WO2020053873A1

WO2020053873A1 - Digital holographic microscopy

Info

Publication number: WO2020053873A1
Application number: PCT/IN2018/050801
Authority: WO
Inventors: Azeem Ahmad; Dalip Singh MEHTA; Balpreet Singh AHLUWALIA
Original assignee: Indian Institute Of Technology Delhi; Uit, The Arctic University Of Norway
Priority date: 2018-09-13
Filing date: 2018-11-29
Publication date: 2020-03-19

Abstract

The present subject matter relates to a device (10) and a method for digital holographic microscopy. The device (10) includes at least one light source (12) for illuminating an object (20), an optical element (22) for collecting an object light beam from the illuminated object (20), a condenser lens (112) arranged in a path of the object light beam and adapted for generating a condensed object light beam, and a beam divider (115) arranged in a path of the condensed object light for generating a first beam (I) and a second beam (II). The device (10) further includes a mask (141) for passage (142) of the first beam (I) and low-pass spatial filtering (144) the second beam (II), and an imaging unit (150) arranged for capturing an interference pattern generated by the first beam (I) and the second beam (II).

Description

DIGITAL HOLOGRAPHIC MICROSCOPY

TECHNICAL FIELD

[0001] The present subject matter relates, in general, to a microscope, in particular, to a device for digital holographic microscopy.

BACKGROUND

[0002] In general, holographic microscopy is a quantitative phase imaging (QPI) technique, which work on the principle of interferometry and helps in retrieving the phase maps, which are further used to measure a refractive index and a dry mass of a sample, such as a biological cell. To produce an interference, at least two coherent beams are utilized, where one is reference beam and another is object beam.

BRIEF DESCRIPTION OF DRAWINGS

[0003] The detailed description is described with reference to the accompanying figures. The same numbers are used throughout the figures to reference like features and components. Some implementations of the system(s), in accordance with the present subject matter, are described by way of examples, and with reference to the accompanying figures, in which:

[0004] Fig. 1A shows a schematic illustration of a device for digital holographic microscopy in accordance with an implementation of the present subject matter.

[0005] Fig. 1B shows a schematic illustration of a mask used in Fig. 1A in accordance with an implementation of the present subject matter.

[0006] Fig. 2 shows a schematic illustration of a device for digital holographic microscopy in accordance with another implementation of the present subject matter.

[0007] Fig. 3 shows a temporal phase noise variation of the device under ambient environmental fluctuation, varying over a range of [- 15, 15] mrad. in accordance with another implementation of the present subject matter.

[0008] Fig. 4A shows a full-frame interferometric image of human RBCs captured using the device in accordance with an implementation of the present subject matter. [0009] FIG. 4B shows an enlarged view of the small portion of RBCs interferogram of Fig. 4A enclosed by the square in accordance with an implementation of the present subject matter.

[0010] FIG. 4C shows an example of a reconstructed phase image of human RBCs in accordance with an implementation of the present subject matter.

[0011] FIG. 4D shows a pseudo 3D phase map of human RBCs in accordance with an implementation of the present subject matter.

DETAILED DESCRIPTION

[0012] Two coherent beams are generally required to produce an interference in quantitative phase imaging (QPI) techniques. In conventional QPI systems, two different geometries are employed to produce the two coherent beams, one for a reference arm and another for an object arm. Such geometries can either work in a reflection mode or in a transmission mode.

[0013] However, said geometries for QPI are non-common path in nature and suffer from several technical problems, such as time-varying phase noise due to vibration, temperature gradient, and air flow. Such time-varying phase noise deteriorates the stability of QPI measurements. Such problems limit the application of QPI systems for the study of live cells dynamics, i.e., measurement of membrane fluctuations, which is a good indicator of several diseases. In addition, the non common path nature makes them difficult to integrate with other existing dynamical behavior studying techniques such as optical tweezers, microfluidics, and optical waveguide trapping etc.

[0014] Conventional QPI techniques also utilize lateral shearing digital holographic imaging to minimize the temporal phase noise. However, such technique require that amount of shear between the reflected wave fronts generated due to the front and back surface of a shear plate has to be greater than the size of an object under study. Therefore, such technique cannot be implemented for closely spaced biological or industrial objects, i.e., the object under study has to be spatially dispersed.

[0015] In further conventional QPI techniques, blazed grating is used to generate two beams having 80% optical power into first order and 10% into zero order and rest of the 10 % into reflected and higher order beams. One of the beam having high energy is passed through the pinhole, which blocks approximately 70% of the input intensity at the pinhole. Therefore, at the output of a pinhole assembly, one beam carries high intensity than another beam generated through the pinhole, and therefore degrades the quality of the interferograms. To equalize the intensity of both the beams, a neutral density filter has to insert into one of the beam path. However, such neutral density filter makes the process complex. In addition, the blazed grating is optimized only for a particular wavelength for which it diffracts the maximum optical power into a specified diffraction order, which further constraints the use of incident wavelength.

[0016] To this end, devices and methods for digital holographic microscopy are proposed, which are highly stable and work both in a transmission mode as well as a reflection mode.

[0017] In an example of the present subject matter, the device for digital holographic microscopy includes at least one light source for illuminating an object. The at least one light source is a source capable of emitting a light beam with specified power and coherence length. In one example, the light source includes multiple lasers with different wavelengths or partially spatially coherent laser. Further, an optical element is disposed for collecting an object light beam from the illuminated object. In one example, the optical element is in the form of an objective lens. The objective lens gathers light and focuses the light rays.

[0018] A condenser lens is arranged in a path of the object light beam and adapted for generating a condensed object light beam, and a beam divider is arranged in a path of the condensed object light beam. In one example, the beam divider is an optical glass plate. The beam divider includes a reflective exterior surface and a reflective interior surface. The beam divider is adapted for generating a first beam from the condensed object light beam reflected from the reflective exterior surface and a second beam from the condensed object light beam reflected from the reflective interior surface.

[0019] Further, the device includes a mask arranged in a path of the first beam and in a path of the second beam. The mask includes a through-hole for passage of the first beam and a pin hole for passage of the second beam. Furthermore, the device includes an imaging unit arranged for capturing an interference pattern generated by the first beam passing through the through-hole and the second beam passing through the pin hole. The device can be easily used with any dynamical behavior studying optical techniques.

[0020] In an example of the present subject matter, the method for digital holographic microscopy includes illuminating the object by at least one light source. An object light beam from the illuminated object is collected by an optical element, and a condensed object light beam is generated by a condenser lens. The condenser lens is arranged in a path of the object light beam. Further, a beam divider generates a first beam by reflecting the condensed object light beam from a reflective exterior surface of the beam divider and the beam divider generates a second beam by reflecting the condensed object light beam from a reflective interior surface of the beam divider. The beam divider is arranged in a path of the condensed object light beam. Yet further, the first beam is passed via a through -hole of a mask and the second beam is passed via a pin hole of the mask, the mask being arranged in a path of the first beam and in a path of the second beam. Furthermore, an interference pattern generated by the superposition of first beam passing through the through- hole and the second beam passing through the pin hole is captured by an imaging unit.

[0021] The implementations of the present subject matter offers a way of generating a reference beam and an object beam of approximately equal intensity. The optical configuration of the present subject matter makes the device compact and provides common path configuration when generating the reference beam and the object beam. The implementations of the present subject matter eliminates the need for using the neutral density filter to equalize the intensity of two beams, which makes it energy efficient and simple in configuration.

[0022] The implementations of the present subject matter provides high contrast interference fringes due to equal intensity of the two beams because of the partially and fully reflecting shear plate. The desired spatial carrier frequency can be generated for high resolution holographic reconstruction. Further, the phase noise of the claimed subject matter under ambient environmental fluctuations found to be quite low (better than 15 mrad.) due to the common path nature.

[0023] The implementations of the present subject matter also removes the need of blazed grating and enables changing of the objective lens with different magnification and numeric aperture without the need of re-alignment. Such implementations of the present subject matter diversify the application thereof in the field of biomedical imaging. Moreover, the use of the back surface silver coated glass slide could help us to combine multi -wavelength lasers into the beam path to perform simultaneous multi-wavelength study of the biological specimens. The implementations of the present subject matter are independent of magnification and numeric aperture of the objective lens used to image the object. The implementations of the present subject matter enables a highly stable common-path digital holographic microscopy for both reflection as well as transmission mode.

[0024] The potential applications of the implementations of the present subject matter can be in the field of profilometry and quantitative phase imaging of various industrial and biological objects. With the digital holographic microscopy of the present subject matter, all transparent objects like biological cells or tissues (human RBCs, cancer tissues, bacteria etc.), urine samples, pathogens in water etc. and industrial transparent objects such as, transparent step like objects, organic solar cells, organic thin film transistors etc. can be imaged quantitatively. However, reflection geometry is suitable for opaque samples like industrial objects (Si integrated circuits, profilometry of solar cells, micro -machined parts, micro-optics, diffractive optics etc.). The reflection geometry can also work for biological applications if the samples are prepared on a reflecting substrate.

[0025] These and other advantages of the present subject matter would be described in a greater detail in conjunction with Figs. 1 -4 in the following description. The manner in which the digital holographic microscopy is implemented and used shall be explained in detail with respect to Figs. 1-4.

[0026] The above mentioned implementations are further described herein with reference to the accompanying figures. It should be noted that the description and figures relate to exemplary implementations, and should not be construed as a limitation to the present subject matter. It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present subject matter. Moreover, all statements herein reciting principles, aspects, and implementations, as well as specific examples, are intended to include other implementations without deviating from the scope of the present subject matter. [0027] Fig. 1A shows a schematic illustration of a device 10 for digital holographic microscopy in accordance with an implementation of the present subject matter. In one example, the device 10 is a reflection type phase microscope. The digital holographic microscopy can be understood as digital holography applied to microscopy. The device 10 includes at least one light source 12 configured for illuminating the object 20. The light source 12 can be understood as a source capable of emitting a light beam with specified power and coherence length. In an example, the light source 12 is a He-Ne laser light source 12 and provides a highly coherent light beam having power - 15 mW and coherence length - 15 cm. In one example, the light source 12 provides a coherent light. In one example, the light source 12 includes multiple lasers with different wavelengths or partially spatially coherent laser. In one example, any laser with different wavelengths can be employed for interferometric imaging provided high temporal coherence length which depends on the spectral bandwidth of the laser. In another example, a spatially partially coherence monochromatic light source can also be used.

[0028] Further, an objective lens 14 and a pinhole 16 are arranged in a path of the emitted light beam from the at least one light source 12. The emitted light beam is passed through a combination of the objective lens 14 and the pinhole 16 to make the emitted light beam spatially filtered, i.e., provide uniform intensity over the entire field of view. The objective lens 14 gathers light from the object being observed and focuses the light rays to produce a real image. The pinhole 16 is a small circular hole used as apertures in optical systems to spatially filter the light beam. In one example, the pinhole 16 acts as a low-pass filter for spatial frequencies in an image plane of the light beam. The spatially filtered light beam diverges when emerges from the pinhole 16 and is collimated by a collimating lens 18 disposed in a beam path of the spatially filtered light beam. The collimating lens 18 is an optical lens that help to make parallel the light beam that enters therein and allow users to control the field of view. The lens is a transparent optical device that collimates or focuses the light beam by means of refraction depending on the nature of the beam whether diverging or collimated beam passing through it. The object 20 located in a path of the collimated light beam being collimated by the collimating lens 18 is illuminated, which avoids inaccuracy in phase measurement. The illuminated object issues an object light beam that is then collected by an optical element 22 disposed in a path of the object light beam. In one example, the optical element 22 is in form of an objective lens. Further, a lens tube 24 is disposed adjacent to the optical element 22 through which the collected object beam is projected to a mirror 26. The collected object beam is further imaged at a first image plane 110 using a tube lens 24. The tube lens 24 is to create an image with an infinity corrected objective and is to focus the image. The mirror 26 is disposed adjacent to the lens tube 24. In one example, an angle of the mirror 26 with respect to an optical axis of the lens tube 24 is 45 degrees to orient the object beam by 90 degrees. The mirror 26 is a surface, typically of glass coated with a metal amalgam, which reflects a clear image. In one example, the mirror 26 is used to bend the output light of a microscope in angle of 90°.

[0029] Further, a condensor lens 112 is disposed at a specified angle with respect to the mirror 26 and below the first image plane 110. In one example, an angle of the mirror 26 with respect to an optical axis of the condensor lens 112 is 45 degrees. The condenser lens 112 can be understood an optical lens which renders a converging beam. The condensor lens 112, in operation, may converge the object beam coming from the mirror 26 via the first imge plane 110 to a second image plane 140. In one example, the second image plane 140 is a mask plane. In another example, the mask plane is a Fourier plane.

[0030] A beam divider 115 arranged in the path of the condensed object light beam is disposed in between the condenser lens 112 and the second image plane 140. The distance between the condenser lens 112 and the beam divider 115 depends on the focal length of the condenser lens 112. In one example, if the condenser lens 112 has the focal length ~ 200 mm, the beam divider 115 lies within 200 mm from the the condenser lens 112 to obtain two focused spots outside the beam divider 115 so that one of the beams can be spatially filtered. In an implementation of the present subject matter, an angle of the beam divider 115 with respect to the optical axis of the condenser lens 112 is 45° to orient the condensed object light beam by 90°. In one example, the angle of the beam divider 115 with respect to the optical axis of the condenser lens 112 can be in a range of 30° to 60°.

[0031] In one example, the beam divider 115 is made of a transparent material, for example, glass, which has an optical density greater than that of air. In one example, the beam divider 115 is made of UV fused silica. The surface flatness of the beam divider 115 is < l/10 at 633 nm wavelength. The beam divider 115 has a thickness of 9.4 mm and can be varied from lmm - 20mm. The temporal coherence length of the light beam must be larger than the optical path length generated by the beam divider to produce interferogram.

[0032] In one implementation of the present subject matter, the beam divider 115 is an optical glass plate having a reflective exterior surface 114 and a reflective interior surface 116 and is arranged such that the condensed object light beam is incident on the reflective exterior surface 114 and the reflective interior surface 116 with an angle >30 degree. In one example, the angle of incidence is approximately 45 degrees.

[0033] The reflective interior surface 116 is provided with a silver coating. The silver coating is at a back surface of the optical glass plate. In one example, the reflective exterior surface 114 and the reflective interior surface 116 are parallel to each other.

[0034] The beam divider 115 generates a first beam I and a second beam II from the object light beam due to the multiple reflections inside the optical glass plate. In one example, the beam divider 115 is adapted for generating a first beam I from the condensed object light beam reflected from the reflective exterior surface 114 and a second beam II from the condensed object light beam reflected from the reflective interior surface 116. In one example, the object light beam reflected from the reflective exterior surface 114 of the optical glass plate carries ~ 20% intensity of the input beam intensity and reflection from the reflective interior surface 116 carries most of the intensity, i.e., ~ 64% of the input beam intensity, and rest of the energy goes into the multiple reflections. The reflections from the reflective exterior surface 114 and the reflective interior surface 116 are isolated at the second image plane 140 by employing a Fourier transform space amplitude mask 141. In one example, the mask 141 is a Fourier transform space amplitude mask 141. The mask 141 is illustrated in detail under Fig. 1B. In one example, the first beam I and the second beam II do not focus at the same plane and converge to different planes depending on the thickness of the beam divider 115..

[0035] Fig. 1B shows a schematic illustration of the mask 141 used in Fig. 1 A in accordance with an implementation of the present subject matter. The mask 141 is arranged in paths of the first beam I and the second beam II from the reflective exterior surface 114 and the reflective interior surface 116 of the beam divider 115 respectively. The mask 141 includes a through-hole 142 and a pin hole 144 such that the first beam I passes through the through-hole 142 and the second beam II passes at through the pin hole 144. In one example, the second beam II passes at least partially through the pin hole 144. In one example, the mask 141 is made of brass. In another example, the mask 141 can be made in any opaque material. In one example, the mask 141 is a pinhole assembly.

[0036] The pin hole 144 can be made by an opaque material. Examples of opaque material include, but is not limited to, stainless steel, black plastic, and composite material. The pinhole is placed in one of the hole of the pinhole assembly. The pinhole can be understood as a small circular hole, as could be made with the point of a pin. In optics, pinholes with diameter between a few micrometers and a hundred micrometers are used as apertures in optical systems.

[0037] In one example, the diameter of the through hole 142 can be in a range of 3 mm- lOmm. Further, the diameter of the pinhole 144 depends on the size of the airy disk produced by the condenser lens 112. The size of the airy disk depends on the input object light beam diameter and the focal length of the lens 112. In one example, the diameter of the pinhole 144 can be in a range of 5 to 50 microns depending on the size of input beam and focal length of the condenser lens 112. In one example, the diameter of the pinhole 144 is 30 microns.

[0038] Returning to Fig. 1A, the mask 141 has an effect that the reflection from the reflective exterior surface 114 of the optical glass plate is spatially low pass filtered by the pinhole 144 such that only the DC component of the second beam II is passed. The second beam II reflected from the reflective exterior surface 114 is completely passed via the through-hole 142. In one example, the diameter of the through-hole 142 is 12 mm. In one example, the through-hole 142 is a circular hole.

[0039] Further, a collimating lens 146 is disposed in a path of the first and second beams I, II when filtered by the mask 141. The collimating lens 146 is employed into the beam path to collimate the first and second beams I and II and subsequently superimpose them at a third image plane 148 for the formation of interferograms. In one example, the focal length of the collimating lens 146 is 200 mm. The distance between the collimating lens 146 and the mask 141 is depends on the focal length of the collimating lens 146. In one example, the distance between the collimating lens 146 and the mask 141 is 200 mm. In one example, the distance between the collimating lens 146 and the mask 141 can be in a range of 150 mm to 250 mm depending on the required magnification and resolution of the device 10 but then the focal length of the collimating lens 146 must be chosen accordingly.

[0040] At the output of the mask 141, the unfiltered first beam I and the spatially filtered second beam II (after passing through the pinhole 144) carries almost equal intensity, which provides generation of good contrast fringes of the resulting holographic image provided by the interference pattern which is captured by an imaging unit 150 at the third image plane 148. The imaging unit 150 is disposed at the third image plane 148. In one example, the imaging unit 150 can be a camera. Any camera can be used for imaging depending of the type of application. Type of camera plays an important role in quantitative phase imaging as it influences signal to noise ratio (SNR) of the recovered phase images of the specimens. If low SNR is sufficient enough for a particular biological application then even webcam can also be employed.

[0041] In one example, the mask 141 is perpendicular to the optical axis of the second beam II to maximize the intensity of the filtered beam after passing through the pinhole 144. Therefore, the beam divider 115 is disposed at an angle of 45° with respect to the optical axis of the condenser lens 112. In addition, such an angle minimizes the unwanted diffraction effects from the edge of the pinhole 144. For other beam divider angles, the angle of the mask 141 must be changed accordingly. Keeping the optical design limitation in mind, the beam divider angle can be tuned from 30° to 60° with change in the position and orientation of the components 141, 146 and 150. In addition, beam divider angle also changes the fringe width of the interferograms.

[0042] Further, a computing unit (not shown) is coupled to the imaging unit 150 for rendering an image of the object 20 based on the interference pattern. In one example, the computing unit is adapted to Fourier transform the interference pattern. The computing unit is a central processing unit (CPU), which is an electronic circuitry within the computing unit that carries out the instructions of a computer program by performing the basic arithmetic, logical, control and input/output (I/O) operations specified by the instructions. [0043] In one example, the spatially modulated interferograms, generated due to the superposition of the unfiltered first beam I and the spatially filtered second beam II and further captured by the imaging unit 150, are further processed by means of a Fourier transform based phase retrieval algorithm to measure the phase shift introduced by the object 20. The measured phase shift is related to the refractive index and height of the sample as:

[0044] Where l is the illumination wavelength, Dh is the refractive index difference between the sample and surrounding medium and h(x,y) is sample thickness.

[0045] The combination of lenses designated by numerals 112, 146 and the beam divider 115 provides a nearly common path geometry, which make the stability of the device 10 high.

[0046] Fig. 2 shows a schematic illustration of the device 10 for digital holographic microscopy in accordance with another implementation of the present subject matter. Elements similar to those of Fig. 1 have the same reference numbers. The device 10 includes at least one light source 12’ configured for illuminating the object 20. In one example, the light source 12’ provides a coherent light. In one example, the light source 12’ includes multiple lasers with different wavelengths or partially spatially coherent laser. Further, an objective lens 14’ and a pinhole 16’ are arranged in a path of the emitted light beam from the at least one light source 12’. The emitted light beam is passed through a combination of the objective lens 14’ and the pinhole 16’ to make the emitted light beam spatially filtered, i.e., provide uniform intensity over the entire field of view. The spatially filtered light beam diverges when emerges from the pinhole 16’ and is collimated by a collimating lens 18’ disposed in a beam path of the spatially filtered light beam. The collimated light beam is further passed through a lens 21 disposed in a path of the collimated light beam. The lens 21 focuses the laser beam at a back focal plane of objective lens 22 using a beam splitter 25. The beam splitter 25 includes an output port (not shown) which is blocked. The objective lens 22 illuminates an object 20 through a collimated beam, which prevents the height measurement error of the opaque sample. The scattered light beam from the object 20 is then collected by the same objective lens 22, and the light beam is further imaged at the first image plane 110 using a combination of mirrors 26, 30 and a tube lens 28. After the imaging of the light beam at the image plane 110, the working principle of the device 10 is similar to the transmission type phase microscope as described above with reference to Figs 1A-1B. All the lenses including the condenser lens 112 are made of N-BK7 material and are highly transparent in the entire visible range (380nm - 800nm).

[0047] Fig. 3 shows a temporal phase noise variation of the device under ambient environmental fluctuation, varying over a range of [- 15, 15] mrad. in accordance with another implementation of the present subject matter. In order to illustrate the temporal stability of the present subject matter, a series of time lapsed interferograms having size 1390 x 1040 pixels without any sample and vibration isolation table were recorded. The time lapsed interferograms were further processed to measure the phase variation at a single spatial location of the retrieved phase map with respect to time. The temporal phase noise distribution shown in Fig. 3 is then calculated as

Af c,g, ί) f{%_. y_s i) - f c, y, t = 0} (2)

[0048] The temporal phase noise of the present invention can be further reduced, possibly to less than 5 mrad., by covering both the interferometric arm and arranging the microscope on a vibration isolation optical table.

[0049] In an example to show the feasibility of an experimental implementation of the present invention, human red blood cells (RBCs) are imaged by using the device 10 of the present subject matter. During the experimental implementation, the blood sample is collected in the EDTA containing tubes, which avoids the coagulation of RBCs. The blood sample is then diluted using PBS and washed twice to isolate the human RBCs from the rest of the blood components such as white blood cells, platelets and plasma. Further, the sample was prepared on a microscopic glass slide and placed under in the device 10.

[0050] Fig. 4A shows a full-frame interferometric image of human RBCs captured using the device 10, 10’ in accordance with an implementation of the present subject matter. [0051] FIG. 4B shows an enlarged view of the small portion of RBCs interferogram of Fig. 4A enclosed by the square in accordance with an implementation of the present subject matter. To avoid the computational load, a small portion A of the interferogram is selected.

[0052] The portion A is then processed further using the Fourier transform based phase retrieval algorithm. Figure 4C and 4D represent the recovered phase map corresponding to the interferogram. FIG. 4C shows an example of a reconstructed phase image of human RBCs in accordance with an implementation of the present subject matter and FIG. 4D shows a pseudo 3D phase map of human RBCs in accordance with an implementation of the present subject matter. The background phase from the recovered phase map is subtracted numerically by employing a polynomial fitting method. The recovered phase map can be further utilized to measure several parameters related to cells such as the cell’s height, refractive index, the cell’s dry mass, hemoglobin content, hemoglobin concentration, membrane fluctuations, etc.

[0053] Although examples for the present disclosure have been described in language specific to structural features and/or methods, it should be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed and explained as examples of the present disclosure.

Claims

I/WE CLAIM:

1. A device (10) for digital holographic microscopy, the device (10) comprising: at least one light source (12) for illuminating an object (20);

an optical element (22) for collecting an object light beam from the illuminated object (20);

a condenser lens (112) arranged in a path of the object light beam and adapted for generating a condensed object light beam;

a beam divider (115) arranged in a path of the condensed object light beam and having a reflective exterior surface (114) and a reflective interior surface (116), wherein the beam divider (115) is adapted for generating a first beam (I) from the condensed object light beam reflected from the reflective exterior surface (114) and a second beam (II) from the condensed object light beam reflected from the reflective interior surface (116);

a mask (141) arranged in a path of the first beam (I) and in a path of the second beam (II), wherein the mask comprises a through-hole (142) for passage of the first beam (I) and a pin hole (144) for passage of the second beam (II); and

an imaging unit (150) arranged for capturing an interference pattern generated by the first beam (I) passing through the through-hole (142) and the second beam passing through the pin hole (144).

2. The device (10) as claimed in claim 1, wherein the beam divider (115) is an optical glass plate having the 20% reflective exterior surface (114) and the 100% reflective interior surface (116).

3. The device (10) as claimed in claim 1, wherein the reflective exterior surface (114) and the reflective interior surface (116) comprise a partially or a fully silver coating.

4. The device (10) as claimed in claim 1, wherein the through -hole (142) has a diameter of 12 mm.

5. The device (10) as claimed in claim 1, wherein the pin hole (144) has a diameter of 30 pm.

6. The device (10) as claimed in claim 1, wherein the at least one light source (12) is to provide a coherent light.

7. The device (10) as claimed in claim 1, wherein the at least one light source (12) comprises multiple lasers with different wavelengths or partially spatially coherent laser.

8. The device (10) as claimed in claim 1, wherein the device (10) comprises a collimating lens (18) arranged between the at least one light source (12) and the object (20).

9. The device (10) as claimed in claim 1, wherein the mask (141) is a Fourier transform space amplitude mask arranged at a Fourier plane of the first beam (I) and the second beam (II).

10. The device (10) as claimed in claim 1, wherein the device (10) comprises a computing unit coupled to the imaging unit (150) for rendering an image of the object (20) based on the interference pattern.

11. The device (10) as claimed in claim 10, wherein the computing unit is to Fourier transform the interference pattern.

12. A method for digital holographic microscopy, the method comprising:

illuminating, by at least one light source (12), an object (20);

collecting, by an optical element (22), an object light beam from the illuminated object (20);

generating, by a condenser lens (112), a condensed object light beam, wherein the condenser lens (112) being arranged in a path of the object light beam; generating, by a beam divider (115), a first beam (I) by reflecting the condensed object light beam from a reflective exterior surface (114) of the beam divider (115);

generating, by the beam divider (115), a second beam (II) by reflecting the condensed object light beam from a reflective interior surface (116) of the beam divider (115), wherein the beam divider (115) being arranged in a path of the condensed object light beam; passing the first beam (I) via a through -hole (142) of a mask and the second beam (II) via a pin hole (144) of the mask, wherein the mask being arranged in a path of the first beam (I) and in a path of the second beam (II);

and

capturing, by an imaging unit (150), an interference pattern generated by the first beam (I) passing through the through-hole (142) and the second beam (II) passing through the pin hole (144).

13. The method as claimed in claim 12, the method comprising:

rendering, by a computing unit, an image of the object (20) based on the interference pattern, wherein the computing unit being coupled with the imaging unit (150).