Patent Description:
In order to aid with the description of the background and the present disclosure, several terms will now be defined:.

<FIG> illustrates a typical monocular mono compound microscope (not to scale - length reduced for clarity). The microscope comprises an objective assembly <NUM> (generally a compound lens made up of several complex lenses) having an aperture stop <NUM>. The objective assembly <NUM> is configured such that it produces an image of the object <NUM> at infinity. A tube lens <NUM> focusses light from the objective assembly <NUM> to produce an intermediate image <NUM> within the microscope. An eyepiece <NUM> magnifies the intermediate image <NUM>, producing a larger virtual image. This virtual image is viewed through an exit pupil <NUM>, which is a reduced image of the aperture stop <NUM>.

A simple binocular stereo microscope can be provided by effectively placing two of the microscopes of <FIG> side by side and angled with respect to each other to provide the parallax required for stereo. However, as the objective assemblies <NUM> are bulky, the working distance of the microscope (i.e. the distance between the objective assembly <NUM> and the object <NUM>) must be large so that there is sufficient space for the objective assemblies <NUM> to be placed side-by-side. The resolution of the microscope is inversely correlated with the aperture and depth of field, and so stereo microscopes with this structure cannot deliver the useful magnification of a monocular mono microscope.

<FIG> illustrates a binocular mono microscope - i.e. a microscope which produces a mono image that can be viewed with both eyes. The objective assembly <NUM>, aperture <NUM>, and tube lens <NUM> are equivalent to those in the monocular mono microscope. A beamsplitter <NUM> is provided within the microscope tube, splitting the light along two paths. Each path comprises an eyepiece <NUM> and mirrors <NUM>, arranged to direct the light to the observer and to ensure that the length of each path is the same. A separate intermediate image <NUM> is produced on each path, and each path has its own exit pupil <NUM> - located such that a viewer can place an eye at each pupil to view the image.

Prior art devices are disclosed in <CIT>, <CIT>, and <CIT>.

The experience of using a binocular mono microscope is rather like looking at a photograph - the viewer is able to see the image with both eyes, but there is no parallax and therefore no depth information and it can be difficult to determine the elevation of features of the image. As such, binocular mono systems may be more comfortable for the user, but they do not replicate the advantages of stereo systems with regards to depth perception. However, because only a single objective assembly is used, the aperture and magnification is not limited in the same way as for a stereo microscope.

According to a first aspect of the invention, there is provided a stereo microscope in accordance with claim <NUM>.

In order to provide a stereo image, a microscope must provide an image to each eye from different perspectives (at an angle to mimic normal stereo vision). In previous designs, this has been done by providing separate objective assemblies, each providing a separate image, one for each eye of the user. This results in the reduced magnification possible with stereo microscopes compared to mono microscopes. An alternative means of providing the different perspectives is shown in <FIG>. The objective assembly <NUM> comprises an aperture stop <NUM> which is illustrated in <FIG>. The aperture stop <NUM> is divided into two separate apertures <NUM> and <NUM>. The light from each aperture <NUM>, <NUM> is then routed via separate optics <NUM> to the eyepeices (not shown).

<FIG> show the ray paths through each of apertures <NUM> and <NUM>, respectively. As can be seen, the perspective of each aperture is different - meaning that the images resulting from each aperture can be directed to different eyes of a viewer so that they are perceived as a stereo image.

However, providing a split aperture raises several challenges. Firstly, the aperture itself is small and generally embedded within the objective assembly, and so the manufacture required to produce such an aperture is complex. Secondly, the apertures <NUM> and <NUM> are each smaller than the single aperture <NUM>. This results in the resolution of the microscope being lower than can be achieved by a single aperture (though the achievable resolution is still greater than for a stereo microscope with two objectives), as well as dimming and distortion of the image compared to a single aperture. This also results in the exit pupils being smaller than they would be with a single aperture, making such a microscope impractical for actual use (as the observer must keep their head extremely still to avoid losing the image and the optical performance of the eye is reduced when the iris aperture is not completely filled ).

<FIG> is a schematic diagram of a microscope configured to provide split apertures. The microscope comprises an objective assembly <NUM> having a (single) aperture <NUM>, and a tube lens <NUM>, arranged to provide an image of an object <NUM>. The microscope further comprises an intermediate lens <NUM>, a beamsplitter <NUM>, and mirrors <NUM>. The lens <NUM>, beamsplitter <NUM>, and mirrors <NUM> together form respective images 512a, 512b of the aperture <NUM> (hereinafter referred to as an aperture image) on each of two optical paths 521a, 521b. A respective stop structure 507a, 507b is provided in the plane of each aperture image 512a, 512b, such that each stop structure 507a, 507b blocks a different portion of the respective aperture image, defining an exit pupil 513a, 513b. Eyepiece lenses 508a, 508b and image sensors 509a, 509b are located such that the image sensors capture a real image of the object <NUM> through each of the exit pupils 513a, 513b. In practice, further optics (not shown) will be required beyond the aperture images to produce real images 531a, 531b of the object beyond the exit pupils 513a, 513b which are then viewed through the eyepieces 508a, 508b. The stop structures 507a, 507b are positioned such that the respective images captured by the image sensors 509a, 509b can be displayed on a stereoscopic viewer as a stereo image of the object, i.e. with one image sensor providing the left eye view and the other providing the right eye view, due to the positioning of the respective stop structures.

The aperture image 512a, 512b will be larger than the aperture itself, and each stop structure only needs to provide one of the perspectives. Therefore, the stop structures can be manufactured much more easily than a split aperture as shown in <FIG>, while having the same effect of providing a stereo image.

The stop structure 507a, 507b may be any suitable shape. Several possibilities are shown in <FIG>. For example, the stop structure 507a, 507b may have an aperture which defines the exit pupil (<NUM>) or may be a "curtain" which blocks only one side of the aperture image 512a, 512b with a flat (<NUM>) or curved (<NUM>) edge. In order to provide a full, pure stereo image, the stop structures 507a, 507b must be located so that the exit pupil for each eye corresponds to a portion of the aperture that is not in the other exit pupil. A less pronounced stereo effect is produced if the exit pupils overlap slightly (i.e. each contains a portion of the aperture which is in the other exit pupil, and a portion which is not in the other exit pupil). If the exit pupils overlap completely, then the result is a binocular mono image.

<FIG> illustrate the effect of different stop structure positions for "curtain"-style stop structures. The same principles apply for other shapes of stop structure. The top part of each figure shows the aperture images and stop structures, the middle part shows the resulting exit pupils (overlaid so that the differences can be seen), and the bottom part shows a representation of the degree of stereoscopy (as much as can be presented in a 2D medium). As shown in <FIG>, where there is no stop structure 507a, 507b occluding the aperture images 512a, 512b, the exit pupils 513a, 513b for each eye correspond exactly and a binocular mono image <NUM> results. This can also occur for stop structures comprising a symmetric aperture, positioned such that the exit pupils correspond exactly. As shown in <FIG>, where each stop structure 507a, 507b occludes the respective aperture image 512a, 512b such that the exit pupils 513a, 513b are completely separate regions of the aperture, a full stereo image <NUM> results. As shown in <FIG>, where each stop structure 507a, 507b occludes a separate portion of each aperture image 512a, 512b, such that the exit pupils 51a, 513b are overlapping regions of the aperture but there is a portion of each exit pupil which does not correspond to a portion of the other exit pupil, then a less pronounced stereo image <NUM> results.

The resolution of the image is dependent on the dimensions of the effective aperture formed by the aperture <NUM> and the stop structure 507a, 507b (i.e. the aperture which, if located at the aperture <NUM>, would form the exit pupil 513a, 513b), with the resolution being lower the smaller the effective aperture is (though the exact value depends on the shape of the effective aperture). As such, the positioning of the stop structure 507a, 507b is a balance between resolution and stereo effect.

Other effects of the stop structures on the stereo image produced can be compensated for prior to display of the images. <FIG> shows a schematic illustration of the microscope <NUM>. The microscope <NUM> comprises stop structures <NUM> and image sensors <NUM> as described above. Additionally, the microscope comprises a digital image processor <NUM> which takes the output <NUM> of the image sensors <NUM> and output of <NUM> of the stop structure control, and adjusts it to compensate for unwanted changes to the image resulting from adjustments to the stop structures <NUM>, with the adjustments being performed on the basis of the shape and position <NUM> of the stop structures. The digital image processor then provides the corrected image as an output <NUM> from the microscope (e.g. to a stereoscopic display).

For example, the intensity of the image will vary with the position of the stop structures <NUM>. This occurs both due to different amounts of area of the aperture <NUM> being blocked, and due to intensity variations across the aperture <NUM> (meaning that there will be intensity variations even for stop structures such as that of <FIG> that always block the same amount of the aperture area). The intensity will depend on both the position and shape of the stop structures.

The aperture <NUM> will have an intensity profile, which is a function describing how much each point on the aperture contributes to the intensity of the final image. The reduction in intensity resulting from the stop structures can be determined by comparing the integral of this intensity profile over the effective aperture formed by each stop structure with the integral of the intensity profile over the whole aperture <NUM>. The digital image processor may then adjust the brightness of the output of each image sensor to ensure that the intensity is apparently constant for the user between different stop structure positions.

The intensity variation will also depend on the shape of the stop structures. The microscope may be provided with multiple different sets of stop structures from which the stop structures to be used are selected. The digital image processor should be configured to apply a different relationship between stop structure position and image brightness adjustment for each set of stop structures. The set of stop structures may be identified by the user in software of the digital image processor, or automatic identification may occur when the stop structures are installed in the microscope (e.g. by providing optical or electronic identifiers on the stop structures which interface with sensors on the microscope, or by other suitable means). Where the microscope is intended to work with only a single type of stop structure, the digital image processor only requires a single relationship between stop structure position and image brightness.

Similarly, occluding different regions of the aperture <NUM> will affect the distortion of the image (due to lens aberrations and other optical effects). This distortion may also be corrected by the digital image processor, with the parameters of the transformation used being dependent on the stop structure shape and position.

The relationship between stop structure position (and shape, if multiple sets of stop structures may be used) and the digital image processing required may be preconfigured, e.g. with a lookup table programmed into the digital image processor, or it may be calculated on-the-fly from the known parameters. The lookup table or predetermined function may be obtained via a calibration step, e.g. measuring intensity, image distortion, or other properties for a range of stop structure positions, and using this data (with suitable interpolation) to compute a lookup table.

The stop structure may be adjustable to allow the user to transition from mono to stereo views, and control the degree of stereoscopy. A setup for achieving this is shown in <FIG>. Each stop structure includes a moveable curtain <NUM>, which is can be introduced into the optical path in a controllable manner so as to occlude a variable amount of the aperture image 512a, 512b. The optical path for the other image has an equivalent system, and the curtains are coupled such that each occludes the same proportion of the respective aperture image 512a, 512b. The moveable curtain <NUM> can be adjusted from a position in which each occludes none of the aperture image (resulting in a binocular mono image) to a position in which the exit pupils are non-overlapping portions of the aperture image 512a, 512b (resulting in a pure stereo image). The moveable curtains <NUM> are configured to move such that each blocks an equal sized portion of the respective aperture image 512a, 512b, on opposite sides of the respective image.

The setup of <FIG> allows a continuous and progressive transition between stereo and binocular mono modes of the microscope, without interruption of the image viewed by the observer. It has been surprisingly found through the use of this apparatus that when transitioning smoothly from a stereo image to a higher resolution mono image, the user experiences a sensation of depth with the mono image which is not present if the mono image is viewed without such a transition. This means that the system described above allows much of the advantage of the stereoscopic image to be retained, while also having the higher resolution of the mono image.

From the above description, it will be noted that the structure of the single objective stereo microscope from the objective assembly <NUM> up to but not including the lens <NUM> is the same as that of the conventional mono microscope from the objective assembly <NUM> up to but not including the eyepiece <NUM>. The head and eyepiece assembly of many commercially available microscopes are removeable, and therefore it is possible to retrofit an existing mono microscope (whether binocular or monocular) with a system comprising the lens <NUM>, beamsplitter <NUM>, mirrors <NUM>, and stop structures 507a, 507b, where the system is configured to attach in place of the head and eyepiece assembly of the mono microscope such that the lens <NUM> is in the light path of the microscope - i.e. in the path which light from the object takes through the microscope. The original mono microscope may or may not apply optical corrections such as field curvature, chromatic aberration etc using the eyepiece - in systems for retrofitting to microscopes where these corrections are applied, the lens <NUM> and/or eyepieces 508a, 508b may be configured to apply equivalent corrections.

The image sensors may be CCDs or other image sensors. A further advantage of the use of image sensors is that there is no requirement for the exit pupils to be arranged to precisely align with the viewers left and right eyes to view them, which allows for simplified structure of the microscope.

One example of a stereoscopic display is that described in <CIT>, and shown in <FIG>. The display comprises two projectors 20a and 20b, which display the left eye and right eye images respectively. Each projector comprises a display <NUM> and an optical arrangement <NUM> (comprising one or more lenses <NUM> and/or mirrors <NUM>) for providing a focussed image of each of the left eye and right eye images on a mirror <NUM>. The mirror <NUM> reflects the exit pupils of the projectors onto a viewing plane (VP) for viewing by an observer, optionally via a viewing lens <NUM>. Optical components other than the mirror <NUM> and viewing lens <NUM> may be placed out of the direct line of sight of the observer, to give a clean viewing experience.

Other examples of stereoscopic displays include "virtual reality" headsets, 3D displays with active glasses (i.e. glasses which are synchronised to the refresh rate of the TV, and block each eye for alternate frames), and 3D displays with passive glasses (e.g. displays that present each of the left eye image and right eye image as a different polarisation, and are used with glasses that have a corresponding polarisation filter for each eye).

An advantage of using an image sensor coupled to a stereoscopic display rather than having the user directly observe the microscope through the exit pupils is that the size of the exit pupils available for the viewer is not limited by the microscope optics, and is not restricted by the stop structures 507a, 507b. Larger exit pupils give a more comfortable viewing experience. This is due to the fact that, where the exit pupils are small, the user must keep their head in a specific position to see the stereo image. Where the exit pupils are smaller than a certain size, as would likely be the case where stop structures are used, the user may have difficulty seeing the image at all, as the human eye does not function well when the exit pupil is smaller than the pupil of the eye. In fact, with the optical systems as used in most existing microscopes, the exit pupil is already smaller than the entrance pupil of the user's eye, which limits the resolution, and causes any inhomogeneity in the eye (e.g. floaters) to have a significantly greater effect on the user's vision.

Claim 1:
A stereo microscope comprising:
an objective assembly (<NUM>) including an aperture (<NUM>);
a lens (<NUM>);
a beamsplitter (<NUM>);
wherein the lens (<NUM>) and the beamsplitter (<NUM>) are configured to form a respective aperture image (512a, 512b) on each of two optical paths;
the assembly further comprising, on each optical path, a stop structure (507a, 507b);
wherein, the lens and beamsplitter are positioned relative to the aperture such that each aperture image (512a, 512b) is located in the plane of the respective stop structure (507a, 507b), and each stop structure (507a, 507b) blocks a portion of the respective aperture image (512a, 512b) in order to provide an exit pupil (513a, 513b), such that a stereoscopic image of an object viewed through the microscope is produceable by the combination of the images of the object visible through each exit pupil (513a, 513b);
the stereo microscope further comprising:
two image sensors, each image sensor being configured to capture an image visible through the respective exit pupil and to output a digital image;
a digital image processor configured to apply a correction to the respective digital image output by each image sensor in order to compensate for a distortion related to the shape and position of the stop structure, the correction being based on the shape and position of the respective stop structure;
wherein the correction comprises performing a spatial transformation of the respective image to correct for distortions introduced by optical structures of the stereo microscope, the spatial transformation having at least one parameter dependent on the position of the respective stop structure.