Patent Description:
Optical microscopes are generally constructed from an objective lens assembly (or simply "objective") and a tube lens assembly (or simply "tube lens"). The objective is typically infinity corrected, meaning that an object plane is imaged to a collimated image, which is then focused by the tube lens to an image plane. In the case of an industrial microscope, a focal plane array image sensor is positioned at the back focal plane of the tube lens. Objectives are often interchangeable, providing a range of different magnifications.

When designing lens systems for both objectives and tube lenses for high performance microscopes, the various optical elements and surfaces should be optimized to minimize multiple types of optical aberration which can impact image quality. These include spherical aberration, coma, distortion, axial and lateral chromatic aberration, astigmatism, and field curvature. Optical surfaces are typically optimized by numerical methods which try to simultaneously minimize multiple parameters corresponding to the different types of aberration. Residual aberrations are often significant, and attempts to reduce them to levels required for certain applications may lead to the use of large numbers of surfaces and consequent high complexity and cost of the microscope.

An optical microscope is known from <CIT>.

The present invention is a unit magnification microscope according to claim <NUM>.

According to the teachings of an embodiment of the present invention there is provided, a unit magnification microscope comprising: (a) a first lens assembly comprising a plurality of lenses forming an infinity-corrected objective for receiving light from an object located at an external focal plane of the first lens assembly; (b) a second lens assembly comprising a plurality of lenses, the second lens assembly being identical to the first lens assembly, the second lens assembly being mounted relative to the first lens assembly in an opposite orientation along a light path; (c) a physical stop located at a central point along the light path between the first lens assembly and the second lens assembly; and (d) a focal plane array image sensor mounted at the external focal plane of the second lens assembly, such that light from the object is focused with unit magnification on the focal plane array image sensor after passing along the light path through the first lens assembly, the physical stop and the second lens assembly.

According to a further feature of an embodiment of the present invention, there is also provided a beam splitter cube and a compensating element deployed in the light path, the beam splitter cube being deployed to allow introduction into the light path of illumination directed towards the object, wherein the beam splitter cube and the compensating element have similar optical properties and are located on opposite sides of the physical stop.

According to a further feature of an embodiment of the present invention, the beam splitter cube and the compensating element are both located between the first lens assembly and the second lens assembly.

According to a further feature of an embodiment of the present invention, there is also provided an illumination source, and an illumination optical arrangement configured to direct illumination from the illumination source via the beam splitter cube into the first lens assembly.

According to a further feature of an embodiment of the present invention, the physical stop is located at a back focal plane of the first lens assembly and of the second lens assembly, and wherein the illumination optical arrangement includes an arrangement of lenses configured to generate an image of the illumination source at a plane of the physical stop or at a plane optically equivalent to the physical stop.

According to a further feature of an embodiment of the present invention, the beam splitter cube and the compensating element are located in the light path between the first lens assembly and the object or between the second lens assembly and the focal plane array image sensor.

According to a further feature of an embodiment of the present invention, the beam splitter cube includes a metallic beam splitter.

According to a further feature of an embodiment of the present invention, the beam splitter cube includes a polarizing beam splitter.

According to a further feature of an embodiment of the present invention, the beam splitter cube includes a dichroic beam splitter.

According to a further feature of an embodiment of the present invention, the compensating element is implemented as a second beam splitter cube.

According to a further feature of an embodiment of the present invention, there is also provided an optical instrument associated with the second beam splitter and employing the second beam splitter as an optical port to the microscope, the optical instrument being selected from the group consisting of: a laser illumination device; a vibrometer: and a spectrometer.

According to a further feature of an embodiment of the present invention, the first lens assembly and the second lens assembly are deployed along a common optical axis.

According to a further feature of an embodiment of the present invention, at least one lens of the first lens assembly is aligned on a first optical axis and wherein at least one lens of the second lens assembly is aligned on a second optical axis, the first and second optical axes being perpendicular.

According to a further feature of an embodiment of the present invention, at least one lens of the first lens assembly is aligned on a first optical axis and wherein at least one lens of the second lens assembly is aligned on a second optical axis, the first and second optical axes being parallel.

According to a further feature of an embodiment of the present invention, there is also provided a rigid housing supporting the first and second lens assemblies, wherein the plurality of lenses of the first lens assembly are deployed within a first hollow channel formed in the rigid housing, and wherein the plurality of lenses of the second lens assembly are deployed within a second hollow channel formed in the rigid housing.

According to a further feature of an embodiment of the present invention, the physical stop is located at a back focal plane of the first lens assembly and of the second lens assembly.

According to a further feature of an embodiment of the present invention, there is also provided at least one diagonally-deployed beam splitter located in the light path between the first lens assembly and the second lens assembly, the beam splitter being deployed to allow introduction into the light path of illumination directed towards the object.

According to a further feature of an embodiment of the present invention, the plurality of lenses of the first and second lens assemblies are refractive lenses.

According to a further feature of an embodiment of the present invention, the first and second lens assemblies are catadioptric assemblies.

According to a further feature of an embodiment of the present invention, the plurality of lenses of the first and second lens assemblies are reflective lenses.

The present invention is a unit magnification microscope.

The principles and operation of unit magnification microscopes according to the present invention may be better understood with reference to the drawings and the accompanying description.

Referring first generically to multiple variant implementations of the present invention as illustrated and described herein, in general terms, a unit magnification microscope according to certain embodiments of the present invention includes a first lens assembly <NUM>, including a plurality of lenses forming an infinity-corrected objective, for receiving light from an object located at an external focal plane (which is also the object plane <NUM>) of first lens assembly <NUM>, and a second lens assembly <NUM>, identical to first lens assembly <NUM>, mounted relative to first lens assembly <NUM> in an opposite orientation along a light path (rays <NUM>). A physical stop <NUM> is located at a central point along the light path between first lens assembly <NUM> and second lens assembly <NUM>. A focal plane array (FPA) image sensor <NUM> is mounted at the external focal plane of second lens assembly <NUM>, such that light from the object is focused with unit magnification on FPA image sensor <NUM> after passing along the light path through first lens assembly <NUM>, physical stop <NUM> and second lens assembly <NUM>.

Embodiments of the present invention according to the above definition provide a number of striking advantages over conventional microscopes. Firstly, the approach taught by these embodiments of the present invention is enabled by the observation that high quality microscopy can surprisingly be achieved using a unit magnification optical arrangement, due to the high pixel resolution available using currently available FPA image sensors. For example, certain particularly effective implementations of the present invention have been achieved using FPA image sensors having pixel dimensions of roughly <NUM> microns square.

Secondly, by choosing to use first and second lens assemblies which are symmetrical (identical lens arrangements arranged in the reverse orientation), it has been found that many types of optical aberration are inherently compensated for by cancelling out due to the symmetry of the optics. Specifically, an arrangement of first and second lens assemblies positioned symmetrically with respect to an aperture stop typically achieves substantially complete cancelation of coma, distortion and lateral color aberration. This greatly simplifies the optical design process, which can address minimizing spherical aberration, astigmatism, axial color and field curvature while ignoring the aforementioned aberrations which are inherently canceled by the symmetry. This results in better optical performance and/or fewer required optical elements compared to conventional microscope designs.

As already mentioned, the first and second lens assemblies <NUM> and <NUM> are infinity corrected, meaning that the image of an object at the object plane <NUM> exits first lens assembly <NUM> towards stop <NUM> as a collimated image, and the collimated image entering second lens assembly <NUM> is brought to focus at FPA image sensor <NUM>. According to a particularly preferred subset of implementations, the location of stop <NUM> is chosen to be the back focal plane of each lens assembly <NUM> and <NUM>, so that the optical system is telecentric.

In the examples detailed herein and illustrated in the drawings, first and second lens arrangements <NUM> and <NUM> are implemented using refractive lenses. It should be noted however that the same principles may be used to advantage in implementations employing reflective lenses for the first and second lens arrangements, or with various arrangements combining refractive and reflective elements, referred to as catadioptric assemblies.

Turning now to the specific example of <FIG>, there is shown a non-limiting exemplary implementation of the present invention in which first and second lens arrangements <NUM> and <NUM> each includes one singlet lens and two doublets, totaling <NUM> surfaces with optical power for each lens arrangement. The surfaces are numbered sequentially from the object plane <NUM> to FPA image sensor plane <NUM>, with the stop <NUM> occupying a position as the <NUM>th element.

The details of this non-limiting exemplary optical arrangement are provided in a table presented here as <FIG>, which provides for each surface: a radius of curvature, a distance along the optical axis (or central ray path) to the next surface, the intervening material for the lens (blank for air spaces), and the diameter of the surface. The object plane ("OBJ") <NUM> and the FPA image sensor ("IMA") <NUM> are planar, and hence have infinite radius of curvature. All dimensions are in millimeters.

<FIG> presents a table quantifying the contribution of each surface to each of <NUM> types of optical aberration as listed across the tops of the columns. The aberrations are quantified by their Seidel aberration coefficients as calculated by the ZEMAX™ Optical Design Program. By looking at the values for coma, distortion and lateral color pairwise, comparing surface <NUM> with surface <NUM>, surface <NUM> with surface <NUM>, surface <NUM> with surface <NUM> etc., it will be noted that the values are very closely equal but opposite. As a result, substantially independent of the net value for each lens assembly separately, the overall cumulative aberration for coma, distortion and lateral color substantially cancel out. These types of aberration can therefore be ignored during optimization of the optical components.

In contrast, the contribution of each surface to spherical aberration, field curvature, astigmatism and axial color are similar but of the same sign, and therefore do not cancel out. In order to minimize these aberrations, the surfaces of each lens arrangement should be optimized to minimize the total sum of spherical aberration, astigmatism, field curvature and axial color for each lens assembly individually, and hence for the overall optical system. The fact that a smaller number of variables need to be minimized greatly facilitates reaching improved optical performance with a given number of surfaces. In the further examples illustrated below, a larger number of surfaces are used, typically three singlet lenses and one doublet lens with a total of <NUM> surfaces for each lens assembly, which allows further optimization of the arrangement for aberration reduction and telecentricity. All of the examples illustrated are non-limiting, and a wide range of lens combinations and numbers of surfaces may be used, as will be clear to one ordinarily skilled in the art.

The FPA image sensor used to implement the present invention is typically a CMOS image sensor, and is provided with the various driving circuitry, readout circuitry, and complementary image processing circuitry to generate images for output to data storage and/or for immediate display on a monitor. All such details are standard practice in the field of microscopy and will not be further described herein.

The basic embodiment of <FIG> is suitable for use with transmissive illumination of a sample or with dark-field illumination, but does not provide for bright-field (EPI) illumination through the microscope. A number of particularly preferred variant implementations described below employ at least one beam splitter element to allow introduction into the light path of illumination directed towards the object.

In the non-limiting example of <FIG>, the beam splitter <NUM> is incorporated on a diagonal plane of a beam splitter cube <NUM>. In order to maintain symmetry of the light paths on both sides of the stop, a compensating element <NUM> have similar optical properties to beam splitter cube <NUM> is also deployed in the light path. Beam splitter cube <NUM> and compensating element <NUM> are located on opposite sides of physical stop <NUM>. In the particularly preferred implementation illustrated in <FIG>, beam splitter cube <NUM> and compensating element <NUM> are both located between first lens assembly <NUM> and second lens assembly <NUM>, on either side of stop <NUM>. Beam splitter cube <NUM> can be on either side of the stop, and is shown here on the side adjacent to first lens assembly <NUM>.

The microscope as illustrated preferably also includes an illumination source <NUM> and an illumination optical arrangement <NUM> configured to direct illumination from illumination source <NUM> via beam splitter cube <NUM> into first lens assembly <NUM> so as to illuminate the image plane <NUM>. The illumination source <NUM> and illumination optical arrangement <NUM> may provide any desired type of illumination, depending on the microscopy technique to be used, using visible light, near infrared or ultraviolet, providing full-field illumination or scanning illumination, polarized or unpolarized, and with or without spectral filters. In the embodiment illustrate here, a slot <NUM> is provided for insertion of a polarizer or filter. One or more mirror <NUM> may be provided to fold the illumination path for convenient placement of the illumination optical arrangement.

According to one subset of illumination arrangements considered particularly valuable for uniformity of illumination, illumination optical arrangement <NUM> includes an arrangement of lenses configured to generate an image of illumination source <NUM> at the back focal plane of first lens assembly <NUM>, thereby generating Köhler illumination, where each point of the illumination source generates a parallel beams of illumination illuminating the object plane. In the preferred case of telecentric optics, the back focal plane of the first (and second) lens assembly is at physical stop <NUM>. so the image of the illumination source is configured to be at the plane of physical stop <NUM>. In the case illustrated here where beam splitter cube <NUM> is on the objective side of the stop, the illumination source image is generated at a plane <NUM> which is optically equivalent to the physical stop (via the alternative light path reflected by beam splitter <NUM>). An illumination stop <NUM> controls the illumination field. Clearly, the Köhler illumination option is only one of many possible illumination arrangements, and other options also fall within the scope of the present invention.

Beam splitter <NUM> may be any type of beam splitter, and is chosen according to the intended application for which the microscope is to be used. In a first group of implementations, beam splitter <NUM> is a metallic beam splitter, e.g., a semi-silvered mirror. In other applications, beam splitter <NUM> is a polarizing beam splitter, while in a further set of applications, beam splitter <NUM> is a dichroic beam splitter with wavelength sensitivity for which wavelengths are transmitted and which are reflected. Again, depending upon the application, various additional filters or polarized analyzers <NUM> may be provided, for example, in a slot between second lens arrangement <NUM> and FPA image sensor <NUM>. Other locations for filters or analyzers are also possible.

As mentioned, compensating element <NUM> is preferably included to maintain symmetry of the light paths through the two halves of the microscope. Advantageously, compensating element <NUM> is implemented as a second beam splitter cube with a diagonally-deployed second beam splitter <NUM>, which may be of the same type, or of a different type, from beam splitter <NUM>, depending on the application. Beam splitter <NUM> allows deployment of an optical instrument, represented schematically by arrow <NUM>. associated with second beam splitter <NUM> and employing the second beam splitter as an optical port to the microscope. Non-limiting but particularly preferred examples of optical instruments <NUM> which may be associated with the optical port include: a laser illumination device: a vibrometer; and a spectrometer.

In the microscope of <FIG>. first lens assembly <NUM> and second lens assembly <NUM> are deployed along a common axis which defines the optical axis of the microscope. This configuration is appropriate for a "tube" configuration microscope. <FIG> illustrate a number of variant implementations which are optically equivalent, or near-equivalent, to the design of <FIG>, but in which the optical axis is folded to achieve alternative form factors which may be advantageous for certain applications, and may offer enhanced compactness of the design.

By way of example, <FIG> illustrates a microscope structure which is essentially the same as that of <FIG>, but in which at least one lens, and preferably the entirety, of first lens assembly <NUM> is aligned on a first optical axis <NUM> and at least one lens, and preferably the entirety, of second lens assembly <NUM> is aligned on a second optical axis <NUM>, perpendicular to the first optical axis <NUM>. This results in what can be referred to as an "L-shape" design. Folding of the optical axis is performed by reflection at second beam splitter <NUM>, which is here implemented as part of second beam splitter cube <NUM> on the objective side of stop <NUM>. The straight-through channel of second beam splitter <NUM> may be used as an auxiliary optical port, as described above. The first (illumination) beam splitter cube <NUM> is here deployed on the tube lens side of the stop.

<FIG> illustrates a microscope structure which is optically and geometrically identical to that of <FIG>. but without an auxiliary optical port. In this case, compensating element <NUM> is advantageously implemented as a prism reflector.

<FIG> and <FIG> illustrate further structural arrangements which are optically equivalent to the devices of <FIG> and <FIG>, respectively, but in which the light path of the microscope is further folded so that at least one lens, and preferably the entirety, of first lens assembly <NUM> is aligned on a first optical axis <NUM> and at least one lens, and preferably the entirety, of second lens assembly <NUM> is aligned on a second optical axis <NUM> parallel to first optical axis <NUM>. In this case, the primary optical channel through the microscope is folded twice, preferably at each of compensating element <NUM> and beam splitter cube <NUM>. The result is a highly compact configuration, where the primary light path through the microscope is twice folded, and may be referred to as a "U-shape" or "horseshoe" design. In order to provide sufficient volume for the FPA image sensor <NUM> (not visible in these drawings), a folding mirror <NUM> folds the light emerging from the second lens assembly <NUM> upwards, out of the plane of the optical axes.

The drawings thus far have represented the various optical arrangements schematically, showing the optical components only. In each case, it is understood that the optical components are housed in a suitable support structure configured to support the components in the desired relative positions. For the in-line configurations of <FIG> and <FIG>, a conventional tube form-factor housing is typically appropriate. For the L-shape and U-shape implementations, the required positions of the components are advantageously maintained by providing a rigid housing, optionally implemented using unitary block of rigid material, supporting the first and second lens assemblies <NUM> and <NUM>. One such non-limiting example, corresponding to the optical configuration of <FIG>, is shown schematically in <FIG> with a cover removed. The lenses of first lens assembly <NUM> are here deployed within a first hollow channel <NUM> formed in the rigid housing <NUM>, and the lenses of second lens assembly <NUM> are deployed within a second hollow channel <NUM> formed in rigid housing <NUM>. In this example, longitudinal axes (i.e., central axes of what is typically a contoured cylinder) of channels <NUM> and <NUM> are parallel, corresponding to the parallel optical axes <NUM> and <NUM> of <FIG>. In this case, illumination optics <NUM> is deployed in a third hollow channel <NUM>, which is again parallel to channels <NUM> and <NUM>. FPA image sensor <NUM> itself is not directly visible in <FIG>, but the reference numeral with an arrow head indicates the sensor housing which extends beneath the assembly as illustrated and houses the image sensor itself.

The position of the various optical components within the channels is typically fixed. Optionally, various adjustment mechanisms (not shown) may be provided for fine tuning alignment of the components during factory assembly, as is known in the art, but no adjustment is typically needed during use. Focus is achieved by adjusting relative positioning of the entire microscope relative to the object being viewed, to bring the object into the object plane of the optics.

In the implementation shown here, in addition to folding mirror <NUM> which folds the optical axis downwards as illustrated, towards FPA image sensor <NUM> which is located beneath the housing in the orientation illustrated, an additional folding mirror <NUM> is illustrated here, deployed to fold the optical axis of the objective upwards as shown, to an object plane located above the device as illustrated. The illustrated structure is closed with another complementary-shaped housing portion which mates with, and closes, the half housing as illustrated here, leaving an aperture above folding mirror <NUM> for the light path to the object plane.

The above positioning of beam splitter cube <NUM> and compensating element <NUM> between the first and second lens assemblies is believed to be particularly advantageous, since the parallel-faces "window" formed by each of these elements has minimal additional aberration-generating effect on the collimated image present between the two lens assemblies. It should be noted however that alternative implementations may position the beam splitter and compensating element outside the lens assemblies, i.e.. with one of them in the light path between first lens assembly <NUM> and the object plane <NUM> and the other between second lens assembly <NUM> and FPA image sensor <NUM>. One such example is illustrated in <FIG>. where beam splitter cube <NUM> is located between first lens assembly <NUM> and the object plane <NUM> for introducing illumination from light source <NUM> passing via illumination optics <NUM> and an illumination folding mirror <NUM>. Compensating element <NUM>, implemented as a second beam splitter cube, provides an optical port as before for integration of an external device <NUM>. In all other respects, this arrangement has analogous components similarly labelled to the above implementations and functions similarly.

As discussed above, particular advantages are achieved by maintaining symmetry of the optical configuration which contributes to cancelling out of various types of aberrations. The symmetry between first and second lens assemblies <NUM> and <NUM> is preferably achieved by using identical lens assemblies, subject to the limitations of manufacturing and assembly tolerances. Similarly, where a beam splitter cube <NUM> is used, the optical impact of the cube is preferably symmetrically offset by a corresponding compensating element <NUM>, as per the above examples. Nevertheless, particularly for relatively thin elements with no optical power, and particularly where they oriented perpendicular to optical axis, asymmetric deployment may not significantly adversely affect the overall optical performance. Hence, for example, deployment of a thin filter or polarizing analyzer <NUM> in various of the above implementations is considered an acceptable departure from symmetry.

A further example of tolerable imperfect symmetry is illustrated in <FIG>, where a flat, diagonally-deployed beam splitter <NUM> is located in the light path between first lens assembly <NUM> and second lens assembly <NUM>. Folding of the light path is here performed by a mirror <NUM>. The overall optical configuration is essentially the same in <FIG> as in <FIG>, described above, but the image illumination passing through beam splitter <NUM> undergoes refraction at the front and back surfaces of the beam splitter. For a relatively thin flat beam splitter, for example, below about <NUM> thickness, the overall impact on image quality has been found to be within acceptable limits. In all other respects, the structure and function of <FIG> is equivalent to that of <FIG>.

<FIG> shows a similar implementation, but in this case, the optical configuration is a U-shape implementation, similar to that of <FIG>. In this case, the use of a diagonally-deployed beam splitter need not impact image quality at all, since the image illumination is reflected from the front surface of beam splitter <NUM> and does not undergo refraction. Any impact on the illumination channel is typically not significant, and can be compensated for by simple refocusing of the illumination source at the plane of the stop (for Köhler illumination). In all other respects, the structure and function of <FIG> is equivalent to that of <FIG>.

Turning finally to <FIG>, this illustrates a microscope system in which the microscope of <FIG> is supplemented with a transmitted--illumination system behind/beyond the object plane, to provide additional functionality, and in the particular implementation shown here, suitable for implementing phase-contrast microscopy. For the purpose of phase-contrast microscopy, the transmitted--illumination system includes a light source having a light-source aperture stop <NUM>, a Köhler field stop <NUM>, first illumination optics <NUM>, optionally an illumination filter <NUM>, a phase stop <NUM>, second illumination optics <NUM>. and optionally a folding reflector <NUM> to direct the illumination towards a specimen in object plane <NUM>. The microscope optical configuration is essentially as described above, but with stop <NUM> between first and second lens assemblies <NUM> and <NUM> supplemented with a phase plate. Aperture stop <NUM>, phase stop <NUM> and phase plate at stop <NUM> are all conjugate planes. The microscope structure illustrated here also includes the aforementioned front illumination arrangement, thereby providing flexibility for multiple imaging modalities. Clearly, if only phase-contrast microscopy is required, the microscope may optionally be implemented as a simplified structure without beam splitters, such as was described above with reference to <FIG>.

The various implementations of the microscopes of the present invention are thus highly suited to a wide range of applications including, but not limited to: inspection of semiconductors, solar panels, LEDs, micro-electro-mechanical systems (MEMS), printed circuit boards, flat panel displays (FPD), optical metrology, wafer dicing, robotics, gemology evaluations, life-sciences, bio-med (including fluorescence microscopy, phase-contrast microscopy, cell imaging), and medical diagnostics.

Claim 1:
A unit magnification microscope comprising:
(a) a first lens assembly (<NUM>) comprising a plurality of lenses forming an infinity-corrected objective for receiving light from an object located at an external focal plane (<NUM>) of the first lens assembly;
(b) a second lens assembly (<NUM>) comprising a plurality of lenses, said second lens assembly being identical to said first lens assembly (<NUM>), said second lens assembly being mounted relative to said first lens assembly in an opposite orientation along a light path;
(c) a physical stop (<NUM>) located at a central point along the light path between said first lens assembly (<NUM>) and said second lens assembly (<NUM>);
(d) a focal plane array image sensor (<NUM>) at the external focal plane of the second lens assembly (<NUM>), such that light from the object is focused with unit magnification on said focal plane array image sensor (<NUM>) after passing along the light path through said first lens assembly (<NUM>), said physical stop (<NUM>) and said second lens assembly (<NUM>); characterized in that the microscope further comprises:
(e) a beam splitter cube (<NUM>) and a compensating element (<NUM>) deployed in the light path, said beam splitter cube being deployed to allow introduction of illumination directed towards the object into the light path, wherein said beam splitter cube (<NUM>) and said compensating element (<NUM>) have similar optical properties and are located between said first lens assembly (<NUM>) and said second lens assembly (<NUM>) on opposite sides of said physical stop (<NUM>).