Patent Description:
Conventional optical microscopy provides high resolution (~<NUM>) images and has a huge range of applications, from inspection of electronic devices to cell biology. In many cases, it is desirable to obtain so-called 'optically sectioned' images, i.e. an image of only an axially thin slice through the sample. The advantages of optically sectioned imaging include reduction of out-of-focus blur, a potential increase in resolution, a reduction in light scattered from out-of-focus planes, and an ability to produce high resolution 3D images. The conventional method for obtaining high quality optically sectioned images is confocal microscopy.

Confocal microscopy involves scanning a point of illumination and detecting the reflected or fluorescent light back to a confocal point detector. This results in high quality confocal imaging, but it is necessary to scan the point source and detection region over the sample in two or three dimensions, depending on whether a 2D or 3D image is required. Such scanning can limit the data acquisition rate or, if rapid scanning is employed, will increase the peak power at the sample, which can lead to increased photodamage and phototoxicity of biological samples.

The scanning speed in confocal microscopy can be increased through the use of multiple excitation and detection spots, e.g. in a Nipkow disk microscope. However, the closer adjacent spots are placed, the greater the chance of crosstalk between neighbouring confocal pinholes, which produces a concomitant increase in the size of side lobes or pedestal on the axial point spread function.

A number of alternative methods to confocal microscopy have been proposed, generally termed 'structured illumination' techniques. However, these all require the acquisition of multiple images using a CCD camera followed by image processing to calculate the sectioned image. Performing calculations on weak (noisy) fluorescence images leads to a compounding of the noise in the final image. All confocal and structured illumination techniques require that the whole sample be illuminated along its axial extent, even though only a single lateral plane in the sample is being imaged, and this leads to unnecessary photobleaching and phototoxic effects.

A recently developed technique for obtaining optically sectioned images is that of Selective Plane Illumination Microscopy (SPIM) [<NUM>,<NUM>], which followed early work by Voie et al. [<NUM>] and Fuchs et al. The SPIM technique [<NUM>] uses two objective lenses, separated by an angle of <NUM>° relative to one another and used to view the same sample. One lens is used to illuminate only a thin 'sheet' within the sample and the second lens is used to produce a diffraction limited image of this sheet. The optical configuration for SPIM is illustrated in <FIG>. The region in the sample where fluorescence is excited is perfectly imaged by the detection optics onto the detection image plane. It should be noted that the image is stretched axially due to the greater (M<NUM>) axial magnification of the detection optical system. SPIM has been used to obtain images of small organisms and embryos and can be used to image both reflected or scattered light and fluorescence [<NUM>].

The drawback of SPIM is that two objective lenses are required and this gives rise to the two main disadvantages of this technique. First, it is mechanically difficult to arrange for the two objectives to be placed close enough to one another so that a high numerical aperture lens can be used to collect the light while still being able to produce a thin sheet of illumination. This can restrict the numerical aperture and hence resolution of the imaging system. Second, the need to illuminate the sample with a lens that is in the plane of the sample being imaged means that conventional sample preparation techniques, e.g. glass microscope slides, cannot be used, and a special sample holder needs to be used instead.

Recent work by Tokunaga et al. [<NUM>] and Konopka et al. [<NUM>] has shown that it is possible to illuminate a thin sheet of a sample using the same objective that is used to collect the fluorescence. This is illustrated in <FIG>. This imaging system was termed Highly Inclined and Laminated Optical sheet (HILO) microscopy and variable angle epi-fluorescence microscopy. A 3D image of the specimen can then be produced by scanning the sheet illumination or specimen in one direction. This system is nearly equivalent to a SPIM system, but with two significant differences; the illumination and detection beams are not at <NUM>° (as is usual for SPIM) and the sheet of illumination does not align in the focal plane of the imaging system used to collect the reflected/scattered light or fluorescence. This is shown in the image plane of <FIG>, where the image of the sample fluorescence (shown as a stripe) lies at significant angle to the image plane (dashed line). The detector cannot simply be tilted with respect to the optical axis due to unwanted spherical aberrations that would arise. This aberration will be most severe for parts of the image of the sample image that are furthest from the image plane.

There is therefore a desire to be able to use a technique similar to SPIM, but using a single objective lens at the sample, and with the illumination and detection beams at <NUM>° at the sample, whilst avoiding (or at least minimising) the aberration affects.

Further background art is provided in <CIT>, which discloses a focusing apparatus for use with an optical system. The focusing apparatus includes a focus adjusting means, which enables the position of a selected axial focal plane to be adjusted within the sample.

<CIT> describes systems and methods for forming an image of a specimen. A system may include a relay lens configured to form an intermediate image of light scattered by a specimen. The relay lens may be positioned at an oblique viewing angle from an upper surface of the specimen. The system may also include a reflection grating positioned such that the intermediate image is imaged on the reflection grating. The reflection grating may be configured to reflect the intermediate image. The reflection grating may be positioned negative to the upper surface of the specimen, at the natural image plane. In addition, the system may include an objective lens configured to focus the reflected intermediate image. The system may further include an area detector configured to produce a signal representative of the focused image. An image of the specimen may be formed from the produced signal.

According to a first aspect of the present invention there is provided an optical arrangement as defined in claim <NUM> of the appended claims.

Optional features are defined in the dependent claims.

Thus, the first optical subassembly comprises a first part arranged to produce a magnified image of the sample, and a second part arranged to de-magnify the image obtained from the first part and thereby form the intermediate image.

Preferably the first optical subassembly is configured to produce the intermediate image with a magnification of unity in both the lateral and axial directions. By recreating the original sample both laterally and axially in the intermediate image, this minimizes the effect of aberrations. However, if the first optical subassembly images the sample whilst in an immersion medium (e.g. water or oil) then the magnification of the first optical subassembly is preferably equal to the refractive index of this immersion medium. If the intermediate image is also formed in an immersion medium, then the total magnification of the first optical subassembly is preferably equal to the ratio of the refractive indices of the two immersion media. That is to say, if the sample is placed in a first immersion medium having a refractive index n<NUM>, or the intermediate image is formed in a second immersion medium having a refractive index n<NUM>, then the first optical subassembly is preferably configured to produce the intermediate image with a magnification of M in both the lateral and axial directions, where M is equal to the ratio (n<NUM>/n<NUM>) of the refractive indices of the first and second immersion media.

Although the first and second optical subassemblies may be formed using separate physical components, according to the invention they share common optical components, thereby making the overall optical arrangement potentially more compact. As defined in the invention, a plane mirror is situated at the focus of the second part of the first optical subassembly (e.g. as is shown in <FIG>).

Preferably the numerical aperture of the first optical subassembly is greater than the numerical aperture of the second optical subassembly.

Preferably the said objective lens has a high numerical aperture.

Preferably a light source is arranged to provide an incident beam of light to illuminate or excite an oblique plane in the sample, the oblique plane illuminated/excited corresponding to the oblique plane being imaged.

More preferably, the incident beam of light is directed through the same objective lens as that which is used to receive light from the sample. Using a single objective lens in this manner reduces the number of components in the overall assembly, and potentially makes it more compact and manoeuvrable, particularly in the vicinity of the sample. Moreover, as a result of having only a single objective lens at the sample, a high numerical aperture lens can be used to collect the light while still being able to produce a thin sheet of illumination. Additionally, as a consequence of using a single objective lens at the sample, conventional sample preparation techniques, e.g. glass microscope slides, can be employed.

Particularly preferably the incident beam of light is directed through the objective lens such that it is incident on the sample at an angle of substantially <NUM>° relative to the beam of light received from the sample through the same objective lens. By selectively illuminating the oblique plane in this manner, and collecting the light from it normal (<NUM>°) to the oblique plane, a thin plane may be imaged, without aberrations, and better spatial resolution and sectioning may be achieved.

The incident beam of light may be directed along the whole of the first optical subassembly. Such a configuration enables all the optical components required for oblique plane microscopy to be placed outside the body of a 'conventional' microscope.

Additionally, or alternatively, the components defining the illumination beam path and the second optical subassembly may be mounted on a common platform, and actuation means may be provided for translating the components defining the illumination beam path and the second optical subassembly together. This enables the plane being imaged to be moved through the sample, without affecting or moving the sample itself.

The optical arrangement may further comprise means for changing the magnification of the said objective lens, and means for changing one or more optical components elsewhere in the optical arrangement in correspondence with the change in magnification of the said objective lens, so as to maintain a desired overall magnification within the first optical subassembly.

The optical arrangement may further comprise an image rotating prism such as a Dove prism behind the said objective lens, in order to be able to change the orientation of the oblique illumination plane and the obliquely imaged plane in the sample simultaneously, without the need to physically rotate relatively large parts of the apparatus.

The optical arrangement may be arranged such that the image contrast arises from light reflected or scattered by the sample, or from the polarization state of the reflected or scattered light.

Alternatively, the optical arrangement may be arranged such that the image contrast arises from fluorescent light emitted from the sample, optionally as a result of a multiphoton excitation process. The fluorescent light may be excited at one or more wavelengths and detected in corresponding detection bands at longer (for single photon excitation) or shorter (for multiphoton excitation) wavelengths than each excitation wavelength. Alternatively, the image contrast may arise from differences in the fluorescence lifetime of the sample, or from the polarization state of the emitted fluorescence.

In use, the optical arrangement may be arranged to image particles or cells flowing through the oblique image plane, for example in a microfluidic device.

Alternatively, it may be set up in combination with an electronically-controlled stage for positioning the sample or for moving the sample in one or more directions. By scanning the sample in one or more directions it is then possible to build up a 3D image of the sample.

Preferably the second optical subassembly is arranged to have a combination of both a long working distance and a high numerical aperture, so as to facilitate the avoidance of collisions with the optical elements that make up the first optical subassembly.

According to a second aspect of the present invention there is provided a method of performing oblique plane microscopy as defined in claim <NUM>. Optional features are defined in the dependent claims.

In embodiments employing fluorescence imaging, the fluorescence may originate from single individually-resolvable molecules. The method may further comprise adjusting the number of fluorescent molecules in the sample by activating or deactivating the fluorescence via a photoactivation or photo-switching mechanism, thereby enabling molecules to be individually resolved. The photoactivation or photo-switching mechanism may be controlled by illumination of the sample (that may be wide-field or may be an oblique illumination) at one or more additional wavelengths.

Also with fluorescence imaging, or when imaging reflected or scattered light, the method may further comprise modifying the excitation sheet so that it is exhibits a more complex form such as a sinusoidal grating. Additional resolution may be obtained by modulating the position or phase of the complex illumination (e.g. sinusoidal grating) and acquiring a plurality of images at different modulations.

According to a third aspect which is not covered by the present invention there is provided a method of performing oblique plane microscopy comprising directing an incident beam of light through an objective lens to illuminate or excite an oblique plane in a sample, and receiving light from the sample through the same objective lens, wherein the incident beam of light is incident on the sample at an angle of substantially <NUM>° relative to the beam of light received from the sample.

According to a fourth aspect of the present invention there is provided a microscope comprising an optical arrangement in accordance with the first aspect of the invention, or which is configured for performing a method in accordance with the second or third aspects of the invention. Optical elements may be added to a conventional microscope to form embodiments of the invention.

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

In the figures, like elements are indicated by like reference numerals throughout.

The present embodiments represent the best ways known to the applicant of putting the invention into practice. However, they are not the only ways in which this can be achieved.

The present embodiments provide a method of correcting for the aberrations occurring in the HILO technique [<NUM>,<NUM>] so that true SPIM can be achieved using a single high numerical aperture lens. High numerical aperture (NA) microscope objectives allow light to be collected over a range of angles that is much larger than <NUM>°, e.g. a water immersion lens with an NA of <NUM> collects light over <NUM>°. The principle of SPIM can therefore be achieved using a single objective lens. The present embodiments also include correction optics that allow an oblique plane in the sample to be imaged without encountering optical aberrations. The use of a conventional microscope objective means that biological samples prepared on conventional glass slides can be imaged with high resolution.

A recent paper [<NUM>] (and patent application <CIT>) describes a technique for 'Aberration-free optical refocusing in high numerical aperture microscopy'. This paper describes a microscope system that can be refocused without moving either the sample or the primary microscope objective. This is achieved by coupling a second (almost) identical microscope to the back of the microscope used to image the sample. A third microscope system is then used to re-magnify the image produced by the second system, and can be positioned so as to image a range of focal planes perpendicular to the optical axis within the specimen. This combination of microscope imaging systems corrects for the severe out-of-plane aberrations (mostly spherical aberration) that prevent refocusing of the detector plane in a conventional microscope. However, neither the concept of imaging of oblique planes nor the concept of oblique illumination is provided for.

A key feature of the embodiments disclosed in the present patent application is that, by angling the third microscope with respect to the second system, it is possible to perfectly image an oblique plane through the sample. This is exactly what is required to correct the aberrations encountered in single objective SPIM or HILO microscopy.

<FIG> is a schematic diagram of an embodiment of the aberration correction principle according to the present invention. The present technique may be referred to as oblique plane microscopy (OPM). In use, the optical arrangement <NUM> of <FIG> may be integrated in a microscope, or provided as a "bolt-on" attachment for existing microscopes.

The optical arrangement <NUM> of <FIG> comprises a first optical subassembly <NUM> and a second optical subassembly <NUM>. At one end of the first optical subassembly <NUM> is an objective lens <NUM> having a high numerical aperture. In use, a sample <NUM> is located in the focal plane of the objective lens <NUM>. A light source <NUM> is arranged to provide an incident beam of light <NUM> to illuminate or excite a selected oblique plane (illustrated as a stripe in <FIG>, <FIG>, <FIG> and <FIG>) in the sample <NUM>. In order to illuminate this oblique plane, the incident beam <NUM> is directed, via mirror <NUM>, through one side of the objective lens <NUM>. In other embodiments of the invention, mirror <NUM> may be replaced by a larger partially reflecting beamsplitter or dichroic mirror that covers the whole of the back aperture of lens <NUM>, e.g. as shown in <FIG>.

The light source <NUM> may be a laser, or some other source of visible light, or a source of light outside the visible region, such as ultraviolet or infrared. The light source may be reflected or scattered from the sample, or fluorescence excited through a one- or multi-photon absorption process may be used.

Excited by the incident beam <NUM>, the selected oblique plane in the sample <NUM> emits fluorescence light <NUM>. The fluorescence light <NUM> is collected by the first optical subassembly <NUM>. (In alternative embodiments, reflected or scattered light instead of fluorescence may be collected. ) At the sample, the group of rays forming the detected beam <NUM> is at substantially <NUM>° to the group of rays forming the incident beam <NUM>, and is collected through the same objective lens <NUM> as is used to illuminate the sample. The detected beam <NUM> passes through the opposite side of the objective lens <NUM> from the path of the incident beam <NUM>. The detected beam <NUM> is then directed through further lenses <NUM>, <NUM> and <NUM> to produce an intermediate image <NUM>. The objective lens <NUM>, lens <NUM> and lens <NUM> are not restricted to operating in air and may use any other immersion medium, such as oil or water.

The first optical subassembly <NUM> may be regarded as comprising a first microscope part <NUM> and a second microscope part <NUM>. The first microscope part <NUM>, which comprises lenses <NUM> and <NUM>, produces a magnified image <NUM> (Image <NUM>) of the sample <NUM>. The second microscope part <NUM>, which comprises lenses <NUM> and <NUM>, is arranged to de-magnify the image <NUM> to produce the intermediate image <NUM> (Image <NUM>) which corresponds to the sample <NUM>. The intermediate image <NUM> is at a magnification of unity in both the axial and lateral directions (in the case that lens <NUM> operates in air) with respect to the sample <NUM>. By recreating the original sample <NUM> both axially and laterally in the intermediate image <NUM>, this is expected to prevent the effect of optical aberrations. In effect, lenses <NUM> and <NUM> compensate for (or "undo") any aberrations produced by lenses <NUM> and <NUM>. The first optical subassembly <NUM> may be implemented with any number of optical elements that achieves the same result.

Lens <NUM> has a sufficiently high numerical aperture such that it does not restrict or reduce the numerical aperture of the first optical subassembly <NUM>.

The second optical subassembly <NUM>, which comprises lenses <NUM> and <NUM>, is arranged such that lens <NUM> focuses on the intermediate image <NUM>. The focal plane of lens <NUM> intersects with the focal plane of lens <NUM> at the centre of the intermediate image <NUM>. In an example which is not part of the invention, lens <NUM> is designed to operate with both a long working distance and high numerical aperture, so that the desired angle between lenses <NUM> and <NUM> can be achieved without the two lenses colliding. The light collected from the image <NUM> is magnified by lenses <NUM> and <NUM>, which also focuses the light, thereby producing a magnified image <NUM> of the sample <NUM>. The resulting magnified image <NUM> (Image <NUM>) may be detected by a charge-coupled device (CCD) detector (e.g. detector <NUM> in <FIG>, <FIG> and <FIG>), or other means for detecting or viewing the magnified image. The second optical subassembly <NUM> may be implemented with any number of optical elements that achieves the same result.

At the point of the intermediate image <NUM> (i.e. at the point where the focal plane of lens <NUM> intersects with the focal plane of lens <NUM>), the optical axis of the second optical subassembly <NUM> is at an angle to the optical axis of the first optical subassembly. The angle between the optical axes of the first and second optical subassemblies at the point of the intermediate image <NUM> corresponds to the angle of the selected plane within the sample <NUM> relative to the optical axis of the objective lens <NUM>. This configuration enables the objective lens <NUM> to receive light normal to the selected plane within the intermediate image <NUM>, along the optical axis of lens <NUM>, even though the selected plane is at an oblique angle in the sample <NUM>. That is to say, the selected plane within the intermediate image <NUM> is aligned with the focal plane of lens <NUM>.

Considering it another way, at the point of the intermediate image <NUM> the rays of light leave lens <NUM> and converge (towards the point of the intermediate image <NUM>) about an angle relative to the optical axis of lens <NUM>, the angle corresponding to the angle of the selected plane within the sample <NUM> relative to the optical axis of the objective lens <NUM>. These converging rays are then collected by the second optical subassembly <NUM>. The focal point of lens <NUM> of the second optical subassembly <NUM> coincides with the focal point of the converging rays leaving lens <NUM>, and the optical axis of lens <NUM> is centred about the rays leaving lens <NUM>.

It will be appreciated that good quality lenses are generally designed to magnify or focus light received at their designed focal plane without introducing optical aberrations. Accordingly, since the objective lens <NUM> of the second optical subassembly <NUM> receives the incoming light centrally about its designed focal plane, rather than at an angle, it is able to magnify the selected plane without introducing optical aberrations.

Also, by virtue of the second optical subassembly <NUM> focusing on and magnifying the intermediate image <NUM> (rather than the sample <NUM> itself), the second optical subassembly <NUM> is able to re-image any plane in the intermediate image <NUM> without the need to adjust or disturb the specimen <NUM>. This concept is similar to the ideas presented in [<NUM>], with the important exception that now an oblique plane in the specimen is imaged.

The range of angles of oblique planes that can be imaged depends on the lenses used. The formulae that can be used to calculate this are provided in the Appendix.

In practice, the light rays <NUM> emitted (or reflected or scattered) from the sample will be emitted in all directions. The numerical aperture of the objective lens <NUM> of the second optical subassembly <NUM> places the restriction on the range of angles of light that are ultimately collected by the detector or CCD camera <NUM>.

Image contrast may be achieved in a number of ways. It may arise from light reflected or scattered by the sample. In fluorescence microscopy, the image contrast may arise from fluorescent light excited at one or more wavelengths and detected in corresponding detection bands at longer wavelengths than each excitation wavelength. Alternatively, the image contrast may arise from differences in the fluorescence lifetime of the sample.

The fluorescence may originate from single individually-resolvable molecules. The fluorescence of the molecules may be switched on or off through any photoactivation or photo-switching mechanism, which may be controlled by illumination of the sample at one or more additional wavelengths.

In alternative embodiments, the image contrast may arise from the polarization state of the reflected or scattered light, or the polarization state of the emitted fluorescence.

Although the second optical subassembly <NUM> may be produced as a distinct set of optical components (e.g. lenses <NUM> and <NUM>), in the embodiments according to the invention, the second optical subassembly shares common optical components with parts of the first optical subassembly <NUM>, whilst also achieving the same level of compensation against aberrations. One example of such an embodiment produces magnified image <NUM> rather than image <NUM>, and involves placing an obliquely angled mirror along the plane indicated by a solid line <NUM> at the intermediate focal plane (image <NUM>), resulting in image <NUM> (Image <NUM>') being produced (via mirror <NUM> and lens <NUM>). The angle of the mirror at the intermediate focal plane is half the angle of the slope of the oblique plane being imaged. Mirror <NUM> may also consist of a larger partially reflective mirror that covers the whole back aperture of lens <NUM>.

The concept of using a second optical subassembly <NUM> at an angle to a first optical subassembly <NUM>, as described above, may be used separately from the concept of a common objective lens <NUM> for directing the illumination and collected beams at <NUM>° to one another, and vice versa.

<FIG> and <FIG> illustrate laboratory prototypes of examples which are not part of the invention. In essence, the optical arrangements in <FIG> and <FIG> function in the same way as that of <FIG> as described above. However, being laboratory prototypes, the arrangements of <FIG> and <FIG> provide additional practical details that are useful for putting the present invention into practice.

The components in <FIG> and <FIG> have been allocated the following reference symbols:.

In <FIG>, the angle of the illumination <NUM> can be controlled by translating the slit <NUM>. The thickness of the 'sheet' illumination at lens <NUM> can be controlled by changing the width of the slit <NUM>.

<FIG> shows a fluorescent sphere as the object at the focus of lens <NUM>. The stripe indicates the region where fluorescence is excited. The subsequent images <NUM>, <NUM> and <NUM> (at lenses <NUM>, <NUM> and <NUM>) indicate how the image of the object is distorted (these images are not to scale and serve only to illustrate the distortion).

In <FIG> the optical configuration has been made more light efficient, as less of the excitation light <NUM> is blocked by the slit <NUM>. The angle of the illumination can be controlled by the angle of mirror M1. As before, the width of the 'sheet' illumination at lens <NUM> can be controlled by changing the width of the slit <NUM>.

An alternative method for providing the illumination sheet is to couple the illumination beam through the whole of the first optical subassembly <NUM>, using an optical configuration such as the one shown in <FIG>. Light from a laser <NUM> (or other illumination source) may be focused by a cylindrical lens <NUM> to produce a sheet of illumination in the focal plane of lens <NUM>. This illumination sheet is then relayed through the first optical subassembly <NUM> to create a sheet of illumination at the sample <NUM>.

A slit <NUM> may be used to adjust the width of the laser beam and hence the thickness of the illumination sheet. Other arrangements to adjust the position and width of the illumination sheet may be employed, as will be known by those skilled in the art. The optical axis of the illumination beam path <NUM> is preferably placed at an angle of <NUM>° to the axis of the second optical subassembly <NUM>.

The advantage of this arrangement is that all of the optical components required for oblique plane microscopy can then be placed outside the body of a 'conventional' microscope <NUM>. For fluorescence microscopy, it would be necessary to add a fluorescence emission filter <NUM> into the beam path of the second optical subassembly <NUM> in order to prevent any excitation light reaching the detector <NUM>.

In some cases, it may be advantageous to mount the components defining the illumination beam path <NUM> and the second optical subassembly <NUM> on the same mechanical platform. This will allow the illumination beam path <NUM> and the second optical subassembly <NUM> to be translated together in one or more dimensions using manual or motorized actuator(s). As the illumination and detection beam paths (<NUM>, <NUM>) are accurately relayed to the sample <NUM> by the first optical subassembly <NUM>, movement of the illumination and detection beam paths (<NUM>, <NUM>) together will cause the plane illuminated and imaged to be moved through the sample <NUM>. This movement of the illumination and detection beam paths (<NUM>, <NUM>) can be achieved without affecting or moving the sample itself, and so will not perturb or cause vibrations in the sample <NUM>.

In many situations conventional microscopes (e.g. <NUM>) are fitted with several different microscope objectives. However, in oblique plane microscopy it is necessary to ensure that the correct magnification is obtained between the sample <NUM> and the intermediate image <NUM>. When the microscope objective <NUM> is exchanged for one of a different magnification it is possible to employ a mechanical system that also changes the effective focal length of lens <NUM> at the same time, thus maintaining the correct desired overall magnification within the first optical subassembly <NUM>. It may also be necessary to move other optical components at the same time in order to achieve the necessary path lengths, i.e. to maintain the correct separation between lens <NUM> and lens <NUM> and subsequent optical components. This can be achieved by mounting elements <NUM>, <NUM> and <NUM> all on the same mechanical platform and translating them together. The same effect could be achieved in other ways that will be apparent to those skilled in the art.

It is also possible to insert an image rotating prism, e.g. a Dove prism or such like, immediately behind the back aperture of the objective lens <NUM> in order to change the orientation of the oblique illumination plane and the obliquely imaged plane in the sample simultaneously, without the need to physically rotate relatively large parts of the apparatus.

Optical arrangements according to embodiments of the present invention may be used to image static samples, or may be employed to image particles or cells flowing through the oblique image plane, e.g. in a microfluidic device. Such particles or cells may be intentionally flowed through the oblique image plane, as part of the imaging procedure.

An optical arrangement embodying the present invention may be integrated in a microscope, or provided as a "bolt-on" attachment for an existing microscope. Indeed, a conventional microscope could be used to provide the functionality of the first microscope part <NUM>, provided a sufficiently high NA objective lens <NUM> is employed. The optical arrangement may be combined with an electronically-controlled stage for positioning the sample or for moving the sample in one or more directions. By scanning the sample in one or more directions it is then possible to build up a 3D image of the sample.

Embodiments of the invention could be extended to exploit the possibility of patterning the excitation sheet illumination in the plane of the illumination sheet. More explicitly, it is possible to pattern the excitation in the direction that is both perpendicular to the direction of propagation of the excitation beam and parallel to the plane of the illumination. For example, illuminating the back aperture of lens <NUM> in the fashion depicted in <FIG> would lead to a sinusoidal patterning of the illumination sheet (i.e. effectively a sinusoidal grating). By varying or modulating this patterned illumination, acquiring multiple images at different modulations (grating positions) and then applying image processing techniques, it would be possible to achieve an enhanced resolution in the direction perpendicular to the grating pattern without compromising the thickness of the thin sheet of illumination. This technique may be termed "resolution enhancement though structured illumination".

The principle of imaging an oblique plane in a sample per se is not new and, for example, is the subject of <CIT>, <CIT> and <CIT>. However, all of these methods make use of a dispersive element, such as a prism or a diffraction grating, to achieve the tilted or oblique image plane. The use of such dispersive elements requires the use of lenses with very low chromatic aberration in order to be able to achieve a high quality final image. The technique presented here is novel inter alia in that no dispersive element is required. Also, in the technique presented here, only a part of the available numerical aperture of the objective lens is used to image the sample at an oblique angle, while another part of the lens is used to illuminate the sample at a different angle.

<FIG> is a diagram showing the angular distribution of beams at the sample. The geometry of <FIG> is as follows:
Excitation and detection rays intersect at <NUM>° at the sample.

The numerical aperture of the objective lens is defined as: <MAT>.

One definition of the resolution of a lens is that of the Rayleigh criterion, <MAT> where d is the position of the first minimum of the point spread function relative to the maximum.

In order that the excitation and emission rays intersect at <NUM>° then the following condition must be satisfied: <MAT>.

As an example, for a lens with NA = <NUM> (water, n = <NUM>) then θ = <NUM>° (dlens = <NUM>). If ϕex = <NUM>° then ϕem = <NUM>° and dex = dz = <NUM>, which is an estimate of the thickness of the illumination sheet, i.e. 'z' resolution. Also, dem = dx = dy = <NUM>, which is an estimate of the resolution achieved in the plane of the sheet illumination, i.e. the 'x' and 'y' resolution.

The range over which the sheet illumination remains thin is determined by the divergence of the illumination beam, which is given by the Rayleigh length: <MAT> then, for this example, zr = <NUM>. The confocal parameter is given by 2zr, which equals <NUM> in this example. Decreasing ϕex will increase the confocal parameter at the expense of increasing the thickness of the illumination sheet (or 'z' resolution).

Claim 1:
An optical arrangement for oblique plane microscopy comprising:
a first optical subassembly (<NUM>), including an objective lens (<NUM>) arranged to receive light from a sample (<NUM>) in use, and configured to produce an intermediate image (<NUM>) of the sample (<NUM>), wherein the first optical subassembly comprises a first part (<NUM>) arranged to produce a magnified image (<NUM>) of the sample, and a second part (<NUM>) arranged to de-magnify the image obtained from the first part and thereby form the intermediate image;
a second optical subassembly focused on the intermediate image (<NUM>), the optical axis of the second optical subassembly being at an angle to the optical axis of the first optical subassembly (<NUM>) at a point of the intermediate image (<NUM>), such that the second optical subassembly images an oblique plane in the intermediate image (<NUM>), corresponding to an oblique plane in the sample (<NUM>), the second optical subassembly comprising a mirror (<NUM>) and a lens (<NUM>) for producing a magnified image (<NUM>); and
a plane mirror (<NUM>) at the focus of the second part of the first optical subassembly, wherein the plane mirror is obliquely angled at half the angle of the slope of the oblique plane being imaged,
wherein the first and second optical subassemblies share a common lens (<NUM>).