An interferometric microscopy arrangement for inspecting a sample using two illumination sources

The present invention relates to interferometric microscopy techniques for inspecting a sample, and more particularly to an interferometric microscopy arrangement that uses two illu ^¬ mination sources in the interferometric microscopy arrange- ment .

Medical technology in recent times has witnessed advent of numerous medical devices and microscopy techniques. A lot of these microscopy techniques are used for imaging microscopic specimens or samples by detecting and analyzing interference patterns formed by superimposition of an object beam and a reference beam for example Interferometric microscopy, also referred to as Digital holographic microscopy (DHM) . An emerging technique in interferometric microscopy is common path interferometry in which a light beam is shone or impinged on a sample to be inspected and then the light beam emerging after interacting with the sample or specimen is split into a reference beam and an object beam. Subsequently, object information is filtered out or deleted from the refer- ence beam and then the filtered reference beam is superim ^¬ posed with the object beam to detect the interference pattern to be studied.

The common path interferometry differs from the commonly known different path interferometry in the sense that unlike the different path interferometry where the light beam is split into the reference beam and the object beam before in ^¬ teracting with the sample, in common path interferometry the light beam is split into the reference beam and the object beam after interaction with the sample. In different path interferometry, only the object beam interacts with the sample, whereas in the common path interferometry, an incoming beam interacts with the sample and emerges from the sample con- taining the object information and then this emergent beam is divided into the reference beam and the object beam, and sub ^¬ sequently the object information from the reference beam is deleted or filtered out before the reference beam and the ob- ject beam are superimposed to form the interference pattern as an output of the interferometry from which the amplitude and phase information are analyzed that represent character ^¬ istics of the sample such as physical structures in the sam ^¬ ple, density of the sample, and so on and so forth. A major advantage of common path interferometry over the different path interferometry is stability because unlike different path interferometry in common path interferometry the light beam interacting with the sample travels as one and is split and subjected to different paths - namely an object beam path and a reference beam path only for short travel distances as compared to different path interferometry where the light beam is split much earlier and thus chances of introduction of error are high. In known common path interferometric techniques, a single in ^¬ coming beam of collimated coherent light is interacted with the sample and then split into the reference beam and the ob ^¬ ject beam by a beam splitter. The object beam is then subjected to spatial filtering to erase or filter out object in- formation from the reference beam and then the reference beam is superimposed with the object beam. Using a collimated beam is necessary for the above described common path

interferometric technique to maintain a desired intensity in the reference beam after spatial filtering, and thus use of any condenser or focusing action of the light beam right before interacting with the sample is highly undesirable as it will lead to having a very low intensity reference beam and thus the interference pattern produced as an output will be unsuitable or of poor quality.

Thus, lateral resolution of such a device or setup of pres ^¬ ently known common path interferometric techniques is low be ^¬ cause of the necessity to use a collimated beam of light to impinge on the sample, as explained above. As is commonly known, equation 1 below presents a measure of lateral resolu ^¬ tion for microscopy: λ

d oc

^^objective ^^Condenser wherein d is measure of distance, λ is wavelength of the in ^¬ coming beam, NA _ob j _ective is numerical aperture of the objective lens of the microscope and NA _Condenser is numerical aperture of the condenser of the microscope. Numerical aperture of the condenser can also be understood as the focusing of the light beam onto the sample - greater the numerical aperture of the condenser more the focusing of the light beam on the sample and smaller the value of d. Smaller the value of d better is the lateral resolution of the microscopy device. In the case of common path interferometry NA _Condenser = 0 because condenser is not used since the use of condenser is detrimental to the intensity of the reference beam and thus to the final ob ^¬ tained interference pattern. Thus it is a challenge in the present day common path interferometric devices and setups to maintain a desired intensity of the reference beam and in ^¬ crease a lateral resolution of the interferometric device.

Thus the object of the present disclosure is to provide a technique for improving present day interferometric devices and setups, more particularly improving a lateral resolution of present day common path interferometric techniques and simultaneously maintaining a desired intensity in the refer ^¬ ence beam. The above objects are achieved by an interferometric microscopy arrangement according to claim 1 of the present tech ^¬ nique. Advantageous embodiments of the present technique are provided in dependent claims. Features of claim 1 may be com ^¬ bined with features of dependent claims, and features of de- pendent claims can be combined together. In the present technique, an interferometric microscopy ar ^¬ rangement, hereinafter the arrangement, for inspecting a sample placed in a sample port, is presented. The arrangement includes a first illumination source, a second illumination source, a selection filter, interferometric unit and an opti ^¬ cal detector system.

The first illumination source provides a first light beam. The first light beam is collimated. The second illumination source provides a second light beam. The second light beam is focused. The selection filter receives and directs each of the first light beam and the second light beam towards the sample port. The interferometric unit, hereinafter the unit, is positioned optically downstream of the sample port. The unit receives the first light beam after interaction of the first light beam with the sample port. The unit also receives the second light beam after interaction of the second light beam with the sample port. The unit generates from the first light beam a first object beam and a first reference beam. The unit then filters out object information from the first reference beam. The unit subsequently directs the first object beam and the first reference beam towards the optical detector system. Furthermore, the unit directs the second light beam towards the optical detector system.

The optical detector assembly detects an interference pattern formed from the first light beam and also detects an optical pattern formed from the second light beam. Thus the inspec ^¬ tion of the sample is performed by superimposing or comparing the interference pattern formed by the first light beam and the optical pattern, formed by the second light beam. Since the optical pattern formed by the second light beam is re- sultant from the second light beam that is focused on to the sample, preferably by a condenser or a condenser lens, the numerical aperture of focusing or condenser contributes to the lateral resolution of the arrangement. Furthermore, an intensity of the reference beam obtained from the first light beam as desired because the first light beam interacts with the sample as collimated light beam. Thus a desired reference beam intensity and a desired lateral resolution is achieved by the arrangement of the present technique.

In an embodiment of the arrangement, the second illumination source includes a light source and a condenser lens. The light source provides a light beam. The condenser lens re- ceives the light beam and generates the second light beam di ^¬ rected towards the sample port by focusing the light beam to ^¬ wards the sample port. This provides a simple construct for the second illumination source. In another embodiment of the arrangement, the first light beam includes light of a first wavelength and the second light beam includes light of a second wavelength. The first wavelength is different from the second wavelength. Thus by using a color detector at the optical detector system, both the wavelengths can be detected at a single color detector and thus the arrangement does not require two separate detec ^¬ tors and this leads to make the arrangement simple, compact and cost effective. In another embodiment of the arrangement, the selection fil ^¬ ter includes a dichroic filter. The dichroic filter allows the light of the first wavelength to pass through the dichro ^¬ ic filter. However, does not allow the light of the second wavelength to pass through the dichroic filter, instead the dichroic filter reflects the light of the second wavelength. This provides a simple construct of the selection filter and a first embodiment of optical path geometry of the arrange ^¬ ment. Since, dichroic filters are readily available; the use of the dichroic filter makes the arrangement simple, easy to setup and cost effective.

In another embodiment of the arrangement, alternate to the preceding embodiment, the selection filter includes a dichro- ic filter that allows the light of the second wavelength to pass through the dichroic filter, but does not allow the light of the first wavelength to pass through the dichroic filter, instead the dichroic filter reflect the light of the first wavelength. This provides a simple construct of the se ^¬ lection filter and a second embodiment of optical path geome ^¬ try of the arrangement. Since, dichroic filters are readily available; the use of the dichroic filter makes the arrange ^¬ ment simple, easy to setup and cost effective.

In another embodiment of the arrangement, the unit defines an object beam path to direct the first object beam towards the optical detector system. The unit also defines a reference beam path to direct the first reference beam towards the op- tical detector system. The object beam path substantially overlaps with the reference beam path. The unit includes a beam splitter/combiner, an object beam reflector, a reference beam reflector, a spatial filter, and a reference beam Fourier optics assembly.

The beam splitter/combiner, hereinafter the BSC, receives the first light beam and splits the first light beam so received into the first object beam and the first reference beam. The object beam reflector is positioned in the object beam path and receives the first object beam from the BSC and reflects the first object beam back towards the BSC. The BSC then di ^¬ rects the first object beam reflected back from the object beam reflector towards the optical detector system. The reference beam reflector is positioned in the reference beam path and receives the first reference beam from the BSC and reflects the first reference beam back towards the BSC. The BSC further directs the first reference beam reflected back from the reference beam reflector towards the optical detec ^¬ tor system.

The spatial filter is positioned optically in front of the reference beam reflector. The spatial filter at least par ^¬ tially filters object information from the first reference beam before the first reference beam is reflected back from the reference beam reflector. The reference beam Fourier op ^¬ tics assembly includes at least a first lens arranged at 4f configuration to a second lens. This presents an optical set- up of the interferometric unit well suited for functioning in the arrangement of the present technique.

In another embodiment of the arrangement, the spatial filter is a pinhole. This presents a simple design of the spatial filter.

In another embodiment, the arrangement further includes an objective lens. The objective lens receives the first light beam after interaction with the sample port. The first lens is positioned such that a Fourier plane of the objective lens coincides with a focal plane of the first lens. The spatial filter is positioned at a other focal plane of the second lens. This presents an effective and simple setup to achieve substantial spatial filtering of the first reference beam, and also possibly to achieve substantial spatial filtering of a second reference beam in suitable embodiments which present the second reference beam.

In another embodiment, the arrangement includes a third lens positioned at 4f configuration with respect to the second lens. The third lens may be a part of the unit or may be in ^¬ dependent of the unit. The third lens is positioned in front of the optical detector and facilitates formation of the in ^¬ terference pattern and/or the optical pattern at the optical detector system.

In another embodiment of the arrangement, the unit generates from the second light beam a second object beam and a second reference beam. The unit directs the second object beam along the object beam path and directs the second reference beam along the reference beam path. Thus a final interference pat ^¬ tern is obtained at the optical detector system. The final interference pattern has reference beam components in form of the first and the second reference beam; and object beam com ^¬ ponents in form of the first and the second object beams.

In another embodiment of the arrangement, the BSC receives the second light beam and splits the second light beam so re ^¬ ceived into the second object beam and the second reference beam. The object beam reflector receives the second object beam from the BSC and reflects the second object beam back towards the BSC. The BSC then directs the second object beam reflected back from the object beam reflector towards the op ^¬ tical detector system. The reference beam reflector receives the second reference beam from the BSC and reflects the se ^¬ cond reference beam back towards the BSC. The BSC then di ^¬ rects the second reference beam reflected back from the ref- erence beam reflector towards the optical detector system. The spatial filter at least partially filter object infor ^¬ mation from the second reference beam before the second ref ^¬ erence beam is reflected back from the reference beam reflec ^¬ tor. This presents an optical setup of the interferometric unit well suited for functioning in the arrangement of the present technique to produce the second object beam.

In another embodiment of the arrangement, the optical detec ^¬ tor assembly includes at least a first optical detector and a second optical detector. The first optical detector detects the interference pattern formed from the first light beam. The second optical detector detects the optical pattern formed from the second light beam. This presents an embodi ^¬ ment with at least two optical detectors dedicated to detect- ing the interference pattern formed by the first optical beam and the optical pattern formed by the second optical beam. Thus possibilities of undesired interference of the first and the second optical beam are at least partially reduced. Fur ^¬ thermore outputs from the first and the second optical detec- tors may be superimposed subsequently to form the final out ^¬ put of the arrangement. In another embodiment of the arrangement, the unit includes an additional selection filter. The additional selection filter receives the first light beam from the selection filter after interaction of the first light beam with the sample port. The additional selection filter directs the first light beam towards the first optical detector. The additional se ^¬ lection filter also receives the second light beam from the selection filter after interaction of the second light beam with the sample port. The additional selection filter directs the second light beam towards the second optical detector. This provides at least one possible optical setup to guide the first optical beam and the second optical beam to their corresponding optical detectors.

In another embodiment of the arrangement, the additional se ^¬ lection filter is a dichroic filter that allows a wavelength of the first light beam to pass through the dichroic filter, but does not allow a wavelength of the second light beam to pass through the dichroic filter, instead the dichroic filter reflects the wavelength of the second light beam. This pro ^¬ vides a simple construct of the additional selection filter and a first embodiment of optical path geometry of the ar ^¬ rangement. Since, dichroic filters are readily available; the use of the dichroic filter makes the arrangement simple, easy to setup and cost effective.

In another embodiment of the arrangement, alternate to the preceding embodiment, the additional selection filter is a dichroic filter that allows a wavelength of the second light beam to pass through the dichroic filter, but does not allow a wavelength of the first light beam to pass through the di ^¬ chroic filter, instead the dichroic filter reflects the wave ^¬ length of the first light beam. This provides a simple con ^¬ struct of the selection filter and a second embodiment of op- tical path geometry of the arrangement. Since, dichroic fil ^¬ ters are readily available; the use of the dichroic filter makes the arrangement simple, easy to setup and cost effec ^¬ tive . The present technique is further described hereinafter with reference to illustrated embodiments shown in the accompany ^¬ ing drawing, in which:

FIG 1 schematically illustrates an exemplary embodiment of an interferometric microscopy arrangement of the present technique;

FIG 2 schematically illustrates another exemplary embodiment of the interferometric microscopy arrangement of FIG 1;

FIG 3 schematically illustrates a detailed scheme for an exemplary interferometric microscopy arrangement comparable to the schematically illustrated exemplary embodiment presented in FIG 2; and

FIG 4 schematically illustrates a detailed scheme for an exemplary interferometric microscopy arrangement comparable to the schematically illustrated exemplary embodiment presented in FIG 1; in accordance with aspects of the present technique. Hereinafter, above-mentioned and other features of the pre ^¬ sent technique are described in details. Various embodiments are described with reference to the drawing, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of ex- planation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodi ^¬ ments. It may be noted that the illustrated embodiments are intended to explain, and not to limit the invention. It may be evident that such embodiments may be practiced without these specific details.

It may be noted that in the present disclosure, the terms "first", "second", "third", etc. are used herein only to fa- cilitate discussion, and carry no particular temporal or chronological significance unless otherwise indicated.

The idea of the present technique is to use two illumination sources - a first illumination source to provide collimated beam upon the sample i.e. to provide straight wavefronts to impinge on the sample to be inspected substantially

parallelly and a second illumination source to provide fo ^¬ cused beam upon the sample, i.e. curved wavefronts, for exam- pie as produced by a microscope condenser, to impinge on the sample to be inspected at increased angles compared to straight wavefronts that impinge on the sample substantially parallelly. As a result of the two illumination sources, de ^¬ sired reference beam intensity is obtained along with a de- sired lateral resolution. The straight wavefronts emerging out of the sample are well suited to spatially filter to de ^¬ lete object information from the wavefronts emerging out of the sample and still have a desired intensity in the refer ^¬ ence beam for creating an interference pattern as an output of the interferometric microscopy arrangement. The curved wavefronts emerging out of the sample are rich in object in ^¬ formation and are well suited for comparing with the reference beam coming from the straight wavefronts or for comparing with an interference pattern formed from the straight wavefronts having the reference beam of desired intensity. The curved wavefronts emerging out of the sample are also well suited for forming an object beam resulting into a high lateral resolution in the interference pattern obtained by interaction of the straight wavefront part of the light emerging from the sample and the curved wavefront part of the light emerging from the sample.

FIG 1 schematically illustrates an exemplary embodiment of an interferometric microscopy arrangement 1 of the present tech- nique for inspecting a sample 99 placed in a sample port 90. The sample 99 may be any sample suited for inspection by an interferometric microscopy device, for example a whole blood of a subject. The interferometric microscopy arrangement 1, hereinafter the arrangement 1, includes a first illumination source 10, a second illumination source 20, a selection fil ^¬ ter 15, an interferometric unit 30, and an optical detector assembly 50.

The first illumination source 10 provides or produces a first light beam 11. The first light beam 11 is a collimated beam of light. The second illumination source 20 provides or pro ^¬ duces a second light beam 21. The second light beam 21 is a focused light beam, i.e. a light beam concentrating towards the sample 99. The first illumination source 10 may be, but not limited to, Laser, superluminescent diode (SLED or SLD) , and so on and so forth. Additional optics may be employed to increase or regulate collimation and/or coherence in the first light beam 11.

In an exemplary embodiment of the arrangement 1, the second illumination source 20 includes a light source 26 that pro ^¬ vides a light beam 27 and a condenser lens 28. The light source 26 may be, but not limited to, Laser, superluminescent diode (SLED or SLD), and so on and so forth. Additional op ^¬ tics may be employed to increase or regulate collimation and/or coherence in the light beam 27. The light beam 27 from the light source 26 is received by the condenser lens 28 and focused towards the sample port 90 or sample 99. The conden ^¬ ser lens 28 may be understood as any light focusing mechanism for example a microscope condenser, a focusing biconvex lens, and so and so forth. A degree or extent of focusing of the light beam 27 towards the sample port 90 is dependent on a numerical aperture of the condensing lens 28. As a result of the focusing of the light beam 27 towards the sample port 90, the second light beam 21 as a focused light beam is generated and directed towards the sample port 90. The selection filter 15 receives the first light beam 11 from the first illumination source 10 and receives the second light beam 21 from the second illumination source 20. The se- lection filter 15 then directs the first light beam 11 and the second light beam 21 towards the sample port 90.

In an exemplary embodiment of the arrangement 1, the selec- tion filter 15 may be, but not limited to a dichroic filter. The dichroic filter also referred to as thin-film filter or interference filter is an optical filter that allows selec ^¬ tive passage of a predetermined wavelength of light or a small range of predetermined wavelengths of light. The di- chroic filter does not allow passage of other wavelengths of light through it i.e. the dichroic filter, instead the di ^¬ chroic filter reflects the other wavelengths acting as re ^¬ flective surface. In optical geometry of the arrangement 1 depicted in FIG 1, the first light beam 11 has first wave- length that is different from a second wavelength of the se ^¬ cond light beam 21, and the selection filter 15 or the dichroic filter 15 selectively allows passage of the second wavelength through the dichroic filter 15 but reflects the first wavelength. Thus, the selection filter 15 directs the first light beam 11 and the second light beam 21 towards the sample port 90.

It may be noted that a person skilled in the art would readi ^¬ ly understand within the scope of the present technique, an- other exemplary embodiment (not shown) of the arrangement 1 having a different optical geometry of the arrangement 1 when compared to the optical geometry of the arrangement 1 depict ^¬ ed in FIG 1. In this another exemplary embodiment of the ar ^¬ rangement 1, the selection filter 15 or the dichroic filter 15 selectively allows passage of the first wavelength through the dichroic filter 15 but reflects the second wavelength. Thus, in this embodiment of the arrangement 1, with reference to position of the interferometric unit 30, relative position of the two illumination sources - namely the first illumina- tion source 10 and the second illumination source 20 - will be reversed or exchanged or swapped as compared to the posi ^¬ tion depicted schematically in FIG 1. Thus collimated light in form of the first light beam 11 and focused light in form of the second light beam 21 are pro ^¬ jected or shined or shone upon the sample 99 that may be pre ^¬ sent in the sample port 90.

The interferometric unit 30, hereinafter referred to as the unit 30, is positioned optically downstream of the sample port 90 and receives the first light beam 11 after interac ^¬ tion of the first light beam 11 with the sample 99 and the second light beam 21 after interaction of the second light beam 21 with the sample 99. The unit 30 generates a first ob ^¬ ject beam 12 and a first reference beam 14 by splitting the first light beam 11. Splitting of light beams is a well known technique and thus not explained herein with reference to FIG 1 for sake of brevity. Furthermore, the unit 30 filters out object information from the first reference beam 14. The fil ^¬ tering of the object information may be performed by a spa ^¬ tial filter 66 present in the unit 30. The unit 30 then di ^¬ rects the first object beam 12 having object information and the spatially filtered first reference beam 14 towards the optical detector assembly 50, as schematically depicted in FIG 1. The unit 30 also directs the second light beam 21 to ^¬ wards the optical detector assembly 50, as schematically de ^¬ picted in FIG 1.

The optical detector assembly 50 detects an interference pat ^¬ tern formed from the first light beam 11 i.e. by the first object beam 12 and the spatially filtered first reference beam 14 and also detects an optical pattern formed from the second light beam 21, the optical pattern formed by the se ^¬ cond light beam may be a microscopy image or may be an inter ^¬ ference component interacting with the first object beam 12 and the spatially filtered first reference beam 14 to con ^¬ tribute to the interference pattern formed from the first light beam 11. The optical detector assembly 50 may comprise a single detector in form of a color detector or may comprise two separate detectors (not shown in FIG 1) for detecting the first wavelength and the second wavelength. Such optical de- tectors, including color detectors and detectors capable of detecting a selected predetermined wavelength of light are well known in the art of optics and thus not explained herein in details for sake of brevity. The optical pattern formed by the second light beam 21 and the interference pattern formed by the first light beam 11 are superimposed to give an in ^¬ creased lateral resolution owing to the focusing of the se ^¬ cond light beam 21 onto the sample 99. AS schematically depicted in FIG 2, in an exemplary embodi ^¬ ment of the present technique, the second light beam 21 may also be split by the unit 30 into a second object beam 22 and a second reference beam 24. FIG 2 does not show the optical geometry or path of the first light beam 11, for sake of sim- plicity and for purposes of facilitating understanding, however, the optical geometry or path of the first light beam 11 may be understood in FIG 2 to be same as the optical geometry or path of the first light beam 11 depicted schematically in FIG 1.

In addition to splitting of the second light beam 21, the unit 30 filters out object information from the second refer ^¬ ence beam 24. The filtering of the object information may be performed by the spatial filter 66 present in the unit 30. It is well within the scope of the present technique, that as a result of the spatial filtering of the second reference beam 24 by the spatial filter 66 the second reference beam 24 may be filtered out entirely (not shown in FIG 2) and in such a scenario the second reference beam 24 will not be directed anymore towards the optical detector assembly 50. The unit 30 then directs the second object beam 22 having object infor ^¬ mation towards the optical detector assembly 50. Furthermore, if the second reference beam 24 is not entirely filtered out by the spatial filter 66, as depicted in FIG 2, then the unit 30 directs the spatially filtered second reference beam 24 towards the optical detector assembly 50, as schematically depicted in FIG 2. Thus the second object beam 22 along with the first object beam 12 creates an interference pattern with the first reference beam 14 and the second reference beam 24, if present. Since the second object beam 22 is part of the second light beam 21 and since the second light beam 21 in ^¬ teracted with the sample 99 as a focused beam or a curved wavefront beam, the lateral resolution of the arrangement 1 is higher as compared to an interferometric microscopy setup (now shown) where only collimated beams interacted with the sample . Thus in the arrangement 1, incoming light beam is a sum of the first light beam 11 and the second light beam 21. Since, the second light beam 21 is focused, for example by using the condenser lens 28 the numerical aperture of the condenser lens 28 contributes to the lateral resolution of the arrange- ment 1. Furthermore, since the numerical aperture of the con ^¬ densing lens 28 is non zero i.e.

^^Condenser ^ 0 the value of d is smaller compared to the value of d ob- tained from equation 1 hereinabove if all other parameters are constant. Since, smaller the value of d better is the lateral resolution of a microscopy device, thus better is the lateral resolution of the arrangement 1 of the present tech ^¬ nique .

Furthermore, since the collimated beam of light that is the first light beam 11 has an extended cross-section and is not just a single ray or very narrow beam of light as usually would emerge from a center of a normal biconvex lens, a de- sired intensity of the reference beam may be achieved simul ^¬ taneously with achieving higher resolution.

Referring to FIG 3, a detailed scheme for an exemplary arrangement 1 comparable to the schematically illustrated exem- plary embodiment presented in FIG 2 is depicted schematically. FIG 3 has been explained hereinafter in combination with FIG 2. The first light beam 11 in the collimated form and the second light beam 21 in the focused form are received by the selec ^¬ tion filter 15 or the dichroic filter 15 as explained in ref- erence to FIG 1. The selection filter 15 directs the first light beam 11 and the second light beam 21 towards the sample 99 or the sample port 90. The first light beam 11 and the se ^¬ cond light beam 21 both fall on or interact with the sample 99 in the sample port 90, and pickup or collect or gather ob- ject information i.e. the information about the sample 99 for example about physical structure of a red blood cell in the sample 99. The first light beam 11 and the second light beam 21 are then received by an objective lens 80, which can be understood as a commonly used microscope objective. As a re- suit of passing through the objective lens 80, the collimated beam or the first light beam 11 is focused on a Fourier plane 82 of the objective lens 80. The first light beam 11 and the second light beam 21 then are incident on a first lens 72 which is arranged such that a focal plane 73 of the first lens 72 coincides with the Fourier plane 82 of the objective lens 80.

The first light beam 11 and the second light beam 21 after passing through the first lens 72 propagate to and are inci- dent upon a second lens 74. The first lens 72 and the second lens 74 are arranged in 4f configuration with respect to each other. The first lens 72 and the second lens 74 form a refer ^¬ ence beam Fourier optics assembly 70. An other focal plane 71 of the first lens 72 coincides with a focal plane 75 of the second lens.

The reference beam Fourier optics assembly 70 may be a part of the unit 30. The unit 30 is positioned optically down ^¬ stream of the sample port 90. The unit 30 includes a beam splitter/combiner 32, hereinafter BSC 32. The BSC 32 receives the first light beam 11 and the second light beam 21 trans ^¬ mitted through the second lens 74. Beam splitters/combiners are well known in the art of optics and thus not explained herein in details for sake of brevity. The BSC 32 splits both the first light beam 11 and the second light beam 21. The first light beam 11 is split into the first object beam 12 and the first reference beam 14, whereas the second light beam 21 is split into the second object beam 22 and the se ^¬ cond reference beam 24.

The first object beam 12 and the second object beam 22 both travel along an object beam path and are directed towards an object beam reflector 62. The object beam reflector 62 functions to reflect the first object beam 12 and the second ob ^¬ ject beam 22 both back towards the BSC 32. The object beam reflector 62 may be a simple reflector such as, but not limited to, a plane mirror, a reflecting prism, or a

retroreflector . The first object beam 12 and the second ob ^¬ ject beam 22 after being received by the BSC 32 as a result of being reflected back by the object beam reflector 62 are directed by the BSC 32 towards the optical detector assembly 50.

The first reference beam 14 and the second reference beam 24 travels along a reference beam path and both are directed to ^¬ wards a reference beam reflector 64. Travelling along the reference beam path, from the BSC 32 towards the reference beam reflector 64, the first reference beam 14 and the second reference beam 24 both encounter the spatial filter 66. The spatial filter 66, for example a pin hole, is positioned op ^¬ tically in front of the reference beam reflector 64 in the unit 30. In an exemplary embodiment of the arrangement 1, as depicted by FIG 3, the spatial filter 66 is arranged at an other focal plane 75 of the second lens 74. The spatial fil ^¬ ter 66 filters out object information from the first refer ^¬ ence beam 14 and the second reference beam 24. In one embodi ^¬ ment of the present technique, as depicted in FIG 3, the se- cond reference beam 24 is filtered out almost entirely by the spatial filter 66 and not reflected back to the BSC 32 by the reference beam reflector 64. In this embodiment, only the first reference beam 14 resulting from the collimated light i.e. first light beam 11 is reflected back to the BSC 32 by the reference beam reflector 64.

The reference beam reflector 64 functions to reflect the first reference beam 14 and the second reference beam 24, if any, back towards the BSC 32. The reference beam reflector 64 may be a simple reflector such as, but not limited to, a plane mirror, a reflecting prism, or a retroreflector . The first reference beam 14 so reflected back to the BSC 32 is spatially filtered i.e. at least parts of the object in ^¬ formation are filtered out. The technique of spatial filter ^¬ ing, including use of pin hole as spatial filter, to delete or filter out object information from a light is well known in the art of optics and thus not explained herein in details for sake of brevity. The spatially filtered first reference beam 14 after being received by the BSC 32 as a result of be ^¬ ing reflected back by the reference beam reflector 64 is di ^¬ rected by the BSC 32 towards the optical detector assembly 50. The first object beam 12, the second object beam 22 and the spatially filtered first reference beam 14 pass through a third lens 76 arranged in a 4f configuration to the second lens 74. The first object beam 12, the second object beam 22 and the spatially filtered first reference beam 14 are super- imposed at the optical detector assembly 50 and form the in ^¬ terference pattern at the optical assembly 50.

Referring to FIG 4, a detailed scheme for an exemplary arrangement 1 comparable to the schematically illustrated exem- plary embodiment presented in FIG 1 is depicted schematically. FIG 4 has been explained hereinafter in combination with FIG 1.

The first light beam 11 in the collimated form and the second light beam 21 in the focused form are received by the selec ^¬ tion filter 15 or the dichroic filter 15 as explained in ref ^¬ erence to FIG 1, and directed by the selection filter 15 to ^¬ wards the sample 99 or the sample port 90. Both the first light beam 11 and the second light beam 21 interact with the sample 99 in the sample port 90, and pickup or collect or gather object information i.e. the information about the sample 99 for example about physical structure of a red blood cell in the sample 99. The first light beam 11 and the second light beam 21 are then received by the objective lens 80. As a result of passing through the objective lens 80, the colli- mated beam or the first light beam 11 is focused on a Fourier plane 82 of the objective lens 80. The first light beam 11 and the second light beam 21 then are incident on the first lens 72.

The unit 30 includes an additional selection filter 40, here ^¬ inafter referred to as the filter 40. The filter 40 receives the first light beam 11 coming from the selection filter 15 after interaction with the sample 99, and more particularly coming from the first lens 72. The filter 40 directs the first light beam 11 towards a first optical detector 52 of the optical detector assembly 50. The filter 40 also receives the second light beam 21 from the selection filter 15 after interaction with the sample 99 and directs the second light beam 21 towards the second optical detector 54 of the optical detector assembly 50, as shown in FIG 4. It may be noted that although in FIG 4, the filter 40 is shown to be positioned in between the first lens 72 and the second lens 74, it is well within the scope of the present technique, that the filter 40 may be inserted at any position in the optical path of the first light beam 11 and the second light beam 21 after the first light beam 11 and the second light beam 21 have interacted with the sample 99 and before the first light beam 11 and the second light beam 21 reach the BSC 32. In an exemplary embodiment of the arrangement 1, as shown in FIG 4, the filter 40 is a dichroic filter that selectively allows a wavelength of the first light beam 11 to pass through the dichroic filter and thus directs the first light beam 11 towards the first optical detector 52 after going through the BSC 32 and associated optical path with the BSC 32 as explained in reference to FIG 3. However, the filter 40 does not allow a wavelength of the second light beam 21 to pass through the dichroic filter 40, instead the dichroic filter 40 reflects the wavelength of the second light beam 21 and thus directs the second light beam 21 towards the second optical detector 54. This embodiment presents the optical ge ^¬ ometry of the arrangement 1 depicted in FIG 4.

However, it may be noted that a person skilled in the art would readily understand within the scope of the present technique, another exemplary embodiment (not shown) of the arrangement 1 having a different optical geometry of the ar- rangement 1 as compared to the optical geometry of the ar ^¬ rangement 1 depicted in FIG 4. In this another exemplary em ^¬ bodiment of the arrangement 1, the filter 40 or the dichroic filter 40 selectively allows passage of the second wavelength through the dichroic filter 40 but reflects the first wave- length. In this embodiment of the arrangement 1, a relative position of the first and second optical detectors 52, 54 and the BSC 32, the reflectors 62 and 64 etc with be reversed or swapped as compared to the embodiment depicted in FIG 4. Referring back to the exemplary embodiment of FIG 4, the se ^¬ cond light beam 21 is removed by the filter 40 and directed towards the second optical detector 54 of the optical detec ^¬ tor assembly 50. However, the first light beam 11 after pass ^¬ ing through the first lens 72 and through the dichroic filter 40 propagates to and is incident upon the second lens 74. The rest of the path of the first light beam 11 is same as the path of the first light beam 11 explained in reference to FIG 3. While the present technique has been described in detail with reference to certain embodiments, it should be appreciated that the present technique is not limited to those precise embodiments. Rather, in view of the present disclosure which describes exemplary modes for practicing the invention, many modifications and variations would present themselves, to those skilled in the art without departing from the scope and spirit of this invention. The scope of the invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.