A PLENOPTICAL DEVICE FOR INTERNAL FOCUSING

Abstract: A focusing device of plenoptical type in which a plate (12) provided with apertures is inserted in the optical path between a lens (11) and, for example, an eyepiece (22). Selected parts of the light that passes through the lens is blocked by the plate. Information of the image that passes through the plate with the apertures is received by the eyepiece. The plate (12) is movable with respect to the lens to allow different optical sections to be received by the eyepiece depending on the position of the focusing plate with respect to the lens. The plate may also be rotatable with respect to the optical axis.

Description: TITLE A plenoptical device for internal focusing TECHNICAL FIELD The present invention relates to a device and a method for focusing in optical equipment. More specifically, the invention relates to a device and a method for internal focusing in a device of so called plenoptical type. PRIOR ART In a conventional camera, telescope or microscope, a scene is reproduced when light from one or several external objects, herein called a scene, is centrally projected in a very small light aperture or is deflected by one or several lenses towards a light receiving surface, or via an eyepiece to the retina of an eye. The very small aperture or the system of lenses will in the following be referred to by the general term "lens". In a camera, the light receiving surface consists of a focusing screen, a photographic film or, for instance, a CCD sensor being placed in or near the focal plane of the lens. In a telescope or a microscope an eyepiece is used, whereby the focal plane of the eyepiece replaces the light receiving surface within the camera. By displacing at least one individual lens in a system of lenses it is possible to focus, i.e. select that distance to the objects being reproduced which represents a sharp image. By altering the size of an aperture of a diaphragm placed in the optical path the diaphragm typically being a device with an adjustable central aperture, it is also possible to shield the outer areas of the incident beam of light to a desired extent. These outer areas of the incident light beams represent reproduction deviations that create a blurred image of objects being positioned in front of or beyond the selected distance for a sharp reproduction, and it is consequently possible to use the diaphragm to adjust the depth of field in the image. A specific type of optical equipment is known as "plenoptical equipment". The plenoptical equipment is characterised in that the incident light passing through the lens is divided into several sub-images by a plurality of miniature lens elements or very small apertures, typically disposed in a plate in the optical path beyond the lens. The basics of this technology is described in the article "Single Lens Stereo with a Plenoptic Camera", Edward H. Adelson and John Y.A. Wang, IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol 14, No 2, Feb 1992. In prior art plenoptical systems the miniature optical elements are disposed in, or adjacent to, the focal plane of the lens. In the case of using apertures in a plate for miniature optics these are so tiny that they generate a mirrored image via a projection centre according to the principle for "camera obscura". Alternatively, miniature lenses are used to create a corresponding image. Beyond the miniature optics a light sensitive surface, typically a CCD sensor, is placed to receive the sub-images at a fixed distance from the miniature optics, and the miniature optics are placed at a fixed distance from the lens. With such a conventional plenoptical equipment more information can be extracted from one exposure than what is possible with a traditional non- plenoptical equipment. The reason for this is that the plenoptical reproduction allows extraction of information that is present in a larger area, namely in a section extending perpendicularly to the optical axis, in contrast to the traditional reproduction that only holds information associated to the state around the focal point. Expressed otherwise, with one exposure using the described plenoptical technique it is possible to receive information which otherwise would have required several exposures, each exposure made after a minor parallel displacement of the photographic film along the optical axis. However, there is more optical information present in the optical path between the input lens and the image surface. The optical section corresponding to a section perpendicular to the optical axis, i.e. a section parallel with the image surface in the conventional plenoptical equipment as described above, differs to some extent from any other such sections that are perpendicular with respect to the optical axis and that are displaced along the optical axis with respect to the first section. This information cannot be registered using conventional plenoptical devices. In addition, this has the effect that a sharp detailed photographic image at an arbitrary depth cannot be obtained without changing the optical section, i.e. by altering the focus setting of the lens. Furthermore, it is not possible to perform good measurements of depth without altering the optical section. Miniature optics of the camera obscura type are regarded by those skilled in the art to be impractical due to the considerable loss of light associated therewith. Thus, there is a need for an optical device that for a pre-selected setting of a lens allows an analysis of an image volume that is present beyond the lens, as well as re-focusing without disturbing the pre-selected setting of the lens. SUMMARY OF THE INVENTION It is an object of the present invention to meet this need by providing a device according to claim 1 of the appended claims. The device according to the invention enables simultaneous registering of a visual image and depth information for a subsequent separation of information and/ or three- dimensional visualisation. The device according to the invention allows, by way of its movable focusing plate being provided with apertures, that optical sections are made in the image space present beyond a lens. Thereby, an effect corresponding to adjusting the focus of a lens is obtained without needing to disturb the lens, even in a case where the lens has no focusing device. With the device an essentially conventional reproduction of the viewed scene is obtained but, due to boundary effects between the apertures, deviations that are associated with depth arise and are possible to analyse, for example, in order to determine a distance. This means that samples of relative depth around several optical sections of image space could be analysed and this information could be compiled. With the movable focusing plate it is possible to register differences in image information between two parallel, or essentially parallel, sections that are displaced along the optical axis with respect to each other. In a specific embodiment, the focusing plate is rotatable around the optical axis in order to register minute differences between images obtained on the same section perpendicular to the optical axis, but using different positions for the apertures of the focusing plate. It is a further object of the present invention to meet the need described above by providing a method for analysing the depth relations in an image space between a lens and an eyepiece. This object is met by the method as defined in claim 7. BRIEF DESCRIPTION OF DRAWINGS The present invention is described in detail below using exemplary embodiments, and with reference to the attached drawings wherein: Fig. 1 is a schematic perspective view of a focusing plate according to the invention; Fig. 2 is a front view of a section of a focusing plate according to the invention, showing an embodiment of aperture positions; Fig. 3 is a cross-sectional view taken along the line I-I in Fig. 2; Fig. 4 is an enlarged cross-sectional view according to Fig. 3; Fig. 5 is a schematic view of a device according to the invention being equipped with an eyepiece; Fig. 6 is a cross-sectional view taken along the line II-II in Fig. 5, Fig. 7 is a cross-sectional view, seen from above, taken through the assembly according to Fig. 1 ; Fig. 8 is a cross-sectional view, seen from above, showing the intensity of light in the image plane for a first position of the focusing plate; Fig. 9 is a cross- sectional view, seen from above, showing the intensity of light in the image plane for a second position of the focusing plate; Fig. 10 is a schematic view of an embodiment comprising fibre optics. In the drawings, like components are given like reference numbers. DETAILED DESCRIPTION OF EMBODIMENTS With reference to Fig. 1, according to the invention a focusing plate 2 is provided with apertures 5 to allow light to pass through the plate. The focusing plate 2 is mounted along the optical axis OA of a lens 1 in such a way that the plate is movable along the optical axis with respect to the lens. This is schematically illustrated in Fig. 1 by the lens 1 being fixedly mounted to a rod 3 that is parallel to the optical axis OA, and a sleeve 4 having the focusing plate 2 fixedly attached thereto is slidably mounted onto the rod. As has been described above, the lens could be a single lens or a lens assembly consisting of several separate lenses. However, it should be understood that other light collecting means such as one or several mirrors could be used instead, or as a complement, to direct light to or from the focusing plate. The purpose of the focusing plate is to block a part of the light received via the lens. That part of the light that is not blocked will continue through the apertures of the focusing plate towards an image plane IP. The term "image plane" refers to a section in the image space where information is registered. A light receiving means (not shown in Fig. 1) which is specific for different embodiments is placed in the image plane, such as the focal plane of an eyepiece or the light receiving section of a fibre optical waveguide having a second end for instance ending in the focal plane of an eyepiece. The image in the image plane can be observed through the eyepiece or be registered with, for example, a camera. Fig. 7 illustrates the image space extending beyond the focal plan LF of the lens, wherein light beams are represented by lines with arrowheads. A scene 31 that is reproduced via the lens generates a mirrored virtual representation 32 having an extension in depth beyond the focal plane of the lens LF. The focusing device 2 according to the present invention allows focusing within this virtual representation, as opposed to focusing using the lens which alters the image space itself. The apertures 5 of the focusing plate 2 are formed to any suitable shape. In an embodiment that is preferred to facilitate manufacturing, the apertures are formed as generally circular through holes in a plate of an opaque material, such as graphite. It is essential that each aperture is not so small that a central projection of the camera obscura type is obtained, or that other not desired phenomena, such as diffraction, occur. The dimensions and the overall shape of the focusing plate should be selected to suit the equipment in which the focusing plate is to be used. A typical shape is a circular disc having a thickness of about 50-200 μm. The thickness of the focusing plate and the dimensions of the apertures should be correlated to each other in such a way that a generally continuous reproduction is obtained in the image plane. An embodiment of the light transferring apertures of the focusing plate is shown in Figs. 2 to 4. According to Fig. 2, the focusing plate 2 is provided with apertures 5, said apertures being formed as through holes in the plate disposed in a generally equalateral triangle pattern, having an edge-to-edge distance of S between an aperture and each of its adjacent apertures. When selecting the shape of an aperture it is essential that the walls of the aperture do not interfere with the light passing therethrough. In the embodiment shown in Figs. 2 and 3 this is achieved by providing each one of the holes extending through the plate with an inlet diameter DI, an outlet diameter DO being larger than the inlet diameter, and intermediate diameter increasing linearly from the inlet diameter to the outlet diameter thereby giving the aperture the shape of a truncated cone. Asymmetric-shaped conical holes are also feasible for non-symmetrical light-cones. The conicity of an aperture, whether symmetric or asymmetric, is selected based on the focal length of the lens and the operative interval of the focusing plate along the optical axis, as well as incident light cone conicities, in order to ensure that no part of the walls of the aperture beyond the inlet opening intersects the beam of light. The inlet opening of an aperture is selected as large as possible with respect to the light blocking area in order to obtain the brightest possible image, e.g. selecting an inlet opening diameter of 50 μm. According to the embodiment illustrated in Fig. 4, the edge-to-edge distance S between the inlet openings having diameter DI is selected such that S=2»DI. The angle α is the cone angle for a symmetrical cone of light in the image space beyond the lens having its axis coinciding with the optical axis of the lens. The distance D between a plane through an inlet opening and the image plane IP, respectively, is in this embodiment selected to be D = DI*cot(α/2) in order to obtain an essentially continuous reproduction in the image plane, and the thickness of the focusing plate is selected to a value less than or equal to D to make the reproduction possible. It should be noted that an embodiment including apertures of hexagonal shape being hexagonally distributed over the focusing plate is preferred for optical reasons. However, the embodiment described above, i.e. including apertures of circular shape that are distributed according to an equalateral triangle pattern, is assessed to be advantageous in that an easily manufactured focusing plate providing an image of generally sufficient quality is obtained. The focusing plate is mounted substantially perpendicular to the optical axis OA of the lens (ref. to Fig. 1) via an attaching device being slidable with respect to the lens. An embodiment of such an assembly is illustrated in Fig. 5 and 6, wherein a lens 11 is mounted at the end of a tube 17 by means of a threaded attaching sleeve 16. A sleeve 18 is inserted into the other end of the tube 17 and is displaceable within the tube according to the telescope principle. A gear rack 19 is fixedly attached to the outer surface of the sleeve 18 and is oriented in the axial direction of the sleeve 18. The cogs of the gear rack protrude out through a longitudinal opening 20 in the tube 17. A feeding means for feeding the tube forward and backward is schematically illustrated in Fig. 5 and 6 with a cog wheel 21 which can be rotated to move the tube 18 into, and out off, the tube 17 along the optical axis OA of the lens. An eyepiece 22 is fixed to the outer end of the sleeve 18 with a threaded attaching sleeve 23. The sleeve 18 is threaded on its inside and an inner sleeve 24 provided with an external thread is screwed into the sleeve 18. The inner sleeve 24 is at one end provided with a flange 25 against which a focusing plate 12 is fixedly attached, for example with an epoxy adhesive or an additional threaded plastic sleeve (not shown) . By rotating the inner sleeve 24 with respect to the outer sleeve 18 during assembly the distance between the focusing plate 12 and the focal plane of the eyepiece 22 can be set as desired. Thus, it is possible to position the inner sleeve such that a selected image plane (not shown in Fig. 5) coincides with the focal plane of the eyepiece. After that, the inner sleeve 24 can be fixed against the outer sleeve 18, e.g. with a locking lacquer. When using a device according to Figs. 5 and 6, the position of the focusing plate 12 is adjusted by rotating the cog wheel 21. Thereby, the reproduction present in the image plane will be focused at different depths in the viewed scene, which is observable via the eyepiece. This effect shall now be described in more detail. According to the invention, the position of the focusing plate, which is displaceable along the optical axis, is selected along the optical axis in such a way that it is positioned within the image space present beyond the lens, i.e. beyond the focal plane of the lens, as is shown in Fig. 7. In the image space, a three-dimensional virtual representation of the scene to which the lens is directed is present. When the focusing plate is displaced through the image space, the image plane moves through this virtual representation. Thus, depending on the position of the focusing plate, different sections of the representation are more or less sharply reproduced in the image plane. In Figs. 8 and 9 is illustrated the measurable light intensity I in the image plane IP from an object 41 of the virtual representation for two different positions of the focusing plate 2 with its apertures 5, one of the positions being shown in Fig. 8 and the other in Fig. 9. Light beams are represented by lines with arrowheads, and the light intensity is schematically illustrated with a graph to the right in each figure. In Figs. 8 and 9 is also illustrated a way in which a variable focusing of the reproduced scene is achieved without the need to focus by the lens. In fact, according to the invention, focusing can be accomplished also using a lens which in itself entirely lacks any focusing device. Thus, the viewed scene is obtained in the image plane, wherein the reproduction is built up of sub-images from each aperture in the focusing plate. The sub-images join essentially edge-to-edge to each other. Thus, the total image that is present in the image plane exhibits a considerable agreement with a similar image of a conventional optical equipment. However, light beams from light cones a distance in front of the focusing plate will give rise to an unsharpness, called a "blur circle", and in consequence a loss of resolution. The blur circle is a pure geometrical effect caused by the light cones that are focused in front of the lens. However, the opaque sections of the focusing plate produce the effect that the unsharp light cones are cut off and chopped into smaller sub cones of light. Sharp patterns, such as sharp edges, lines or dots, in arbitrary sections of the image space are converted into periodic patterns of some degree of sharpness (cf. Fig. 8). As is also shown in Figs. 8 and 9, when the focusing plate 2 is displaced somewhat along the optical axis OA, other parts of the beams of light that pass through the lens will collect in the image plane IP. In this case, the total reproduction in the image plane is essentially the same as before the displacement, but a change of the deviations described above will occur at the same time as the focused part of the reproduced scene is moved to another depth. For example, rays of light from the object 41 in Fig. 8 enter through several apertures 5A-C and produce a relatively blurred image and, in addition, patterns associated to depth, while in an other position as shown in Fig. 9 light beams from the same object will be enter only one aperture 5 of the focusing plate, whereby these light beams will produce a sharp image in the image plane. Preferably, the focusing plate is coupled to a suitable means for position registering in order to determine its position with respect to the lens. Thereby it becomes possible to relate an exposure to the actual position of the focusing plate and in the case of different exposures during 3-D-measurements relate these to each other. Such position adjusting and position registering means are known to anyone skilled in the art, and is for example obtainable from Melles Griot AB, Taby, Sweden, or from Physik Instrumente GmbH & Co, Waldbronn, Germany. In a further developed embodiment the focusing plate is rotatable around the optical axis of the lens, thereby allowing alternation of depth dependent blur patterns while keeping focusing distance constant, which alternations can be analysed. It is not new in itself to introduce a rotatable plate in an optical path and suitable means for this, with or without position read-out feed back, are available from Melles Griot. The expression "exposure" has been used to represent a reproduction for a specific setting of the focusing plate according to the present invention. Although this expression suggests a reproduction obtained with a camera, the expression "exposure" should be understood to include visual perception through an eyepiece of the light in the image plane associated with a selected setting of the focusing plate. Such a device has been described above with reference to Figs. 5 and 6, wherein an eyepiece is arranged in such a way that its focal plane coincides with the image plane. In Fig. 10 is illustrated an embodiment of the present invention, for example adapted to endoscopic applications, wherein light is guided from the image plane IP to the eyepiece 6 via a fibre optical waveguide 7. Thus, the present invention makes it possible to analyse a series of sections of the image space being present between a lens and an eyepiece along the optical axis, as compared to previously known plenoptical equipment that only allows a single section in a corresponding image space to be analysed. Furthermore, the present invention is adapted to reproduce a readily visible image of the scene to which the lens is directed. It is obvious for anyone skilled in the art that the present invention may be varied in many ways with respect to the detailed description above within the scope of the invention as claimed. Such variations include non-planar focusing plates, multiple eyepieces, additional position adjusting and position registering means for the focusing plate, different types of registering means beyond or in front of the eyepiece etc. In a specific, not illustrated embodiment of the present invention the apertures of the focusing plate are combined with a transparent and coloured material, thereby enabling wavelength-associated analysis. Such an embodiment could, for example, be practised by forming the focusing plate as a thin cover plate, in which apertures are formed, disposed adjacent to a continuous coloured transparent sheet of glass or plastic. In a corresponding way, it is possible to provide a polarising filter.

Claims: CLAIMS 1. A focusing device disposed between a light collecting means (1) and a light receiving means (6, 7, 22), along the optical axis (OA) of the light collecting means (1), and wherein the focusing device includes an opaque plate (2, 12) being provided with numerous apertures (5) for allowing light to pass through, characterised in that the plate (2, 12) is coupled to a means (4, 19, 21) for adjusting the position of the plate (2, 12) along the optical axis (OA). 2. The device according to claim 1, characterised in that at least one of the apertures (5) of the plate (2, 12) is a hole of a conical shape extending through the plate, whereby the inlet opening of the hole being directed toward the light collecting means (1) is of a smaller diameter (DI) than the diameter (DO) of the outlet opening being directed toward the light receiving means. 3. The device according to anyone of the previous claims characterised in that the light receiving means comprises an eyepiece (6, 22). 4. The device according to anyone of the previous claims characterised in that the light receiving means comprises a fibre optical waveguide (7). 5. The device according to anyone of the previous claims characterised in that the plate (2, 12) is rotatably arranged with respect to the optical axis (OA). 6. A plate (2, 12), provided with apertures (5) for passing light, for use in a device according to anyone of the previous claims, characterised in that the apertures (5) are of sufficient size to substantially avoid central projection via the apertures. 7. A method for analysis of the depth relations of an image space along the optical axis (OA) between a lens (1) and an eyepiece (6) characterised by the steps of disposing an opaque plate (2, 12) in the optical path substantially perpendicular to the optical axis (OA) of the lens, said opaque plate (2, 12) being displaceable along the optical axis and provided with apertures (5) for allowing light to pass through; displacing said plate (2, 12) along the optical axis (OA); registering selected positions for said plate (2); registering the reproductions obtained through the apertures (5) of said plate at the selected positions; and compare said registered reproductions with each other and with respect to said registered positions for said plate. 8. The use of an opaque plate (2, 12) provided with apertures (5) for passing light therethrough for assessing depth relations in an image space of an optical equipment, whereby the plate is disposed in the optical path between a lens (1) and an eyepiece (6) and whereby reproductions of light passing through the apertures are registered, characterised in that during the assessment of the depth relations the plate (2, 12) is disposed at different positions along the optical axis of the lens (OA).

Inventors: ARNSPANG JENS, BERGHOLM FREDRIK, HENRIKSEN KNUD, ARNSPANG, JENS, BERGHOLM, FREDRIK, HENRIKSEN, KNUD
Applicants: ARNSPANG JENS, BERGHOLM FREDRIK, HENRIKSEN KNUD, ARNSPANG, JENS, BERGHOLM, FREDRIK, HENRIKSEN, KNUD
Application year: 2000
Main classification: G02B5
IPC codes: G02B7, G02B21, G02B27, G03B13, H04N5