Patent Description:
Existing instruments for medical diagnosis rely on visual or photographic examination. In general, they provide a means of observing, in real-time, a magnified image of a subject anatomical structure. Some examinations are often coupled with procedures for which tool access is required, such as removing ear wax from the outer ear canal. Detailed visualisation of the structures being examined is critical for both accurate diagnosis and minimising potential pain or damage caused during the procedures. Performing procedures with simultaneous visualisation of the subject structures further increases the quality of care provided, offering the ability to record the whole process for subsequent analysis. Such apparatus is expensive and difficult to operate.

Some instruments incorporate traditional optical systems with mobile user devices, providing improved ergonomics, usability and cost for wider access to associated procedures such as micro-suction of the ear canal. However, such instruments often exhibit poor image magnification and detail, and do not provide sufficient space for tool access.

<CIT> discloses a handle, a speculum mount, a smartphone mount and a spacing element. The spacing element is configured to maintain an optical separation distance between the speculum mount and the smartphone mount. The apparatus also comprises optical paths for the smartphone's camera and light source and optical elements to focus the image and/or direct the light. The optical element may be configured to provide dual images to the smartphone's camera. In use the apparatus enables a clear view of the ear canal while allowing access of a micro-suction tool to be inserted into the ear canal. A cannula grip and guide for the micro-suction tool is also disclosed.

<CIT> discloses a zoom lens system for a mobile phone camera including two or more zoom lens groups and an intermediate real image plane. <CIT> discloses an optical system for an endoscope including a reversal system for projecting a distal intermediate image onto a proximal intermediate image plane, where the system enables sharp imaging in the near infrared range.

The invention is defined in independent claim <NUM> and preferred features are set out in the dependent claims.

There is described herein apparatus for providing magnification of an object for a device having a camera, the apparatus comprising:.

The apparatus can provide greatly increased magnification of objects undergoing examination, while providing clear space for tool access to the objects. In particular, the use of two optical elements arranged as described significantly increases the magnification of the object while the spacing element maintains the object at a fixed distance from the first optical element thus allowing a space for tool access to be provided.

The second distance may be at least twice the first distance and preferably around <NUM> times larger than the first distance. Alternatively, the second distance may be at least around a decimal order of magnitude greater than the first distance. It is noted that the distance along the optical path may be significantly greater than the physical straight-line distance, in particular if the optical path is bent and diverted as described in more detail below.

While the device may simply comprise a camera and hardware and software for operating the camera, optionally, the device having a camera comprises a mobile user device such as a smartphone or tablet device. It may also comprise a custom device that includes a camera, hardware and software and optionally a screen and wireless communication capabilities.

The apparatus may further comprise a body for defining an aperture through which human or animal anatomical structures are examined. The optical path preferably passes through the aperture. The body may be a speculum for placing in a patient's ear canal. The apparatus may comprise an otoscope. The speculum may be disposed at the distal end of the spacing element, and the spacing element may be configured to provide a gap for tool access to the ear canal through the speculum.

The apparatus may further comprise a third optical element disposed on the optical path at the intermediate image plane. The third optical element may contain the entire image at the intermediate image plane, and may be arranged such that the first optical element and the second optical element lie in conjugate planes. Advantageously, this feature reduces in the final image vignetting effects resulting from light at higher field angles missing the second optical element.

At least one, preferably each, optical element may comprise a lens. The first optical element may comprise two doublets.

The apparatus may further comprise an aperture stop disposed on the optical path, wherein the aperture stop effects a reduction in the diameter of the entrance pupil of the apparatus, optionally wherein the first optical element comprises two doublets and the aperture stop is disposed between the two doublets. This increases the depth of field in the final image by increasing the f-number of the optics.

The apparatus may further comprise a mirror arrangement comprising a plurality of mirrors arranged to divert the optical path away from and then towards an axis extending between the first optical element and the final image plane, such that the optical path is longer than the distance between the first optical element and the final image plane. The plurality of mirrors may divert the optical path to a plane substantially parallel and proximate to a distal surface of the device, that is, along a path parallel to the back of the mobile device. This offers the advantage of being able to fold a section of the optical path, providing a more compact and ergonomic apparatus while maintaining sufficient clear space for tool access.

The first optical element may be achromatic. The front focal length of the first optical element may be no less than around <NUM>, preferably no less than <NUM>. The front focal length of the first optical element may be no greater than around <NUM>, preferably no greater than <NUM>. The power of the second optical element may be no less than around <NUM> dioptres, preferably no less than <NUM> dioptres. The power of the second optical element may be no greater than around <NUM> dioptres, preferably no greater than <NUM> dioptres. The optical magnification factor of the apparatus may be no less than around <NUM>, preferably no less than <NUM>. It is noted that <NUM> dioptre = <NUM>-<NUM>.

The apparatus may further comprise a lighting arrangement, comprising a powered light source, preferably a plurality of white light LEDs. The apparatus may further comprise an optical arrangement configured to guide light from the powered light source towards the object. The powered light source may be provided by the device, for example it may be the flash light associated with the camera of the device. The lighting arrangement may further comprise electronic circuitry for controlling the powered light source and further comprise means for attaching the powered light source and the electronic circuitry to the spacing element, wherein the optical arrangement comprises a collimator. Advantageously, the lighting configuration increases the illumination of the final image for better image quality. The apparatus may further comprise a power source for powering the light source, preferably a rechargeable lithium-ion battery, and further comprise electronic circuitry for controlling the power source. The apparatus may further comprise a handle. The power source and electronic circuitry for controlling the power source may be disposed in the handle.

An accompanying application may be provided for the mobile computing device having a camera receiving an image from the apparatus described above and such an application may be configured to crop the image, digitally magnify the image and/or invert the image in real-time.

The application may be further configured to control and set other parameters associated with the system. For example, it may be arranged to control the intensity, frequency and beam width of the powered light source and control the diameter of one or more apertures in the apparatus.

According to another aspect, there is described herein a method for magnifying an object for a device having a camera, the method comprising the steps of:.

As noted above, embodiments can enable greatly increased magnification of objects undergoing examination, while providing clear space for tool access to the objects.

The method may further comprise the step of arranging a third optical element on the optical path in the intermediate image plane, the third optical element containing the entire image at the intermediate image plane and being arranged such that the first optical element and the second optical element lie in conjugate planes.

The method may further comprise the step of arranging a plurality of mirrors to divert the optical path away from and then towards an axis extending between the first optical element and the final image plane, such that the optical path is longer that the distance between the first optical element and the final image plane. The method may further comprise the step of arranging the plurality of mirrors to divert the optical path to a plane substantially parallel and proximate to a distal surface of the device. The method may further comprise the step of installing the optical elements and the plurality of mirrors in a housing, wherein the housing is partially or totally sealed and provides space for the optical path through one or more internal cavities.

The method may further comprise the step of illuminating the object using a powered light source and an associated optical arrangement, the light source preferably being a plurality of white light LEDs. The powered light source may be provided by the device. The method may further comprise the steps of controlling the powered light source using electronic circuitry and attaching the electronic circuitry to the spacing element. The method may further comprise coupling a handle to the spacing element. The method may further comprise powering the powered light source using a power source and associated electronic circuitry. The power source and the associated electronic circuitry may be disposed in the handle.

Any system feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to system aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.

It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:.

Referring to <FIG>, apparatus <NUM> for providing magnification of an object <NUM> for a device, in this embodiment a mobile user device, having a camera according to a first embodiment will now be described. The apparatus comprises a first optical element <NUM> at a first location <NUM>, and a second optical element <NUM>. The first optical element <NUM> provides an image <NUM> of the object <NUM> in an intermediate image plane <NUM>. The image <NUM> is then magnified optically by the second optical element <NUM> to provide a final image <NUM> in a final image plane <NUM>. The intermediate image plane <NUM> and the second optical element <NUM> are arranged along an optical path <NUM> extending between the first optical element <NUM> and the final image plane <NUM>. The focal lengths of the optical elements <NUM>, <NUM> are such that the distance from the first optical element <NUM> to the intermediate image plane <NUM> is greater than the distance from the intermediate image plane <NUM> to the second optical element <NUM>.

As shown in <FIG>, a third optical element <NUM> may be provided at the intermediate image plane <NUM>, such that the third optical element <NUM> contains the entire image <NUM>. The third optical element <NUM> is arranged such that the first optical element <NUM> and the second optical element <NUM> lie in conjugate planes.

In both embodiments, a spacing element (not shown in <FIG>) maintains a fixed separation between the first optical element <NUM> and the object <NUM>. Means for mounting the apparatus <NUM> in a fixed position relative to the mobile user device is provided (also not illustrated in <FIG>) such that a camera aperture of the mobile user device is supported in the final image plane <NUM>. At least one, preferably each, optical element is a lens or a system of lenses.

The length of the optical path <NUM> taken between two locations will be referred to as a distance along the optical path <NUM>, and is distinct from the shortest spatial distance between the two locations. The distance along the optical path <NUM> from the intermediate image plane <NUM> to the final image plane <NUM> is significantly smaller than the distance along the optical path <NUM> from the intermediate image plane <NUM> to the first optical element <NUM>.

It is noted that <FIG> are not drawn to scale and the distance between the first optical element <NUM> and the plane of the object <NUM> may be greater than the spatial distance between the optical elements <NUM>, <NUM> and the spatial distance between the first optical element <NUM> and the image plane <NUM>. That is, the spacing element is arranged to provide a gap large enough to allow access for surgical tools or other devices.

For the avoidance of doubt, the end of the apparatus <NUM> nearest to the object <NUM> will be referred to as the distal end, i.e. the end which is furthest from the user, and the end of the apparatus <NUM> nearest to the mobile user device will be referred to as the proximal end, i.e. the end closest to the user.

The second optical element <NUM> is arranged to reduce the minimum focusing distance (and effective focal length) of the camera, allowing the image <NUM> to be placed closer to the final image plane <NUM> for an in-focus final image <NUM>.

The apparatus <NUM> may be used for medical examinations and any coupled procedures. Accordingly, by way of example, the apparatus <NUM> may be incorporated into or implemented in conjunction with medical devices such as otoscopes, endoscopes or ophthalmoscopes, for examination of internal or external anatomical structures.

In one example, the first optical element <NUM> has a focal length of approximately <NUM> -<NUM> and is placed along the optical path <NUM> approximately one focal length away from the object <NUM>. The intermediate image plane <NUM> is therefore approximately <NUM> along the optical path <NUM> from the first optical element <NUM>. As in this example, magnification can be achieved using the first optical element <NUM>, found as the ratio of the distances along the optical path <NUM> from the first optical element <NUM> to the object <NUM> and from the first optical element <NUM> to the intermediate image plane <NUM>. The second optical element <NUM> has a power of approximately <NUM> dioptres and reduces the minimum focusing distance of the camera by a factor of <NUM>, and in combination with the first optical element <NUM> has a magnification factor of approximately <NUM> (equal to <NUM> x <NUM> where the ratio of the distance between the object <NUM> and the first optical element <NUM> to the distance between the first optical element <NUM> and the intermediate image plane <NUM> is <NUM>:<NUM>). Alternatively, if the power of the second optical element <NUM> is <NUM> dioptres, the minimum focussing distance is reduced by a factor of approximately <NUM> and a magnification factor of approximately <NUM> is achieved. Advantageously, compared to using a camera alone at a similar separation from the object <NUM>, this apparatus <NUM> achieves a magnification approximately <NUM> times greater with a <NUM>-dioptre second optical element <NUM> and approximately <NUM> times greater with a <NUM>-dioptre second optical element <NUM>.

Referring to <FIG>, apparatus <NUM> comprising an otoscope according to a second embodiment will now be described. The means for mounting the apparatus <NUM> in a fixed position relative to the mobile user device <NUM> comprises a housing <NUM> from which the spacing element <NUM> extends distally. A handle <NUM> and a speculum <NUM> are coupled to the spacing element <NUM>, the speculum <NUM> defining an aperture through which human or animal anatomical structures, such as a patient's ear canal, may be examined. The speculum <NUM> is disposed on the distal end of the spacing element <NUM> and is placed in a patient's ear canal during operation of the apparatus <NUM>. This provides a fixed separation between the object <NUM> (ear canal) and the first optical element <NUM>. The gap <NUM> is sufficiently large for tool access to the ear canal through the speculum <NUM>, suitable for medical procedures such as micro-suction.

The apparatus <NUM> further comprises a mirror arrangement <NUM> comprising a plurality of mirrors <NUM> to divert the optical path <NUM> away from and then towards an axis extending between the first optical element <NUM> and the final image plane <NUM>. Accordingly, the optical path <NUM> is longer than the distance between the first optical element <NUM> and the final image plane <NUM>. The plurality of mirrors <NUM> is preferably a plurality of plane mirrors, for example of about <NUM> or <NUM> diameter. An aperture of the camera <NUM> is shown supported in the final image plane <NUM> in <FIG>. In one example corresponding to <FIG>, <FIG> and <FIG>, the bounding dimensions of the housing <NUM> are approximately <NUM> × <NUM> × <NUM>.

As shown in <FIG>, the plurality of mirrors <NUM> may divert the optical path <NUM> to a plane substantially parallel and proximate to a distal surface (that is the rear surface) of the mobile user device <NUM>. A number of other folding arrangements exist, for example a further arrangement is shown in <FIG>. In the example of <FIG>, the bounding dimensions of the housing <NUM> are approximately <NUM> x <NUM> x <NUM>.

Referring to <FIG>, the first optical element <NUM> in the apparatus <NUM> according to an alternative embodiment will now be described. In this embodiment, the first optical element <NUM> comprises two achromatic doublets <NUM>, <NUM> arranged in opposing orientations. As illustrated in <FIG> by example light rays <NUM>, the achromatic doublets <NUM>, <NUM> have one infinite conjugate, meaning the light rays <NUM> form a collimated beam between the achromatic doublets <NUM>, <NUM>. Using the achromatic doublets <NUM>, <NUM> reduces achromatic aberrations that could give rise to colour fringing artefacts. The performance of the achromatic doublets <NUM>, <NUM> is diffraction limited provided the apparatus <NUM> has field angles less than around <NUM> degrees, meaning other common forms of aberrations are reduced, such as spherical aberrations, coma and astigmatism. The achromatic doublets <NUM>, <NUM> may be non-identical, wherein their focal lengths (equivalent to the front and rear focal lengths of the first optical element <NUM> respectively) are different according to the respective distances of the object <NUM> and the intermediate image plane <NUM> from the first optical element <NUM>.

The first optical element <NUM> further comprises an aperture stop <NUM> disposed in between the achromatic doublets <NUM>, <NUM>. The aperture stop <NUM> effects a reduction in the diameter of the entrance pupil of the apparatus <NUM>, wherein the entrance pupil is the smallest optical aperture in the apparatus <NUM> along the optical path <NUM>. This has the effect of increasing the f-number of the apparatus <NUM>, i.e. the ratio of its focal length to the diameter of the entrance pupil. The depth of field in the final image <NUM> is reduced as a result of magnification. To compensate, greater depth of field is achieved by increasing the f-number. In some embodiments, the diameter of the aperture stop <NUM> may be adjustable between a plurality of different sizes.

In one example, the distal <NUM> and proximal <NUM> achromatic doublets both have an effective focal length of <NUM>-<NUM> and are separated from each other by a gap of <NUM>. The aperture stop <NUM> has an aperture diameter of <NUM>. The third optical element <NUM> has an effective focal length of <NUM> and is disposed around <NUM> proximally along the optical path <NUM> from the proximal achromatic doublet <NUM>. The second optical element <NUM> has an optical power of <NUM> dioptres, and is disposed around <NUM> proximally along the optical path <NUM> from the third optical element <NUM> and <NUM> distally along the optical path <NUM> from the final image plane <NUM>. The plane of closest focus exists around <NUM> distally from the distal achromatic doublet <NUM> and <NUM> from the distal end of the speculum <NUM>; at the plane of closest focus the depth of field is <NUM> and the f-number is <NUM>. The plane of furthest focus exists <NUM> distally from the plane of closest focus; at the plane of furthest focus the depth of field is <NUM>. The optical elements <NUM>, <NUM>, <NUM> in this example are of diameter <NUM>. The optical properties of the apparatus <NUM> can be designed to match the particular optics of a particular mobile user device. This allows the quality of the final image <NUM> to be tuned appropriately, especially with reference to depth of field and focus range.

In a similar example, the aperture stop <NUM> has an aperture diameter of <NUM> and the optical elements <NUM>, <NUM>, <NUM> have a diameter of <NUM>. In this example, the plane of closest focus exists <NUM> from the distal end of the speculum <NUM>, and the plane of furthest focus exists <NUM> distally from the plane of closest focus. The depths of field at the plane of closest and furthest focus are, respectively, <NUM> and <NUM>. The f-number at the plane of closest focus is <NUM>.

Referring to <FIG> and <FIG>, the apparatus <NUM> further comprises a lighting arrangement <NUM> according to a preferred embodiment which will now be described. The lighting arrangement <NUM> comprises a powered light source <NUM> and an optical arrangement <NUM> configured to guide light from the powered light source <NUM> towards the object <NUM>. The lighting arrangement <NUM> is mounted to the apparatus <NUM>, for example in the speculum (see <FIG>) or substantially adjacent to the mobile user device <NUM> (see <FIG>). As in <FIG>, the powered light source <NUM> is provided by the mobile user device <NUM>; otherwise, the powered light source <NUM> is a plurality of white light LEDs. The optical arrangement <NUM> is a collimator for collimating the light emitted from the powered light source <NUM>. Directing additional light to the object <NUM> in this way increases the illumination of the object <NUM> in the final image <NUM>, providing good image quality despite parts of the apparatus <NUM> which may reduce the available light, such as the aperture stop <NUM>.

The lighting arrangement <NUM> further comprises electronic circuitry <NUM> for controlling the powered light source <NUM>, and means for attaching the powered light source <NUM> and electronic circuitry <NUM> to the apparatus <NUM> including positioning internal to parts mentioned above. The lighting arrangement <NUM> further comprises a power source <NUM> for powering the light source <NUM>, and electronic circuitry <NUM> for controlling the power source, the power source <NUM> preferably being a rechargeable lithium-ion battery. Additionally, the apparatus <NUM> comprises a charging stand.

In one example, as shown in <FIG>, <FIG>, the lighting arrangement <NUM> is disposed in the speculum <NUM>. In this arrangement, the powered light source <NUM> comprises a ring of white light LEDs disposed on a circular printed circuit board extending around a circumference of the speculum <NUM>. The optical arrangement <NUM> extends around a section of the internal contour of the speculum <NUM> and is coupled to the powered light source <NUM> to direct light along the length of the speculum <NUM> towards the object <NUM>. Electronic circuitry <NUM> and a user-operable button <NUM> for activating the powered light source <NUM> are disposed in the spacing element <NUM>. The power source <NUM> and its associated circuitry <NUM> are disposed in the handle <NUM>, providing a counterweight to the mobile user device <NUM>. The skilled person will appreciate that other arrangements of the light source and its associated power source may be provided.

As shown in <FIG>, a charging stand <NUM> is configured to receive the handle <NUM> of the apparatus <NUM> in order to charge the power source <NUM>. The charging stand <NUM> comprises a charging stand cover <NUM> and a cabling <NUM> for connection to an external power supply, wherein the cabling <NUM> is coupled to the charging stand <NUM> via a cable retraction system <NUM> for biasing mechanically the cabling <NUM> towards the cover <NUM>. For example, the cabling retraction system <NUM> may comprise a plurality of springs and pulleys. Electronic circuitry <NUM> for controlling the charging of the power source <NUM> is disposed within the charging stand <NUM>. The charging stand <NUM> therefore provides a means for charging the power source <NUM> of the apparatus <NUM>.

In the above embodiments of the apparatus <NUM>, the optical elements <NUM>, <NUM>, <NUM> may be further configured to provide a stereoscopic final image <NUM>, for example via one or more arrangements of mirrors, lenses and/or prisms. The camera <NUM> may be a stereo camera and the mobile user device <NUM> may comprise built-in hardware for displaying such stereoscopic images, for example an autostereoscopic screen. Alternatively, the mobile user device <NUM> may be coupled to suitable external viewing hardware for the user, such as binocular spectacles. Advantageously, this allows the user to view dual 2D images of the object <NUM>, providing depth perception.

With reference to <FIG>, an application <NUM> operable with all previous embodiments will now be described. The application <NUM> receives the final image <NUM> in a digital format via one or more image sensors of the camera <NUM> of the mobile user device <NUM>. The application <NUM> is configured to perform any number of a selection of operations <NUM> on the image to produce an output image <NUM>. Via the graphical user interface of the mobile user device <NUM>, the application <NUM> displays the output image <NUM> on a screen. In addition, the application <NUM> is operable to effect configuration changes in the apparatus <NUM> using any number of a selection of configuration controls <NUM>. The application <NUM> therefore allows for further digital image enhancement and can be used to optimally display the image <NUM> of the object <NUM> for the user.

In one example, the operations <NUM> include cropping <NUM>, digital magnification <NUM> and/or real-time image inversion <NUM>. Cropping <NUM> and digital magnification <NUM> allow the area of interest in the image <NUM> to be maximised on the screen. The magnification <NUM> can over-zoom the image, i.e. enlarge it beyond a one-to-one mapping between sensor pixels in the camera <NUM> and screen pixels. The optical elements of the apparatus <NUM> may be configured such that the final image shows an inverted version of the object <NUM>. Real-time image inversion <NUM> allows for this misrepresentation to be corrected.

In another example, the configuration controls <NUM> include controlling the illumination <NUM> of the object <NUM> and the aperture <NUM> of the optics in the apparatus <NUM>. The illumination control <NUM> communicates with the lighting arrangement <NUM> via the electronic circuitry <NUM>, <NUM>, for example to alter the intensity, frequency or beam width of the powered light source <NUM>. The aperture control <NUM> adjusts the diameter of one or more adjustable apertures present in the apparatus <NUM>, such as the aperture stop <NUM> or internal apertures of the camera <NUM> of the mobile user device <NUM>. The skilled person will appreciate that other adjustable elements of the apparatus <NUM> may be controlled by the application <NUM> by way of appropriate electro-mechanical communication. Equally, the application may be implemented without configuration controls <NUM>.

In all of the above embodiments and examples, some or all of the magnification may be implemented with optical elements other than lenses, such as mirrors and/or prisms. Optical elements (including lenses, mirrors and prisms) of a sufficient quality are used such that the optical performance of the apparatus <NUM> is assumed to be diffraction-limited. Effects such as field curvature and geometric distortions are not of importance provided sufficiently small field angles are used, as described above.

The apparatus <NUM> may be used for a range of other medical examination procedures other than that of the ear canal. For example, the apparatus <NUM> may be used to inspect the nose, throat and mouth, including for assisting with dental procedures. The apparatus <NUM> may also be used for external examination, such as that of the eye or skin surface, for which specialised attachments such as restraints or supports may be required. For different specific use cases, different bodies for defining an aperture may be provided at the distal end of the spacing element. For example, a speculum designed for use in a patient's mouth or a body that defines an aperture through which the surface of a patient's skin may be inspected are envisaged. The apparatus can be used equally for veterinary as well as for human medical applications.

While many of the above examples are directed towards medical examination, the apparatus <NUM> as described may equally be used for a range of applications in other industries. These may include but are not limited to manufacturing quality control, inspecting electronic circuitry and examining items for forensic purposes. The apparatus <NUM> can be used for detecting surface defects such as cracks in manufactured materials as part of quality control processes. Alternatively, physical and biological items in can be examined non-invasively for forensic investigation using the described apparatus <NUM>.

Claim 1:
Apparatus (<NUM>) for providing magnification of an object (<NUM>) for a device having a camera, the apparatus comprising:
a first optical element (<NUM>) arranged at a first location to provide an image (<NUM>) of the object in an intermediate image plane (<NUM>);
and a second optical element (<NUM>) arranged at a second location to magnify optically the intermediate image (<NUM>) to provide a final image (<NUM>) in a final image plane (<NUM>);
means for mounting the apparatus in a fixed position relative to the device such that a camera aperture of the device is supported in the final image plane;
a spacing element (<NUM>) for maintaining the first optical element at a fixed distance from the object;
wherein the intermediate image plane and the second optical element are arranged along an optical path (<NUM>) extending between the first optical element and the final image plane;
wherein the second optical element is disposed on the optical path between the intermediate image plane and the final image plane;
characterised in that
a first distance along the optical path from the intermediate image plane to the final image plane is significantly smaller than a second distance along the optical path from the first optical element to the intermediate image plane.