras increases the back focal distance for a given focal length. The ability of the prism to fold the optical path, particularly with a high index prism, however, can more than offset this increase. It should also be noted, however, that this "foldability" does not necessarily result in compactness.

In particular, this disclosure teach the desirability of spacing the exit face of the prism at a practical distance from the film plane in order to avoid abrasion and to avoid having dust or other irregularities present on the face of the prism cast a shadow on the film. The large spacing also has the advantage of allowing the use of prisms having a smaller size, which in turn reduces the absorption of the light within the prism and reduces the weight which the prism elements add to the overall optical system.

It is also clear from this disclosure that a material such as glass with a relatively high index of refraction, typically 1.6 or 1.7, is preferable compared to a plastics material -- typically with an index of refraction only near 1.5 - both in terms of optical efficiency and of "foldability." A camera according to the present invention, for producing an image of an object on film having its photosensitive surface coincident with a focal plane in the camera, comprises: a housing enclosing an exposure chamber, a lens mounted on the housing for focusing at the focal plane light from within a selected angular field of view and optical means within the exposure chamber for forming a folded optical path between the lens and the focal plane, the optical means including prismatic means having a first face arranged to receive light from the lens, a second face overlying and spaced from the focal plane and having at least one portion arranged to receive light which has travelled along the said optical path from the lens and to redirect the said light by total internal reflection, and at least one additional face arranged to receive light which has travelled along the said optical path from the second face for reflecting the said light back on to the second face transverse thereto for transmission through the second face to the focal plane.

In the preferred form of camera embodying the invention, the second face is closely spaced from the focal plane, and forms an acute angle with respect to the additional face. In this preferred form, the said first, second and additional (third) faces are the only major faces in the optical path and the angle of the first face with respect to the second face is also acute but is greater than the acute angle between the second and third faces. If desired, light from the lens may be reflected by a mirror prior to entry into the prism through the said first face, depending upon whether an odd or an even number of reflections are required in the camera.

In a further form of camera embodying the invention, the additional reflecting face is within the prism and redirects light from the first face on to the second face.

If desired, in a prism having three optical faces in the optical path, the first face can be obliquely arranged with respect to the second. The prismatic means may be of glass or of a synthetic plastics material or may comprise a liquid encased in a thin transparent shell.

In another advantageous form of camera embodying the invention, the imagerepresenting light undergoes two internal reflections within the prismatic means between its total internal reflection by the said second face and its subsequent transmission through the second face to the film plane, one of the said two further internal reflections being at the abovementioned additional face. If desired, a reflection can take place at a mirror or one or more reflections may take place within an entry prism prior to the introduction of the light into the abovementioned prismatic means. The second face of the prism is preferably planar and closely spaced from the film plane.

However, in another form of camera embodying the invention, there is a beam-splitter dividing the image-representing light into two optical paths and the second or exit face of the said prismatic means comprises two planar surfaces inclined to one another at an oblique angle along the centreline of the film plane; each of these surfaces is transmissive to light which has been totally internally reflected from the other surface of the exit face. An emergent prism is interposed between the exit face of the abovementioned prismatic means and the film plane, the exit face of the emergent prism being closely spaced from the film plane.

The invention permits the construction of a camera which is both rigid and compact and is suitable for use with self-developing film.

In order that the invention may be better understood, some examples of cameras embodying the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a simplified perspective view of a compact prism camera embodying the invention Figure 2 is a view in vertical section taken along the line 2-2 of Figure 1; Figure 3 is a top plan view of the prism element shown in Figure 2; Figure 4 is a view corresponding to Figure 2 showing an alternative embodiment of the invention in which light emergent from the objective lens undergoes an even number of reflections before reaching the film plane; Figure 5 is a simplified perspective view of a further compact, rigid prism camera embodying the invention; Figure 6 is a view in vertical section taken along line 6-6 of Figure 5 and showing the prism elements within the camera and the highly folded optical path they provide; Figure 7 is a view corresponding to Figure 6 showing an alternative embodiment of the invention; Figure 8 is a simplified perspective view of another compact, rigid camera embodying the invention employing split field and split aperture; Figure 9 is a view in horizontal section taken along line 9-9 of Figure 8 and showing the optical elements and the highly folded optical path they provide; Figure 10 is a view corresponding to Figure 9 showing an alternative embodiment of the invention where the prism elements are of plastic material; and Figures 11 and 12 are view corresponding to Figures 6 and 9, respectively, and showing alternative embodiments of the invention in which the light emergent from the objective lens undergoes an even number of reflections before reaching the film plane.

Figures 1 and 2 show a camera 10 having a rigid housing 12 that is light tight except at selected openings and that supports various optical and mechanical elements. The housing forms an exposure chamber 13, and a compartment 14 for supporting, along one wall of this chamber, a cassette 15 of self-developing film units. The. film cassette 15 is preferably of the type available for the camera marketed by the Polaroid Corporation under the registered designation SX-70. The uppermost film unit in the cassette is presented for exposure with its upper lightsensitive surface lying in and coextensive with a film plane 16.

With reference to Figures 2-3, the housing 12 supports an objective lens 18 having an optical axis 19 and mounted in a support structure 20. A frontal portion 22 of the structure 20, adjacent the front-surface of the lens 18, forms a field stop, the design of which is well known to those skilled in the art. Opening 241 in the structure 20 adjacent the exit surface of the lens 18 forms an aperture stop, also of well known design. The lens 18 and the stops define an angular field of view in object space external to the camera, and form an exit pupil or diverging beam of light rays in image or camera space. The beam of useful light rays is represented in FIGURE 2 by three light rays, a chief ray 28 that is generally coincident with the optical axis 19, and a pair of extreme rays 30 and 32 that represent light rays originating from the extreme upper and lower edges, respectively, of the angular field of view. Each of these rays is directed along a folded optical path between the lens 18 and the film plane 16 by a mirror 134 and a prism 136 of generally triangular cross section in a plane such as that of FIGURE 2.

The camera 10 and the other embodiments of the invention described below typically include other elements which for clarity of description are not shown.

These elements, which may be of conventional design, include a shutter, a viewfinder, focus adjustment means, and means for advancing units out of the film cassette 15 for processing. An opposed pair of spread rollers 43 receives an exposed film unit removed from the cassette in this manner, and initiates the development process in a well known manner. Means, not shown, are also provided for opening the film compartment 14 to facilitate insertion and removal of the cassette 15.

A principal feature of this invention is the beam folding prism 136 in the exposure chamber 13 and which defines at least part of the optical path between the lens and the film plane. With particular reference to FIGURES 2 and 3, the prism 136 is a block of a transparent optical material with a generally triangular cross section. The prism has three main optically active faces, an entrance face 144, an exit or reflective/transmissive face 146 that intersects the entrance face 144 at an acute angle a, and a reflective roof face 148 that intersects the exit face at an acute angle b smaller than angle a. The exit face has a front portion 146a (i.e., a portion located, along the length of the exit face, adjacent the angle a and shown on the left side of FIGURE 2) that is coated with a reflective material, and a rear portion 146b that is preferably optically finished and coated for low reflection. The entrance face 144 preferably also has a clear polish finish, and each optically active face 144, 146,148 is preferably highly flat, especially the entrance face 144 and the reflective roof face 148.

In general the dimensions of the prism are selected to enhance the compactness of the camera, reduce its weight, and facilitate the folding of the light beam while avoiding vignetting. More particularly, as shown in FIGURE 3 the sides 49 of the illustrated prism 136 are trimmed to reduce the prism bulk. At least one lateral step-like serration 152 extends between the upper edge of the entrance face 144 to the forward edge of the roof face 148.

The exit face portion l46b on the prism overlies and is closely spaced from the film plane 16. This face portion is at least coextensive with the film plane 16 and preferably extends laterally beyond all the edges of the film 17.

The exit face portion 146a and the roof face 148 are both coated, using well known techniques, with a reflective material such as aluminium or silver. While a silver coating reflects approximately 98% of the incident light and a similar aluminium coating is 90% efficient, aluminium is generally preferred for reasons of durability and adhesion. The configuration of the prism 136 of this invention is particularly conducive to use of aluminium since it requires few front surface reflectors and therefore the light intensity is not unduly diminished by these reflections. The serration 152 and the side surfaces 49 are preferably coated with a light absorbant material, preferably a black, index-matching paint.

The operation of the foregoing prism optical system can be understood through a consideration of its action on the rays 28, 30 and 32, as FIGURE 2 shows.

As it emerges from the lens 18, the principal or chief ray 28 is coincident with the lens optical axis 19 and forms an angle c with respect to the vertical, indicated by the arrow 57 and chosen to be coincident with the perpendicular or normal to the film plane. The ray 28 proceeds in air to the mirror 134 where it is reflected onto the entrance face 144. The optical axis 19, the angle of inclination d of the mirror 134 with respect to the vertical (arrow 57), and the angle of inclination a of the entrance face 144 are preferably chosen so that the ray 28 strikes the entrance face substantially along its perpendicular. With this configuration, since the exit pupil or image space light ray beam is generally in the shape of a diverging cone, the extreme rays 30 and 32 strike the face 144 at symmetrically small angles of incidence and hence enter the prism with slight refraction. In general it is desirable to minimize the angle of incidence to maximize the light transmission into the prism and to minimize the introduction of aberrations by the prism, such as chromatic aberration. While it will be understood that the lens 18 is designed to compensate for these prism aberrations, minimizing them facilitates the lens design and improves the image quality. It should also be noted that any undulations or departures from flatness in the entrance surface 144 will distort portions of the image formed at the film plane.

Within the prism 136, the principal ray 28 strikes the exit face portion 146b at an angle e that exceeds the critical angle of the prism material so that the ray is totally internally reflected toward the reflective surface 148. (For the purposes of this description, "internally reflected" means reflected by total internal reflection.) The total internal reflection transmits 100% of the incident light energy to the face 148.

The extreme rays 30 and 32 are refracted upon entering the prism and then proceed along straight line paths to the exit face 146. Although refraction by the prism increases the optical path length for these rays between the lens and the image plane, this lengthening is more than compensated by the folding of the optical path by the prism. The extreme ray 32 undergoes total internal reflection at the exit face portion 146b. However, to optimize the compactness of the prism, and hence of the camera 10, the extreme ray 30 strikes the exit face 146 at an anglef which is less than the critical angle. The reflective coating on the exit face portion 146a nevertheless redirects this ray to the face 148 The exit face portion 146a similarly reflects other rays of the light beam which lie between the ray 28 and the ray 30 and which would be at least partially transmitted from the prism but for the reflective coating. It will be understood that the longitudinal extent of the reflectively-coated face portion 146a is dependent in a known manner on a number of factors such as the index of refraction of the prism material, the slope of the entrance face (the angle a), the parameters of the objective lens 18, and the film format.

The rays 28, 30 and 32 reflected at the exit face 146 proceed to the roof face 148 where they are reflected back towards the exit face. The roof face 148 is angled with respect to the exit face 146 (angle b) so that ray 28 is reflected to travel to the exit face substantially along the perpendicular to that surface. As shown, the chief ray 28 strikes the face portion 146b directly along the perpendicular and the rays 30 and 32 strike it at high angles of incidence. All rays are accordingly transmitted through the face 146b with minimal disturbance, and travel across narrow air gap 158 to expose the film unit lying in the film plane. Although it is not essential that the transmitted rays be perpendicular to the exit face, this relationship is desirable to minimize dispersion, distortion, and astigmatism.

The exit face preferably is closely spaced from the film plane to minimize the overall height of the camera and to avoid magnification of image distortions. On the other hand, the spacing should be sufficiently wide to avoid abrasion of the exit face by the film as each unit is withdrawn from the cassette and to avoid imaging of dust particles on the exit face. A suitable spacing is around 2.0 millimeters. Since the exit face is extremely close to the image plane, it is important that this face be free of dust, scratches, finger prints and other foreign matter or irregularities that would cast a shadow on the film. While the gap or spacing 158 is preferably uniform, i.e. the face 146 is preferably parallel to the image plane 16, the exit face 146 can be inclined with respect to the film plane to yield a wedge-shaped gap. As with the parallel spacing, the face to film distance should not be narrow enough to abrade the face or wide enough to magnify image distortions or foreign matter on the exit face.

One advantage of the prism arrangement is that relatively little of the light reflected from the film is redirected back to it to expose another area of the film.

This improvement stems from the fact that light reflected by the film back into the prism through the exit face is precluded by the exit face (by internal reflection) from re-exiting from the prism to the film and the action on the film-reflected light of the low reflection coating applied to the exit face. The present camera arrangement thus reduces by at least a factor of ten the "flare" problem associated with some prior instant cameras employing a path-folding reflective element.

In the image path optics of a camera embodying the invention it is desirable to control unwanted light that may. reach and hence undesirably expose the film. A particularly troublesome problem in photographic apparatus is stray light which enters the objective lens from outside its angular field. With the alignment of optical elements shown in FIGURE 2, light originating from the lower portion of the field and proceeding generally in an upward direction along the arrow 57 does not present a problem. This stray light lies "outside" the extreme ray 30 from the chief ray 28. It will either fail to strike the mirror 134 and therefore be absorbed by the inner surface of the housing or some other light absorptive surface, or it will strike the forward edge of the mirror 134 and be reflected at an angle such that it fails to enter the prism. Stray light from the lateral portions of the field are readily absorbed by the prism side surfaces 49 (Figure 3). For this purpose it is desirable to limit the maximum width of the prism at its base to a dimension equal to or slightly in excess of the width of the film plane. It should be understood, however that an advantage of this invention is that comparatively large, wide film formats can be readily accommodated by increasing the width of the prism. Thus, for example, while a common film format is 80 millimeters by 80 millimeters, a camera according to this invention can be readily constructed to accommodate an 80 millimeter by 120 millimeter format.

On the other hand, stray light from the upper portion of the field, i.e. which lies outside the lower extreme ray 32 of the desired bundle of light, is more difficult to control since it can enter the prism either directly or by reflection from the mirror 134. It is to eliminate these rays that the serration 152 is provided between the prism entrance face 144 and the roof face 148. Stray rays strike the serration - 152 at an angle of incidence less than the critical angle so that they are transmitted to the light absorbent coating on the serration or to some other light absorbing surface within the camera.

Further, a space or gap 160 is provided between the rear edge of the mirror 134 and the face 144 to control stray rays from the upper portion of the field. The gap allows certain of these stray rays to pass from the lens 18 to light absorptive surfaces within the camera, without either impinging on the mirror or entering the prism. The space 160 is as large as practical, but not so large as to eliminate useful rays.

The illustrated camera 10 includes a baffle plate 62 which extends to the intersection of the two extreme rays 30 and 32. This efficiently limits stray light from reaching the film. The cameral also can have a hood or shade (not shown) secured above the entrance face of the leading lens element to block certain light rays which exceed the desired angular field of view. Generally the shade and the baffle plate 62 perform the same function and therefore would not be used together.

The prism 136 is preferably formed from a transparent material having a low optical dispersion such as crown glass, or a plastic such as the acrylic resin material commonly termed "Plexiglass" (Registered Trade Mark). In general, glass is a more efficient transmitter of light energy than plexiglass, and because of its higher index of refraction it is generally more conducive to folding a given optical path in a smaller volume. Plexiglass nevertheless is a preferred material where it is desired to produce the camera with relatively low weight and low cost of manufacture.

By way of illustration and not limitation, the following Table I gives representative parameters of the optical system according to the invention shown in FIGURES 2 and 3, where the prism material is plexiglass having an index different lens 18, a different lens location and a different film format.

It is further contemplated that in the embodiment shown in FIGURES 1--3, the region bounded by the mirror 134, the prism face 144 the extreme ray 30 and the plane of the face 146 can be occupied by a forwardly extending portion of the prism 136 rather than air. With this alternative arrangement, the upper surface of the forwardly extending prism portion is reflectively coated to perform the same function as the mirror 134. It is also possible, but considered less desirable, to replace the serration 152 with a substantially flat surface extending between the prism faces 144 and 148 provided that this surface is coated with an index matching absorbant material.

The optical systems in FIGURES 1--3 subject light to an odd number of reflections, which is appropriate for self developing film of the type marketed by the Polaroid Corporation. FIGURE 4, on the other hand, illustrates a simplified embodiment of modifications of the optical system shown in FIGURE 1 for use with self-developing film units which require an even number of light reflections.

As shown in FIGURE 4, the objective lens 18' has its optical axis 19' aligned directly with the entrance face 144' of the prism 136' so that light rays enter the prism substantially along an axis perpendicular to the entrance face and are internally reflected from prism face 146' overlying the film plane 16'. In the FIGURE 4 embodiment, a bezel 170 before the lens and an aperture plate 172 positioned between the lens and the prism entrance face are shaped and located to block stray rays that would otherwise enter the prism and impinge on the film plane. It will be understood that the function of the aperture plate 172 could also be performed by an optically absorbent coating on the prism entrance face.

There has been described a compact, rigid camera that utilizes a prism with the camera exposure chamber and which has a reflective and transmissive exit face coextensive with and closely spaced from the film plane to achieve a highly folded optical path in a relatively small volume. A significant advantage of the invention is that it provides a camera construction adapted for use with large-format selfdeveloping film but having a relatively small overall height dimension, i.e.

perpendicular to the film plane. This vertical compactness lends itself to a flat, streamlined configuration which is both aesthetically pleasing, readily handled, and conveniently stored and carried. Another advantage is that the optical path can be folded by a prism formed of relatively low weight and low cost plastics material. A further advantage of the invention is that stray and other unwanted light can be selectively controlled within acceptable limits through several features which do not significantly increase the cost of the camera's manufacture.

Referring now to Figures 5 and 6 the housing 12 forms an exposure chamber 13 and a compartment 14 supporting a cassette 15 of selfdeveloping film units. The objective lens 18, which typically is a series of aberration-correcting optical elements aligned along a common optical axis indicated generally by the arrow 19, forms a bundle of light rays which come to focus in an image plane substantially coincident with the film plane 16. The camera includes prisms 24 and 26 housed in the exposure chamber 13 and which define the optical path between the objective lens and the film plane 16.

This optical path is depicted by the chief or principal ray 28 of the diverging bundle of useful light rays in the exit pupil of the lens 18 and transmitted through the prisms 24 and 26 by two extreme rays 30 and 32 at the periphery of the bundle. The prism 24, and more particularly the prism 26, fold the optical path numerous times within a relatively small compact volume.

As shown in FIGURE 6, the prism 24 has an entrance face 34, a reflective and transmissive face 36, a reflective and adsorptive face 38 and an apsorptive face 40.

The faces 34, 36 and 38 each have portions with optically different properties which define and control the optical path. The entrance face 34 forms an entrance window 34a, that is preferably optically finished (a "clear polish" finish) and is coated for low reflection. Around this entrance window the face 34 preferably has a surrounding portion 34b that is optically absorptive. The exit face 36 of the prism has a reflective portion 36a in optical alignment with the entrance windows 34a and offset therefrom has a clear, transmissive exit portion 36b. Exit face 36 and adjacent face 42 of prism 26 can be coated with anti-reflection films. The reflective face 38 aligned opposite the face 36 has an absorptive portion 38a outside the optical path, i.e. adjacent the face 34. The absorptive face 40 extends between the faces 36 and 38.

The prism. 26 has, in adjacent sequence, an entrance face 42, an absorptive face. 44, a reflective face 46, an absorptive face 48, a reflective face 50, an absorptive face 52, and an exit face 54. Entrance face 42 is separated from exit face 36 of prism 24 by a thin air space 58. The exit face 54 has an absorptive portion 54a adjacent the entrance face 42 and a clear polished portion 54b which overlies and is closely spaced from the film plane 16. Portion 54b can be coated with an antireflection film to increase efficiency and to reduce ghost reflections. Each of the optically active faces of the prisms 24 and 26 is preferably flat, particularly the reflective surfaces, to avoid image distortions. The absorptive faces and face portions of the prisms 24 and 26 can be rendered absorptive by coating them with conventional material that absorbs white light and adheres to the glass or plastics material forming the prisms. A black index-matching paint is preferred since it enhances the transmission of light from within the prism to the absorptive materials dispersed in the paint, particularly at large incidence angles. The lateral faces of the prisms not shown in FIGURE 6, i.e. which extend transverse to the faces shown and longitudinal to the plane of the drawing, are also preferably coated with an absorptive material. The reflective faces and face portions are formed using standard techniques by coatings that form front surface optical reflectors.

While the prism 26 can be formed from a single block of material, it is preferably formed from a minor segment 26a of triangular section in the plane of FIGURE 6, joined to a major segment 26b by an index-matching cement along the interface which is designated with a dashed line 56. The prisms 24 and 26 are positioned with the thin air gap 58 between the light transmitting surface portions 36b and 42b. The air gap 58 must have a thickness of at least several wavelengths to avoid interference effects, and is preferably in the range of .025 millimeter (.001 inch).

The transmission of light along the optical path through the prisms 24 and 26 can be considered with reference to the chief ray 28. The ray 28 enters the prism 24 at the entrance window 34a along its perpendicular. While some deviation from the perpendicular is permissible, in general increases in the angle of incidence (measured from the perpendicular or normal) tend to reduce the transmission of light energy into the prisms and to introduce image distortions which become increasingly difficult to correct. Within the prism, the ray is reflected from the reflective surface 36a to the face 38 where in reflects to the exit face portion 36b substantially along its perpendicular. The ray emerges from the prism 24, traverses the air gap 58, and enters the prism 26 at the entrance face portion 42b along its perpendicular.

Within the prism 26, the ray is directed, in succession, obliquely to face 54, reflected to face 50, reflected to face 46 and reflected back to face 54, from which it exits the prism. The exit face portion 54b thus first reflects and then transmits the light rays following the optical path of the camera. The ray 28 strikes the face portion 54b at an angle of incidence A that exceeds the critical angle so that the ray is totally internally reflected within the prism 26. The reflective faces 50 and 46 then successively reflect the ray and direct it back onto the exit face portion 54b at a substantially perpendicular angle of incidence. The ray accordingly is transmitted essentially undisturbed through the face portion 54b, and traverses an air gap 59 to strike the film plane 16, where it exposes a film unit. While the ray 28 is reflected back into the prism 26 at faces 54, 50 and 46 the phrase "internally reflected" for the purposes of this description is limited to total internal reflection as distinguished from standard front surface reflection from a mirror-like surface. While the gap 59 is preferably of uniform thickness, the exit face 54 can be inclined with respect to the film plane to yield a wedge shaped gap. In either case, the gap should be wide enough at all points to avoid abrasion of the exit face by the film but narrow enough to avoid magnification of image aberrations or foreign matter on the exit face. A recommended minimum width is approximately 0.7 millimeter.

The overall height of the prism 26 measured in a direction perpendicular to the film plane 16, indicated by the arrow 60, is substantially less than the maximum linear dimension of the film plane, typically its diagonal. For a film unit having an 80 millimeter by 80 millimeter format, this maximum dimension is approximately 112 millimeters. For this film, and when made with a material having an index of refraction of approximately 1.5, the maximum height of the prism 26 as shown is approximately 93 millimeters. The camera 10 is also compact in its longitudinal and lateral directions since the exit face 54b of the prism 26 typically is only slightly longer than the film plane 16 which it overlies. The relative narrowness, i.e.

dimension transverse to the plane of FIGURE 6, of the prism 26 is significant since in general it is much more difficult to fold a wide, diverging beam of light than a similar narrow beam of light. The dimensions of the prism must, nevertheless, be sufficiently large to avoid vignetting.

While for a given film format, prism material and objective lens, tracing the chief ray 28 is a valuable design tool, it is also necessary to consider the action of the prism on the extreme rays 30 and 32 which strike the film plane at its opposite lateral edges 16a and 16b, respectively. A particularly important consideration is that the angle of incidence A' of the ray 30 on the face portion 54b is equal to or exceeds the critical angle of the prism material so that the ray 30 is not lost by transmission through the face 54b, particularly since such transmission would expose the underlying film. In order to have the angle A' at least equal to the critical angle, the angle A associated with the chief ray must be at least equal to the critical angle plus the sum of the chief ray half-angle in image space in the medium and the convergence half-angle in the medium. The chief ray half-angle can be viewed as the angular difference, in image space, in the direction between the optical axis 19 of the lens 18 and the ray in the medium passing obliquely through the center of the lens and striking the film plane at the same edge 16a as the ray 30.

The convergence half-angle is a function of the angular difference within the prism of the rays striking the edge 16a and which pass through the center of the lens 18 and those which pass through the periphery of its exit aperture. Given that the ray 28 strikes the entrance face portion 42b substantially along its perpendicular, the angle B formed by the intersection of the prism faces 42 and 54 is substantially equal to the angle A.

The dimensions and angular relationships of the reflective surfaces 46 and 50 are chosen to direct those rays which internally reflect at the exit face 54b back onto the exit face substantially along its perpendicular. This perpendicular incidence on the exit face avoids distortion or astigmatism due to refraction of the emerging rays as they pass from the prism to air. By displacing the reflective surface 50 in a direction away from the entrance face 34, and suitably extending the reflective surface 46, it is possible to accommodate a longer focal length lens and a larger film format. This process; however, is limited by considerations of compactness, weight, cost of materials, and loss of light intensity through absorption by the prism material. Similar considerations limit increasing the dimensions of the prism 26 to shift the film plane 16 longitudinally away from the lens 18 to increase the angle A'. It is also possible to increase the angle A' by tilting the exit face with respect to the film plane.

The optically absorbent surfaces of the prisms 24 and 26 serve principally to eliminate unwanted or nonuseful light. This includes both stray rays which enter the prism from outside the normal angular object field of the objective lens as well as light which is scattered, reflected or refracted within the prism structure. For example, light entering the prism 24 at a larger angle of incidence than the ray 30 can be absorbed by the surface 38a upon reflection from the surface 36a or, if it is transmitted through the portion 36b, it may strike the absorbing exit face portion 54a or be absorbed by the face 48. Similarly, rays entering with an angle of incidence greater than that of ray 32 entering the prism 26 will be internally reflected by the face 54b so that they strike and are absorbed by the face 52. Similar stray rays with an even larger angle of incidence will be absorbed by the face 40.

The light absorbing function of the side surfaces of the prisms 24 and 26 also eliminates stray light from the extreme lateral portions of the object field.

It is important that the prism material has a low dispersion. Suitable materials include crown glass and the acrylic resin material commonly termed plexiglass.

Glass is an extremely optically efficient material having for many types a low dispersion and a high index of refraction, usefully in the range of 1.5 to 1.7, which affords a smaller critical angle and therefore can be used to fold the optical path in a more compact volume. Plexiglass, on the other hand, reduces the light transmitted therethrough to a greater extent and has a lower index of refraction, typically in a narrower range around 1.5. As a result, glass is a preferred material where high opticalquality or extreme compactness are the prime considerations.

Plexiglass, however, is the preferred material where weight and cost of manufacture are relevant considerations. It should be noted that plexiglass is desirable from a cost standpoint not only because of the cost of the material itself, but also because it can be fabricated by relatively lower cost moulding and finishing techniques. A significant advantage of the invention is that the optical path is so highly folded that it is possible to utilize, with prisms formed from plexiglass, lenses having a relatively long focal length, even allowing for the fact that the plexiglass increases the back focus in air of the lens by approximately one-third.

By way of illustration and not limitation, the following Table I gives representative parameters for the prisms 24 and 26 and the rays 28, 30 and 32 where the prism material is plexiglass (index of refraction of 1.5), the film format is 80 millimeters by 80 millimeters, and the objective lens is an f/I 1 telephoto lens with a 230 millimeter focal length, similar, except for the focal length, to the lens marketed by the Polaroid Corporation for its Pronto brand of self-developing instant camera.

TABLE I

Chief Ray 28 Prism 24 Optical Path Length (Typically) Angle F 550 Angle G 97.50 Path Segment 28a lOmm Path Segment 28b 1Smm Path Segment 28c 38.5mm Prism 26 Path Segment 28d 85mm Path Segment 28e 47mm Angle B 51.90 Path Segment 28f 75.5mm Gap 59 0.68mm (minimum) Total 271mm Angle A 51.90 Angle C 550 Angle D 660 Angle E 640 Extreme Ray 30 Angle A' 41.80 FIGURE 7 illustrates a modification of the camera 10 as shown in FIGURES 5 and 6 and which achieves a highly folded optical path within an even more compact volume and with a simpler and hence more readily manufactured prism configuration. Elements in FIGURES 5 and 6 bear the same reference numeral in FIGURE 7 plus a prime, thus FIGURE 7 depicts a prism 26'. A principal difference between these embodiments is that the absorbing surfaces 44, 48 and 52 of the FIGURE 6 prism 26 have been eliminated from the FIGURE 7 camera so that the reflecting faces 46' and 50' are contiguous. The prism 26' shown in FIGURE 7 therefore has a four-sided, generally wedge shaped cross section. As in the previous embodiment, the side surfaces are light absorbant and lie slightly beyond the side edges of the film plane to facilitate the folding and eliminate unwanted light. In addition to the light absorbing side faces and the face portion 54a', the prism 26' has a light absorbing face portion 50a' which intercepts stray rays entering the prism with an incidence angle at the exit face portion 54b' that exceeds that of the extreme ray 32'. The prism 24' shown in FIGURE 7 is the same as that shown in FIGURE 6 except that it is repositioned on the entrance face 42'.

Assuming a film format of 80 millimeters by 80 millimeters, and an index of refraction of approximately 1.5, the maximum height of the prism 26, (as shown) in the direction W' is approximately 75 millimeters as compared to 93 miflimeters for the prism shown in FIGURE 6.

For purposes of illustration only, the following Table II gives values for many of the same parameters listed in Table I for a corresponding construction according to FIGURE 7. The embodiment of FIGURE 7 enhances compactness and design simplicity, at the expense of the focal length. In FIGURE 7 the corresponding f/l 1 objective lens has a focal length of 189 millimeters.

TABLE II

Chief Ray 28 u Prism 24 Optical Path Length (Typically) I Angle F 550 Angle G 97.5 Path Segment 28a' lOmni Path Segment 28b' 15mm Path Segment 28c' 38.5mm Prism 26 Path Segment 28d' 67.5mm Path Segment 28e' 39mm Angle B 53.60 Path Segment 28f' 52mm Angle.SJ. 980 Angle J 270 Total 222mm Angle K 820 Angle L 62.50 Angle A 53.60 Angle C 550 Gap 59' 0.68mm Angle D 540 (minimum) Angle E 710 Ray 30 Angle A' 4t .80 Figures 8 and 9 show another embodiment of the invention in the form of a compact split field prism camera 62. A rigid camera housing 64 which forms a normally light tight exposure chamber 66 mounts a pair of mutually perpendicular mirrors 68 located adjacent the exit surface of an objective lens 69 to split the field into two beams 70 and 71, one on each side of the centerline of the camera, which is coincident with the optical axis 72 of the lens. The camera has a generally diamondshaped beam folding prism 74 that has an exit face 76 formed from a pair of exit surfaces 78 and 80. Each of these surfaces both internally reflect useful light rays and transmits them to a common film plane 82.irrors direct 82.

More particularly, as Figure 9 shows, the mirrors direct each beam generally transverse to the optical axis 72 to a prism 84 having an entrance face 86, a first reflecting face 88, a second reflecting face 90, and an exit face 92. The end faces of the prism 84 (i.e. the upper and lower surfaces which extend longitudinal to the plane of Figure 5 and transverse to the faces shown there), prism faces 94 and 96, and the face portion 92a near the lens to be outside the optical path, are preferably coated with an index-matching optical absorbant.

The prism 74 has a pair of entrance surfaces 98 and 100, each of which has a reflective portion 98a and 100a respectively adjacent the optical axis 72 and extending in a "vertical" direction 102 (Figure 8) the full height of the prism 74.

There is a thin air gap 104 between the exit faces 92 of the prism 84 and the uncoated portions of the prism 74 entrance surfaces 98 and 100. An emergent prism 106 is located between the exit faces 78 and 80 of the prism 74 and the film plane.

The entrance face of the prism 106 is constituted by a pair of mutually inclined surfaces 108 that are spaced from the exit surfaces of the prism 74 by a thin air gap.

(The air gaps along the faces of the prism 74 are preferably about .025 mm (.001 inch) wide.) An exit face 110 of the prism is parallel to and closely spaced from the film plane. The exit face 110 is preferably optically finished, has an antireflection film and is coextensive with the film plane. The emergent prism 106 aids in recomposing the split beams in the image plane without a dark or bright line or band appearing down the middle of the film plane along the optical axis 72 which would otherwise be caused by the unequal refraction of the light rays emerging from the inclined exit surfaces 78 and 80 of the prism 74. It should be noted that the prism 106 can be formed from two separate prism units joined along the optical axis 72 at their narrow prism faces. In addition, prism 106 can be made significantly thinner than shown (a smaller angle R) provided that the camera is made sufficiently wider.

This prismatic structure in the camera of FIGURES 8 and 9 forms optical image paths as follows. Useful light rays from within the angular field of the objective lens are split by the mirrors 68, 68 into two light beams 70 and 71 each carrying an image from its own one half of the object field. A diaphragm 111 interposed between the lens and the mirrors defines the exit aperture and preferably the real stop of the objective. As shown, representative rays 112, 113 and 114 of the beam 70 enter the prism 84 substantially perpendicular to the entrance face 86, and are successively reflected from the surfaces 88 and 90 so that they strike the exit and entrance faces 92 and 98 substantially along their perpendiculars. Within the prism 74 the light rays undergo two reflections that direct them to the exit surface 80 generally along its perpendicular, so that each is transmitted through the prism 106 to the film plane 82 where it exposes a film unit held in a film cassette 116 (FIGURE 8). The first reflection in prism 74 is a total internal reflection at the exit surface 78. The second reflection is also a total internal reflection for the ray 114, which strikes the edge 82a of the film plane 82, and for the ray 112. The second reflection for the ray 113, however, is from the reflectively coated portion 100a (see 113c, 113d and 1 l3e).

The lateral dimension of the reflectively coated face portions 98a and 100a depend on several factors such as the objective lens, the prism index of refraction and configuration and the picture format. While these factors are interrelated, the nonreflective portions of the faces 98 and 100 must receive all the useful rays transmitted from the prisms 84 and each must also reflect all useful rays reflected from the refelctive/transmissive exit face surfaces 78 and 80.

Further, as with the embodiments discussed with respect to FIGURES 5-7, the configuration and dimensions of the prisms 84 and 74 vary with the film format, prism material and lens parameters to ensure the total internal reflection and transmission of all useful light rays at the exit surfaces 78 and 80.

The optical system of the camera provides a highly folded optical path in a small volume. In particular, this optical system provides a self-developing type camera which has a relatively small front-to-back dimension and accommodates a lens having a sufficiently long focal length to take portrait photographs.

By way of illustration only, Table III gives the parameters describing the optical system of the cameras shown in FIGURE 9 and assuming a film format of 80 millimeters by 80 millimeters, an f/I 1 telephoto lens system having a focal length in air of approximately 158 millimeters, and prisms formed of lanthanum crown glass having an index of refraction of approximately 1.70.

TABLE III

Extreme Ray 113 Diaphragm 111 Optical Path Length (Typically) Clear aperture radius of exit 5.6mm Path Segment 113a 69.0 pupil Path Segment 113be 5.0 Path Segment 113c 42.4 Prism 84 Path Segment 113d 52.6 Path Segment 113e 38.5 Angle M 450 Angle N 90 Total 207.5mm Angle A 450 Prism 74 Angle S 600 Angle 0 450 Angle P 600 Extreme Ray 114 Angle Q 750 Angle T Prism 106 Width of Exit Face 110 91mm Angle R 150 FIGURE 10 illustrates an embodiment of the camera 62 in which the prisms are plexiglass. As noted with respect to the embodiments in FIGURES 5-7, although plexiglass has a relatively low index of refraction and is otherwise less optically efficient than glass, it has significant weight and cost advantages which can make it a preferred material. The construction and operation of the prism system shown in FIGURE 10 is essentially the same as that described above with respect to FIGURE 9 and corresponding elements are identified with the same reference number as used in FIGURES 8 and 9 except they are primed. One difference in this embodiment is that the lower index of refraction of the plexiglass in this instance accomodates a longer focal length lens resulting from the larger critical angle at n 1.5. Another difference is that reflections from the faces 98' and 100' are exclusively by means of reflection coatings on the portions 98a' and 100a'. Since the uncoated portions of faces 98' and 100' are not used for total internal reflections, they can be secured directly, i.e. without an intervening air gap, to the mating faces 92' of the prism 84' with an index-matching cement.

In order to facilitate recomposing the two image beams at the film plane, a vertically extending rectangular portion of the prisms 74' and 106' adjacent the film plane at the line of recomposition can be a separate, continuous block 116 (shown in phantom) of plexiglass which is secured to the prisms by a suitable index matching cement. The block 116 should extend the full height of the prisms and can extend l

The optical systems in FIGURES 5-10 subject light to an odd number of reflections, which is appropriate for self-developing film of the type currently marketed by the polaroid Corporation. FIGURES Il and 12 on the other hand, illustrate simplified embodiments of modifications of the optical systems shown in FIGURES 7 and 9, respectively, for use with self-developing film units which require an even number of light reflections. As shown in FIGURE 11, the objective lens 18" has its optical axis 19" aligned with respect to a mirror 122 and the entrance face 42" of the prism 26" so that light rays enter the prism 26 substantially along a perpendicular to the entrance face 42" and are internally reflected from prism face 54b" overlying the film plane 16". In FIGURE 12 a mirror 124 angled at 45 with respect to the optical axis 72" introduces an extra reflection to light emerging from the objective lens 18" oriented perpendicular to the axis 72" before it is split by the mirrors 68". The mirror 124 ensures that the number of reflections is even.

It is also contemplated that for an odd number of reflections with the embodiments shown in FIGURES 5-7, the initial ray-orienting prism 24 can be eliminated provided the optical axis of the objective lens is placed in direct optical alignment with the entrance face of the prism 26. These and similar modifications offer a wide degree of flexibility in the practice of the invention as set forth above.

There has been described a compact, rigid camera that utilizes a prism within the exposure chamber to provide a reflective and transmissive surface overlying the film plane. This structure achieves a highly folded optical path in a relatively small volume. One advantage of the design is that it provides a camera for use with selfdeveloping film having a relatively large film format. Another advantage of the invention is that a relatively long focal length objective lens with an acceptable numerical aperture for use in instant photography can be employed. In particular, cameras constructed according to the invention can be used for portrait photographs. The cameras can also be manufactured of plastics materials to have relatively low weight and cost. Also the prisms and the other optical elements have few critical dimensional tolerances or optical alignments, which reduces the cost of manufacture.

WHAT WE CLAIM IS: 1. A camera for producing an image of an object on film having its photosensitive surface coincident with a focal plane in the camera, the camera comprising a housing enclosing an exposure chamber, a lens mounted on the housing for focusing at the focal plane light from within a selected angular field of view and optical means within the exposure chamber for forming a folded optical path between the lens and the focal plane, the optical means including prismatic means having a first face arranged to receive light from the lens, a second face overlying and spaced from the focal plane and having at least one portion arranged to receive light which has travelled along the said optical path from the lens and'to redirect the said light by total internal reflection, and at least one additional face arranged to receive light which has travelled along the said optical path from the second face for reflecting the said light back on to the second face transverse thereto for transmission through the second face to the focal plane.

2. A photographic camera for producing an image of an object on film having its photosensitive surface coincident with a focal plane, the camera comprising a lens focusing at the focal plane light from within a selected angular field of view; and prismatic means located within an exposure chamber forming a folded optical path between the lens and the focal plane and having a first face arranged to receive light travelling along the said optical path from the lens, a second face that overlies and is closely spaced from the focal plane, the second face having at least one portion arranged to receive and to redirect by total internal reflection light travelling along the said optical path from the first face, and a third face that forms an acute angle with respect to the second face, the third face being arranged to receive light travelling along the said optical path from the second face and to reflect the said light on to the second face for transmission through the second face to the focal plane.

3. A camera according to claim 2, wherein the first face forms an acute angle with respect to the second face and light from the lens transmitted by the first face travels directly to the second face.

4. A camera according to claim 3, further comprising optical means between the lens and the said first face for directing light from the lens to the first face so that light passing along the axis of the lens is incident substantially normally upon the first face.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (35)

**WARNING** start of CLMS field may overlap end of DESC **. The optical systems in FIGURES 5-10 subject light to an odd number of reflections, which is appropriate for self-developing film of the type currently marketed by the polaroid Corporation. FIGURES Il and 12 on the other hand, illustrate simplified embodiments of modifications of the optical systems shown in FIGURES 7 and 9, respectively, for use with self-developing film units which require an even number of light reflections. As shown in FIGURE 11, the objective lens 18" has its optical axis 19" aligned with respect to a mirror 122 and the entrance face 42" of the prism 26" so that light rays enter the prism 26 substantially along a perpendicular to the entrance face 42" and are internally reflected from prism face 54b" overlying the film plane 16". In FIGURE 12 a mirror 124 angled at 45 with respect to the optical axis 72" introduces an extra reflection to light emerging from the objective lens 18" oriented perpendicular to the axis 72" before it is split by the mirrors 68". The mirror 124 ensures that the number of reflections is even. It is also contemplated that for an odd number of reflections with the embodiments shown in FIGURES 5-7, the initial ray-orienting prism 24 can be eliminated provided the optical axis of the objective lens is placed in direct optical alignment with the entrance face of the prism 26. These and similar modifications offer a wide degree of flexibility in the practice of the invention as set forth above. There has been described a compact, rigid camera that utilizes a prism within the exposure chamber to provide a reflective and transmissive surface overlying the film plane. This structure achieves a highly folded optical path in a relatively small volume. One advantage of the design is that it provides a camera for use with selfdeveloping film having a relatively large film format. Another advantage of the invention is that a relatively long focal length objective lens with an acceptable numerical aperture for use in instant photography can be employed. In particular, cameras constructed according to the invention can be used for portrait photographs. The cameras can also be manufactured of plastics materials to have relatively low weight and cost. Also the prisms and the other optical elements have few critical dimensional tolerances or optical alignments, which reduces the cost of manufacture. WHAT WE CLAIM IS:

1. A camera for producing an image of an object on film having its photosensitive surface coincident with a focal plane in the camera, the camera comprising a housing enclosing an exposure chamber, a lens mounted on the housing for focusing at the focal plane light from within a selected angular field of view and optical means within the exposure chamber for forming a folded optical path between the lens and the focal plane, the optical means including prismatic means having a first face arranged to receive light from the lens, a second face overlying and spaced from the focal plane and having at least one portion arranged to receive light which has travelled along the said optical path from the lens and'to redirect the said light by total internal reflection, and at least one additional face arranged to receive light which has travelled along the said optical path from the second face for reflecting the said light back on to the second face transverse thereto for transmission through the second face to the focal plane.

2. A photographic camera for producing an image of an object on film having its photosensitive surface coincident with a focal plane, the camera comprising a lens focusing at the focal plane light from within a selected angular field of view; and prismatic means located within an exposure chamber forming a folded optical path between the lens and the focal plane and having a first face arranged to receive light travelling along the said optical path from the lens, a second face that overlies and is closely spaced from the focal plane, the second face having at least one portion arranged to receive and to redirect by total internal reflection light travelling along the said optical path from the first face, and a third face that forms an acute angle with respect to the second face, the third face being arranged to receive light travelling along the said optical path from the second face and to reflect the said light on to the second face for transmission through the second face to the focal plane.

3. A camera according to claim 2, wherein the first face forms an acute angle with respect to the second face and light from the lens transmitted by the first face travels directly to the second face.

4. A camera according to claim 3, further comprising optical means between the lens and the said first face for directing light from the lens to the first face so that light passing along the axis of the lens is incident substantially normally upon the first face.

5. A camera according to claim 4, wherein the optical means comprises a

mirror in the optical path between the lens and the said first face.

6. A camera according to claim 5, wherein the mirror is oriented with a first edge proximate to the first face and further comprising means forming an optically non reflective spacing between the first face and the said first mirror edge.

7. A camera according to claim 2, in which the prismatic means comprises a further face located in the optical path between the first face and the second face and adapted to reflect light passing through the first face from the lens on to the second face.

8. A camera according to any one of the preceding claims, wherein a portion of the second face remote from the third face is coated with a light-reflective material to direct light, which is incident on the said portion of the second face at an angle of incidence less than the critical angle of the material of the optical means to the third face.

9. A camera according to claim 8, wherein the portion of the second face which is not coated with the light reflective material is coated with an anti reflection material.

10. A camera according to any one of the preceding claims, further comprising means for preventing stray light from propagating within the prismatic means in a manner to impinge on the film plane.

11. A camera according to claim 10, wherein the means for preventing stray light from propagating within the prismatic means comprises at least one serration extending across the prism at the junction between the first and third faces and having a surface oriented generally perpendicular to the first prism face to transmit stray light incident on the said serration surface outwardly from the prismatic means.

12. A camera according to claim 10 or 11, wherein the means for preventing stray light from propagating within the prismatic means comprises a coating of light absorptive material applied to the exterior surface of the prismatic means except that first, second and third faces.

13. A camera according to claims 10, 11 or 12, wherein the means for preventing stray light from propagating within the prismatic means comprises a baffle extending to a point adjacent the first intersection of the extreme usable rays.

14. A camera according to any one of the preceding claims, wherein the prismatic means includes side walls canted inwardly as they extend from the said second face.

15. A photographic camera according to claim 2, in which the prismatic means comprises only three faces in the folded optical path, the first face being substantially perpendicular to the said optical path to receive and introduce into the prismatic means light rays from the lens, the second face being acutely angled relative to the first face.

16. A camera in accordance with claim 1, in which the image-representing light undergoes two internal reflections within the prismatic means between its total internal reflection by the said second face and its transmission through the second face to the film plane, one of the said two further internal reflections being at the said additional face.

17. A camera in accordance with claim 16, in which the first of the said two further internal reflections is from a face of the prismatic means which makes an angle with the said second face which is greater than the angle made with the said second face by the face at which the second of the said two further internal reflections takes place.

18. A camera in accordance with claim 16 or 17, in which prior to the entry of the light from the lens into the prismatic means, the light is reflected by a mirror.

19. A camera in accordance with claim 16 or 17, in which prior to the entry of light from the lens into the prism it is twice reflected by the faces of an entry prism, the exit face of which is substantially parallel with the said first face of the said prismatic means.

20. A camera in accordance with claim 1, comprising a beam splitter dividing light which enters the camera through the lens and transmitting the said light along two separate optical paths, in which the said second face of the prismatic means comprises first and second exit surfaces at an oblique angle to one another and together overlying the whole of an exposure aperture in the said film plane, the camera comprising further prismatic means whereby light passed by the beam splitter along one optical path is directed into the first prismatic means so as to be transmitted through one of the said exit surfaces of the said second face and light passed by the beam splitter along the other optical path is directed into the first prismatic means so as to be transmitted through the other of the said exit surfaces of the second face.

21. A camera in accordance with claim 20, wherein the said prismatic means containing the second face has two other faces each of which serves as the said first face for light travelling along the first and second optical paths, respectively, and each of which serves as the said additional face for light travelling along the said second and first optical paths, respectively.

22. A camera in accordance with claim 20 or 21, in which the said further prismatic means includes a pair of entry prisms which respectively reflect light entering the camera and following the said first and second optical paths into the said prismatic means containing the said first, second and additional faces.

23. A camera in accordance with claim 20, 21 or 22, comprising additional prismatic means located between the said obliquely arranged exit surfaces of the said second face and the focal plane.

24. A camera according to claim 1, in which the said first face receives light from a first portion of the said angular field, the additional face receives light from another portion of the said angular field, different from the first portion, and the said first face reflects such light entering the prismatic means at the additional face to the said second face for transmission to the focal plane alongside the light transmitted by the said second face following reflection at the said additional face.

25. A camera in accordance with claim 1, in which the said second face has two planar mutually inclined surfaces each of which is both reflective and transmissive to light.

26. A camera in accordance with claim 25, wherein the said optical means includes an emergent prism having an entrance face closely spaced from the said mutually inclined surfaces of the second face and an exit face closely spaced from the said focal plane.

27. A camera according to any one of the preceding claims, wherein the prismatic means is of a transparent optical material having a low dispersion characteristic.

28. A camera according to claim 27, wherein the said material is a synthetic plastics material and the angle between the second and third sides is approximately 28".

29. A camera according to claim 27, wherein the said material is a liquid encased in a transparent shell.

30. A camera in accordance with any one of the preceding claims, in which the said optical means subjects the light to an even number of reflections in its passage from the lens to the focal plane.

31. A camera in accordance with any one of claims 1-29, in which the optical means subjects the light to an odd number of reflections during its passage from the said lens to the focal plane.

32. A camera in accordance with any one of the preceding claims, in which the said prismatic means is a prism having a maximum direction measured perpendicular to the film plane which is less than the maximum linear dimension of an exposure aperture in the said focal plane.

33. A camera in accordance with claim 1, in which the second face is substantially planar and spaced closely from the said focal plane.

34. A camera in accordance with claim 1, in which the said second face of the prismatic means overlying the focal plane is obliquely angled relative to the said first face.

35. A photographic camera, substantially as herein described with reference to Figure 1 to 3, Figure 4, Figures 5 and 6, Figure 7, Figures 8 and 9, Figure 10, Figure 11 or Figure 12 of the accompanying drawings.