FIELD AND BACKGROUND

The present invention generally relates to the field of optical measurement and/or inspection techniques and more specifically relates to an auto-focus method and device, particularly useful when viewing non planar surfaces.

Auto focusing is an essential feature in many automated inspection fields such as the computer chip industry, biomedical research, data reading/recording in optical information carriers, etc. Specifically, when analyzing samples in multiwell plates including a plurality of wells on a single plate, auto focusing of a microscope viewing the contents of the wells can enable more efficient work procedures as the operator need not focus the objective on each well in the plate separately.

Various auto focusing methods for inspection of a multiwell plate have been disclosed in the past, such as in US Patent No. 7,109,459. However, when using wells having a non- planar bottom, such as a multiwell plate having wells with a U-shaped bottom, such as are used, for example, for growing living cells into spheroids, existing autofocus methods may require image analysis which is time-consuming.

There is thus a need for a method for auto focusing a microscope on a multiwell plate, which is suitable for multi-well plates having wells with a non-planar bottom surface.

SUMMARY

The present invention generally relates to the field of optical measurement and/or inspection techniques and more specifically relates to an auto-focus method and device, particularly useful when viewing non planar surfaces.

There is provided in accordance with an embodiment of the invention an auto- focusing method for determining an in-focus position of a plurality of wells in at least a portion of a multi-well plate, the method including:

using a first objective lens having a first magnification to identify, in each of at least three wells of a selected subset of the plurality of wells, an in-focus position of each said well with respect to the first objective lens;

on the basis of at least three said in-focus positions, computing a plane along which the at least three wells will be in focus with respect to at least one objective lens having a second magnification that is not greater than the first magnification; and using the at least one objective lens to scan, along the plane, at least some of the plurality of wells in the portion of the plate.

In some embodiments, the at least one objective lens is the first objective lens, and the first magnification is equal to the second magnification. In some embodiments, the computing a plane includes computing a plane along which the at least three wells are in focus with respect to the first objective lens.

In some embodiments, the at least one objective lens is a second objective lens, different from the first objective lens, wherein the second magnification is smaller than the first magnification. In some embodiments, computing a plane includes translating at least three of the in-focus positions identified using the first objective lens to corresponding second in-focus positions with respect to the second objective lens based on optical characteristics of the second objective lens; and computing the plane on the basis of at least three the second in-focus positions. In some embodiments, computing a plane includes: on the basis of at least three said in-focus positions, computing a first plane along which the at least three wells will be in focus with respect to the first objective lens; and translating the first plane to a corresponding plane along which the at least three wells will be in focus with respect to the second objective lens based on optical characteristics of the second objective lens, thereby to compute the plane.

In some embodiments, scanning using the at least one objective lens is carried out without carrying out additional focusing operations.

In some embodiments, the subset of the plurality of wells includes more than three of the plurality of wells.

In some embodiments, identifying an in-focus position includes identifying an in- focus position for each well in the subset.

In some embodiments, each of the wells includes generally cylindrical side walls, and a bottom surface including a portion of at least one of a sphere, a parabola, and an ellipse. In some embodiments, each of the wells has a U-shaped cross-section.

In some embodiments, each of the wells includes generally cylindrical side walls, and a planar bottom surface. In some embodiments, the planar bottom surface lies generally parallel to a top surface of the multiwell plate, such that the well has a rectangular cross section.

In some embodiments, each of the wells is frusto -conical.

In some embodiments, each of the wells has inclined side walls, a planar bottom, and a trapezoidal cross section. In some embodiments, the method further includes, prior to the using a first objective lens, aligning the first objective lens to lie axially over the center of one of the wells.

In some embodiments, the portion of the plate includes a quadrant of the plate. In some embodiments, the portion of the plate includes an entirety of the plate.

There is also provided, in accordance with an embodiment of the invention, an auto- focusing method for determining an in-focus position of at least a portion of a well in a plate, the method including: using a first objective lens having a first magnification to identify, at at least one location of the well, a first in-focus position of at least a portion of the well with respect to the first objective lens; identifying, for the first in-focus position, a corresponding in-focus position with respect to at least one objective lens having a second magnification, based on optical characteristics of the at least one objective lens; and using the at least one objective lens to scan, at a height corresponding to the corresponding in-focus position, at least the portion of the well, wherein the second magnification is not greater than the first magnification.

In some embodiments, the at least one objective lens is the first objective lens, the first magnification is equal to the second magnification, and the corresponding in-focus position is the first in-focus position.

In some embodiments, the at least one objective lens includes a second objective lens, different from the first objective lens, wherein the second magnification is smaller than the first magnification. In some such embodiments, the identifying includes translating the first in-focus position to the corresponding in-focus position with respect to the second objective lens based on optical characteristics of the second objective lens.

In some embodiments, scanning using the at least one objective is carried out without carrying out additional focusing operations.

In some embodiments, the well includes generally cylindrical side walls, and a bottom surface including a portion of at least one of a sphere, a parabola, and an ellipse. In some embodiments, the well has a U-shaped cross-section.

In some embodiments, the well includes generally cylindrical side walls, and a planar bottom surface. In some embodiments, the planar bottom surface lies generally parallel to a top surface of the plate, such that the well has a rectangular cross section.

In some embodiments, the well is frusto-conical. In some embodiments, the well has inclined side walls, a planar bottom, and a trapezoidal cross section.

There is also provided, in accordance with an embodiment of the invention, an auto- focusing device for automatically determining an in-focus position of a plurality of wells located in at least a portion of a plate containing wells, the device including: a computation component programmed to compute a plane along which at least three wells in the portion of the plate would be in focus with respect to an objective lens; a first objective lens functionally associated with the computation component, the first objective lens having a first magnification, images from the first objective lens being used by the computation component for identifying an in-focus position for each of at least three wells of a selected subset of the plurality of wells; and at least one objective lens having a second magnification, the second magnification not being greater than the first magnification, for scanning at least some of the plurality of wells in the portion of the plate along the plane, wherein the computation component is configured to compute the plane along which the at least three wells would be in-focus with respect to the at least one objective lens on the basis of at least three the in- focus positions.

In some embodiments, the at least one objective lens is configured to scan the plurality of wells along the plane without carrying out additional focusing operations.

In some embodiments, the at least one objective lens is the first objective lens, and the second magnification is equal to the first magnification.

In some embodiments, the at least one objective lens is a second objective lens, different from the first objective lens, and the second magnification is smaller than the first magnification.

In some embodiments, the computation component is programmed to compute the in- focus plane by: translating at least three of the in-focus positions identified using the first objective lens to corresponding second in-focus positions with respect to the second objective lens based on optical characteristics of the second objective lens; and computing the in-focus plane on the basis of at least three the second in-focus positions.

In some embodiments, the computation component is programmed to compute the in- focus plane by: on the basis of at least three the in-focus positions, computing a first plane along which the at least three wells will be in focus with respect to the first objective lens; and translating the first plane to a corresponding plane along which the at least three wells will be in focus with respect to the second objective lens based on optical characteristics of the second objective lens, thereby to compute the plane.

In some embodiments, the computation component is programmed to identify an in- focus position for each well in the subset.

In some embodiments, the device is adapted for use with a plate in which each of the wells includes generally cylindrical side walls, and a bottom surface including at least one of a portion of a sphere, a parabola, and a portion of an ellipse. In some embodiments, the device is adapted for use with a plate in which each of the wells has a U-shaped cross section.

In some embodiments, the device is adapted for use with a plate in which each of the wells includes generally cylindrical side walls, and a planar bottom surface. In some embodiments, the planar bottom surface lies generally parallel to a top surface of the plate, such that each of the wells has a generally rectangular cross section.

In some embodiments, the device is adapted for use with a plate in which each of the wells is frusto-conical. In some embodiments, the device is adapted for use with a plate in which each of the wells has inclined side walls, a planar bottom, and a trapezoidal cross section.

In some embodiments, the portion of the plate includes a quadrant of the plate. In some embodiments, the portion of the plate includes an entirety of the plate.

There is also provided, in accordance with an embodiment of the invention, an auto- focusing device for automatically determining an in-focus position of at least a portion of a well, the device including: a computation component programmed to compute an in-focus position of the portion of the well; a first objective lens functionally associated with the computation component, the first objective lens having a first magnification, images from the first objective lens being used by the computation component for identifying, in at least one position of the well, a first in-focus position of the well with respect to the first objective lens; and at least one objective lens having a second magnification, the second magnification not being greater than the first magnification, for scanning at least a portion of the well at a height of a corresponding in-focus position of the well with respect to the at least one objective lens, wherein the computation component is programmed to identify the corresponding in-focus position based on optical characteristics of the at least one objective lens.

In some embodiments, the at least one objective lens is configured to scan the portion of the well without carrying out additional focusing operations.

In some embodiments, the at least one objective lens is the first objective lens, the second magnification is equal to the first magnification, and the corresponding in-focus position is the same as the first in-focus position.

In some embodiments, the at least one objective lens is a second objective lens different from the first objective lens, and wherein the second magnification is smaller than the first magnification. In some embodiments, the computation component is programmed to identify the corresponding in-focus position by translating the first in-focus position to the corresponding in-focus position with respect to the second objective lens based on optical characteristics of the second objective lens.

In some embodiments, the device is adapted for use with a well including generally cylindrical side walls, and a bottom surface including at least one of a portion of a sphere, a parabola, and a portion of an ellipse. In some embodiments, the device is adapted for use with a well having a U-shaped cross section.

In some embodiments, the device is adapted for use with a well including generally cylindrical side walls, and a planar bottom surface. In some embodiments, the planar bottom surface lies generally parallel to a top surface of the plate, such that the well has a generally rectangular cross section.

In some embodiments, the device is adapted for use with a frusto-conical well. In some embodiments, the device is adapted for use with a well having inclined side walls, a planar bottom, and a trapezoidal cross section.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. In case of conflict, the specification, including definitions, will take precedence.

As used herein, the terms "comprising", "including", "having" and grammatical variants thereof are to be taken as specifying the stated features, integers, steps or components but do not preclude the addition of one or more additional features, integers, steps, components or groups thereof. These terms encompass the terms "consisting of" and "consisting essentially of .

As used herein, the indefinite articles "a" and "an" mean "at least one" or "one or more" unless the context clearly dictates otherwise.

Embodiments of methods and/or devices of the invention may involve performing or completing selected tasks manually, automatically, or a combination thereof. Some embodiments of the invention are implemented with the use of components that comprise hardware, software, firmware or combinations thereof. In some embodiments, some components are general-purpose components such as general purpose computers or monitors. In some embodiments, some components are dedicated or custom components such as circuits, integrated circuits or software.

For example, in some embodiments, some of an embodiment is implemented as a plurality of software instructions executed by a data processor, for example which is part of a general-purpose or custom computer. In some embodiments, the data processor or computer comprises volatile memory for storing instructions and/or data and/or a non-volatile storage, for example, a magnetic hard-disk and/or removable media, for storing instructions and/or data. In some embodiments, implementation includes a network connection. In some embodiments, implementation includes a user interface, generally comprising one or more of input devices (e.g., allowing input of commands and/or parameters) and output devices (e.g., allowing reporting parameters of operation and results.

BRIEF DESCRIPTION OF THE FIGURES

Some embodiments of the invention are described herein with reference to the accompanying figures. The description, together with the figures, makes apparent to a person having ordinary skill in the art how some embodiments of the invention may be practiced. The figures are for the purpose of illustrative discussion and no attempt is made to show structural details of an embodiment in more detail than is necessary for a fundamental understanding of the invention. For the sake of clarity, some objects depicted in the figures are not to scale.

In the Figures:

FIGS. 1A and IB are, respectively, a top plan view of a multiwell plate and a sectional view of a single well in a multiwell plate, the well having a non-planar bottom surface, for which embodiments of the teachings herein may be useful;

FIG. 2 is a block diagram of an embodiment of an imaging device for auto-focusing on samples in a multiwell plate in accordance with an embodiment of the teachings herein; and

FIG. 3 is a flow chart of an embodiment of a method for auto-focusing an imaging device on samples in a multiwell plate in accordance with an embodiment of the teachings herein.

DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

The principles, uses and implementations of the teachings herein may be better understood with reference to the accompanying description and figures. Upon perusal of the description and figures present herein, one skilled in the art is able to implement the invention without undue effort or experimentation.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its applications to the details of construction and the arrangement of the components and/or methods set forth in the following description and/or illustrated in the drawings and/or the Examples. The invention can be implemented with other embodiments and can be practiced or carried out in various ways. It is also understood that the phraseology and terminology employed herein is for descriptive purpose and should not be regarded as limiting.

Reference is now made to Figures 1A and IB which are, respectively, a top plan view of a multiwell plate and a sectional view of a single well in a multiwell plate, the well having a non-planar bottom surface, for which embodiments of the teachings herein may be useful.

As seen in Figure 1A, a multiwell plate 10 has a top surface 11, side surfaces (not shown) and, in some embodiments, a bottom surface (not shown). The plate 10 includes a plurality of wells 12, arranged in a grid formed of columns 14 and rows 16 and accessible via apertures 17 in top surface 11. Typically, the rows and columns are enumerated or otherwise labeled so as to enable a user to easily reference a specific well 12. The multiwell plate 10 in the illustrated embodiment includes 96 wells, though other types of plates, which include, for example, a different number of wells, such as 12, 24, or 384 wells, may be used with the teachings herein as described in further detail hereinbelow. Typically, the wells 12 have fixed distances between one another, and thus are distributed on plate 10 at regular intervals. Specifications as to the distanced between wells are standard in the art, and are typically also provided by the manufacturer of the plate. Often, the number of wells in the plate have a 3:2 aspect ratio. As such the wells may be arranged, for example, as a 3x2 grid, 6x4 grid, 12x8 grid, or 24x16 grid.

Turning to Figure IB, it is seen that the cross section of each well 12 in the plate 10 may be non-rectangular, such that the well has a non-linear bottom surface. In the illustrated embodiment, the well 12 includes a cavity 18 and has a U-shaped cross section, such that side walls 20 of the well generally form a cylinder, and a bottom portion 22 of the well forms part of a sphere, part of a parabola, or part of an ellipse, thereby defining a curved bottom surface to the well. As such, the well typically has a U-shaped cross-section or a cross-section somewhat resembling a parabola. Typically, the thickness of side walls 20 and of bottom portion 22 is uniform. A rim 26, typically forming part of, and being flush with or being raised with respect to, top surface 11 of plate 10, often surrounds the well 12.

Multiwell plates including wells having non-planar bottom surfaces are well known in the art, and are commercially available from many manufacturers, such as Corning Incorporated Life Sciences of Tewksbury, Massachusetts. Such multi-well plates are used for many types of samples, including for growing spheroids, for growing non-adherent cells such as lymphocytes and other blood cells, for analysis of 3-dimensional samples and for handling of compounds. Oftentimes, analysis of such samples requires imaging of the samples within the wells.

It will be appreciated that due to the curvature of the bottom surface of well 12 the area at which a microscope viewing the well would be in focus is typically very small, and in some cases comprises a single point. As such, existing auto-focusing mechanisms, such as that disclosed in US Patent No. 7,109,459 often do not succeed in focusing on a sample disposed within the well. As explained hereinbelow, the method of the teachings herein enables an operator to autofocus an imaging device on a well having a non-planar bottom, such as U-shaped wells 12 of Figure IB, without having to manually focus the imaging device on each individual well.

It will be appreciated that though the exemplary illustration shows wells having a U- shaped cross section, the method of the teachings herein as described hereinbelow may be used for other types of wells, such as wells having a planar bottom surface and a rectangular cross section, or wells of a frusto-conical shape, i.e. comprising a cutoff cone having inclined side walls and a planar bottom, and having a generally trapezoidal cross section.

Reference is now made to Figure 2, which is a block diagram of an embodiment of an imaging device 200 for auto-focusing on wells in a multiwell plate in accordance with an embodiment of the teachings herein.

It will be appreciated that the disclosure herein discusses auto-focusing on wells including samples as an example only, and that the same method and device may also be used to auto-focus on wells not containing a sample, or on a multiwell plate in which some wells include a sample and other wells do not.

As seen in Figure 2, imaging device 100 includes a scanning microscope 202, functionally associated with a sample platform movable along the X, Y, and Z axes. The sample platform is configured to have disposed thereon a sample plate 205, which may be for example a plate like plate 10 of Figures 1A and IB.

Microscope 202 further includes a plurality of objective lenses 206 functionally associated with an objective lens exchanger 208. At any given time, a single one of lenses 206 is aligned with a sample platform (not shown) and is operational, such that the sample plate disposed on the sample platform may be viewed through the objective lens. Objective lens exchanger 208 is configured to change the operational lens, used for viewing the sample, when a change of objective is required. An example of such an exchanger is described, for example, in WO 2012/097191, the contents of which are incorporated herein by reference. Microscope 202 is functionally associated with at least one illumination source, controlled by a controlling unit (not shown). In some embodiments, the microscope includes a first illumination source comprising a transmission light source 210a, such as an LED lamp, configured to illuminate the sample platform during imaging of a sample plate 205 disposed thereon. In some embodiments, the microscope further includes a second illumination source comprising an excitation light source 210b configured to provide illumination to yield a response in a sample carried on or in the sample plate 205, such as providing illumination to excite a fluorescent or a luminescent component of the sample. In some embodiments, illumination from light source(s) 210a and/or 210b impinges upon one or more optical elements 212, such as a mirror, a dichroic cube, a beam splitter, a filter, and the like, prior to impinging upon a sample disposed on the sample plate 205. In some embodiments, illumination from illumination source(s) 210 travels through an optic fiber 213 before impinging on the sample.

In some embodiments, the image visible by microscope 202 is captured by an image capturing unit (not shown), and is transferred to a processing unit 214 for further processing and analysis.

Reference is now made to Figure 3, which is a flow chart of an embodiment of a method for auto-focusing an imaging device on samples in a multiwell plate in accordance with an embodiment of the teachings herein.

The method described hereinbelow may be used in an imaging device, such as imaging device 200 of Figure 2, to automatically determine an in-focus position of a plurality of samples disposed in a sample plate, such as plate 10 of Figure 1A, the plate containing a plurality of wells. The method may be carried out on a plate including wells having a non- planar bottom surface, such as wells 12 of Figure IB, or on other types of wells, such as wells having a planar bottom surface, or frusto-conical wells having inclined side walls and a planar bottom, and the like.

As seen at step 300, a subset of the wells in the plate is selected. In some embodiments, the subset includes at least three wells that each contains a liquid or a sample, though this is not necessary for the method disclosed herein. For at least three of the wells in the subset, and in some embodiments for all the wells in the subset, an in-focus position of the sample included in the well is identified with respect to a first objective lens having a first magnification, such as an objective lens 206 of Figure 2, at step 302.

Typically, the first objective lens has a fairly large magnification, such as for example 20x, lOx or the like. In some embodiments, the subset includes more than three wells, but in-focus positions are identified only for three of the wells in the subset. In some embodiments, the subset includes more than three wells, and in-focus positions are identified for more than three wells in the subset, but not for all the wells in the subset. For example, the subset may contain at least five wells, and in-focus positions are identified for at least four wells but not for all the wells in the subset. In some embodiments, in-focus positions are identified for all the wells in the subset.

The in-focus positions of the samples in the wells of the subset may be identified using any suitable method known in the art, including both manual and automatic methods. In some embodiments, the in-focus positions are identified substantially as described in US Patent No. 7,109,459, which is incorporated by reference as if fully set forth herein.

In accordance with the teachings of US Patent No. 7,109,459, in order to identify the in-focus positions, the focal plane of the first objective is spaced from a surface of the plate, such as a bottom surface of the plate, a certain distance, for example about one millimeter. The focal plane of the objective is then displaced towards the plate, for example by displacing the objective or the plate relative to one another. For example, the objective lens may be disposed below the plate, such that the focal plane of the objective lens is disposed below the surface of the plate and the focal plane is displaced vertically upward toward the surface of the plate.

During displacement of the focal plane of the objective lens, control hardware of the microscope records the intensity of light reflected from the plate, until the intensity of the detected light reaches a maximal value, which, in some embodiments, is higher than a preset threshold. This maximal value of the detected light intensity corresponds to an in-focus position of a surface of the plate.

Without wishing to be bound by theory, in the example described above, in which the objective lens is disposed below the plate and the focal plane is initially disposed below the plate and is displaced toward the plate, it is believed that the location at which maximal light intensity is observed corresponds to a point at which the focal plane of the objective lens is tangential to the curved surface of the well bottom.

Subsequently, in some embodiments, the focal plane of the objective continues to be displaced toward the plate, until another peak in the intensity of reflected light is detected, the peak being defined by a respective threshold value in accordance with the environment and the sample being tested. Without wishing to be bound by theory, in the example described above, in which the objective lens is disposed below the plate and the focal plane is initially disposed below the plate and is displaced toward the plate, it is surmised that this second peak in the intensity of the reflected light occurs when the focal plane of the objective lens is tangent to the intra-well plate bottom, and typically represents an offset from an in-focus position of the sample. The magnitude of the offset may be determined manually by the user, or may be determined automatically using methods known in the art.

In some embodiments, the offset is computed from the first peak in the intensity of detected light, without continuing the search for a second peak in the intensity of detected light. In such embodiments, the magnitude of the offset may be determined manually by the user, or may be determined automatically using methods known in the art.

It will be appreciated that the direction in which the focal plane is displaced toward the plate, and the order in which the peaks in intensity of detected light are identified, is dependent on the setup of the imaging device. For example, in some embodiments, the objective lens is disposed below the sample plate, but the focal plane of the objective lens is disposed above the well bottom, such that the focal plane would be displaced downward toward the well bottom. Without wishing to be bound by theory, in such embodiments, it is surmised that the first peak in the intensity of the reflected light occurs when the focal plane of the objective lens is tangent to the intra-well plate bottom, and typically represents an offset from an in-focus position of the sample while the location at which the second peak in intensity of the reflected light is detected corresponds to a point at which the focal plane of the objective lens is tangential to the curved surface of the well bottom. A corresponding situation occurs in other embodiments in which the objective lens is disposed above the sample plate, and the focal plane of the objective lens is disposed above the well bottom, such that the focal plane would be displaced downward toward the well bottom.

As another example, in some embodiments, the objective lens is disposed above the sample plate, but the focal plane of the objective lens is disposed below the well bottom, such that the focal plane would be displaced upward toward the well bottom. Without wishing to be bound by theory, in such embodiments, it is surmised that the location at which the first peak in intensity of the reflected light is detected corresponds to a point at which the focal plane of the objective lens is tangential to the curved surface of the well bottom while the second peak in the intensity of the reflected light occurs when the focal plane of the objective lens is tangent to the intra-well plate bottom, and typically represents an offset from an in- focus position of the sample.

In some embodiments, the center of the well, at which the in-focus position would lie, is identified based on the plate specifications provided by the manufacturer. In some embodiments, the center of the well is determined using X-Y displacement of the plate or X- Y displacement of the objective lens, until the center of a well or the edge of a well are identified using suitable light detection parameters and characteristics, as is known in the art.

At step 304, at least three of the in-focus positions identified at step 302 are used to compute a plane along which at least some of the plurality of wells in the plate, and typically all the wells in the plate, are in-focus or close to in-focus with respect to a second objective lens, such as an objective lens 206 of Figure 2. The second objective lens has a second magnification which is not greater than the first magnification of the first objective lens. As described hereinbelow, the wells are scanned using the second objective lens based on the location computed plane, by maintaining the position of the second objective lens during scanning so that for any given well scanned, the calculated plane and the focal plane of the second objective lens are coincident or close to coincident.

In some embodiments, the plane is computed by translating at least three of, and typically each of, the in-focus positions identified in step 302 using the first objective lens to corresponding second in-focus positions with respect to the second objective lens, based on optical characteristics of the second objective lens, and computing a plane including at least three of the second in-focus positions.

In some embodiments, the plane is computed by computing, on the basis of at least three of the in-focus positions, a first plane along which at least some of the plurality of wells in the plate, and typically all the wells in the plate, are in-focus or close to in-focus with respect to the first objective lens. The first plane is then translated into the corresponding plane along which at least some of the wells, and typically all the wells, are in-focus or close to in-focus with respect to the second objective lens, based on optical characteristics of the second objective lens.

As mentioned above, the second objective lens has a magnification that is not greater than the first magnification of the first objective lens. As such, in some embodiments the second magnification is smaller than the first magnification, and may be, for example, 4x or 2x. In some embodiments, the second magnification is equal to the first magnification, but the numerical aperture value of the second objective lens is higher than the numerical aperture value of the first objective lens.

In some embodiments, the plane is computed using all the in-focus positions identified at step 302. In other embodiments, the plane is computed using fewer than all the in-focus positions identified at step 302. In some embodiments, the plane is computed for a section of the plate, for example for a quadrant, using at least three in-focus positions identified, using the first objective lens, within that section of the plate. In such embodiments, the method described herein is repeated for each section or quadrant of the plate using a different set of in-focus positions for each such section.

At step 306, which may occur before or after step 304 above, the first objective lens is changed to the second objective lens, for example by a suitable hardware mechanism such as objective lens exchanger 208 of Figure 2. In some embodiments the first and second objective lenses are identical, and step 306 of Figure 3 is omitted.

Finally, at step 308, the wells of the multiwell plate are scanned, or imaged, along the plane computed at step 304, using the second objective lens, without carrying out any additional focusing operations.

The scanning at step 308 may be carried out using any suitable method known in the art, including capturing an image stack, which is particularly useful when imaging a three dimensional construct such as a spheroid. In some embodiments, the teachings herein may be carried out on a plate having a single well, or on a single well within a multi-well plate. In such embodiments, the first objective is used to find an in-focus point of the sample in the plate. The in-focus point found using the first objective is translated into an in-focus point for the second objective, based on the optical characteristics of the second objective. The second objective is then used to scan the plate, when placed at the translated in-focus point, or at the height thereof.

It will be appreciated that the teachings herein allow the imaging device to be in-focus with respect to the plate regardless of the "expected height difference" and of the "unexpected height difference" within the plate. The "expected height difference" is defined as the curvature of the plate listed in the specifications of the plate and that is intended by the manufacturer to be in the plate, such as having a curved bottom due to the structure. The "unexpected height difference" is defined as lack of planarity which is not intended in the specification of the plate. Such "unexpected height difference" may be, for example, due to differences in the relative heights of the bottoms of the wells; or may be, for example, due to deviations from planarity in the virtual surface traced by the scanning components as the objective is moved; or the surface upon which the plate rests being non-parallel with the virtual surface traced by the scanning components as the objective is moved.

It will be appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the scope of the appended claims.

Citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the invention.

Section headings are used herein to ease understanding of the specification and should not be construed as necessarily limiting.