WO1996036908A1

WO1996036908A1 - Holographic imaging

Info

Publication number: WO1996036908A1
Application number: PCT/GB1996/001196
Authority: WO
Inventors: Paul Michael William French
Original assignee: Imperial College Of Science, Technology And Medicine
Priority date: 1995-05-19
Filing date: 1996-05-20
Publication date: 1996-11-21
Also published as: GB9510200D0

Abstract

A holographic imaging apparatus using a pulsed laser light source (1) and a real-time interferogram recording medium (8), such as a photorefractive crystal or semiconductor quantum well device, that is viewed by a CCD camera 15 to yield a 2-D image for a particular time-of-flight (depth) to the object. The time-of-flight may be varied to produce a sequence to 2-D images from different depths that may then be processed to yield a 3-D digital model of the object being imaged. This model may be transmitted elsewhere to a 3-D rendering apparatus that can produce a 3-D copy of the original object.

Description

HOLOGRAPHIC IMAGING

The present invention provides apparatus for generating real time or near real time 2-D and/or 3-D images of objects, particularly objects obscured by diffusing media.

The apparatus according to the invention is expected to have applications in medical diagnostic equipment for imaging internal tissues in human, animal or plant bodies in-vivo or in-vitro, in micro-fabrication, microscopy and various engineering applications. Various techniques for producing 3-D holographic images are well known but a brief explanation is included for the sake of completeness and to assist in the readers understanding of the present invention. Briefly, an object is illuminated by an object beam of coherent light from a laser source. An image recording medium, commonly a photographic plate is exposed to the object light reflected from the object. Simultaneously, a reference beam derived from the source, is directed on to the surface of the photographic plate. By careful arrangement of the apparatus the coincidence of the object beam and the reference beam generates a pattern of interference fringes which record the intensity and phase of the wavefront of the object light. By developing the plate and illuminating the resulting interferogram using a reconstruction beam similar to the reference beam, a virtual or a real image of the object can be created.

The closeness of the fringes in a conventional hologram means that the recording will be very sensitive to movement of the object during exposure, causing severe degradation of the hologram record. When photographic plates are used for recordal, exposure times of seconds are required for the light levels commonly used.

In an experiment conducted by Abramson N.H. and Spears K.G. entitled

"Single pulse light-in-flight recording by holography" published in Applied Optics Vol

28, No.10/15:05:89, Abramson and Spears disclose apparatus and a method whereby light-in-flight techniques allow temporal information to be encoded into the hologram as position along the holographic plate. To achieve this a pulsed laser source is used and the reference beam illuminates the holographic plate at oblique incidence so that interference occurs for different relative delays between the object and reference beam. Once the hologram is recorded the holographic plate can be developed and illuminated with a reconstruction beam (which may be similar to the reference beam although this beam can be a continuous wave and of a different wavelength). For a three dimensional object different views (two dimensional images) of the object at different positions along the holographic plate will correspond to different depths in the object.

Thus a contour map can be obtained, giving three dimensional information about the object.

The time needed to record a hologram onto photographic plate seriously limits the usefulness of Abramson and Spears technique. Because of the low light levels, prolonged exposure times of the order of seconds are required. Also, film requires long developing times, of the order of thirty minutes. This is very inconvenient and impractical for many possible applications. The holographic film used is expensive and cannot be reused.

Accordingly there is provided apparatus comprising a light source to generate an object beam and a reference beam whereby an object can be illuminated with the object beam, a real-time or quasi-real time recording medium arranged to be exposed to and record an interferogram formed by the coincidence of light reflected from the object and the reference beam, and means for viewing and recording the holographic image which is reconstructed from the interferogram. The real-time recording medium may be a photorefractive recording medium such as photorefractive crystals

(for example a crystal of Bi,₂ SiO₂₀) which has the advantageous property of responding to light by locally changing the refractive index. Thus the interferogram pattern to which the photorefractive medium is exposed is recorded and can be read by illuminating the medium with a suitable reconstruction beam. Because photorefractive crystals and other photorefractive media are faster than photographic plates and have an appropriate spatial resolution it is possible to expose the photorefractive medium for a very brief duration and then, using a suitable reconstruction beam, to read the hologram stored in the medium and record the holographic image using an electronic camera (for example) between exposures or simultaneously. Preferably the electronic camera is provided by a charge coupled device. The holographic image is conveniently recorded electronically via the CCD and the electronic record data thus produced can be processed to reconstruct an image _• of the object. Alternatively the "real time" holographic recording medium may be a medium which exhibits a non-linear optical response such that its absorption or refractive index changes locally according to the intensity of light incident upon it. In this case the holographic grating would be written by the interference between the object and reference beams and the hologram would be read out simultaneously or soon afterwards by the reconstruction beam and this read out image would be viewed directly or recorded using a suitable imaging device such as an electronic CCD camera. An example of such a medium would be a liquid crystal device or a semiconductor device incorporating multiple quantum well structures or a bulk semiconductor device. Figures 7 and 8 show possible embodiments of photorefractive semiconductor devices.

The present apparatus can be used to record light-in-flight images by using a configuration such as that disclosed by Abramson and Spears. By recording multiple exposures and superposing images or otherwise processing the image data, objects obscured by speckle noise in diffuse media may be imaged. Also since only coherent light contributes to the hologram, objects obscured by diffusing media may be imaged. This may be achieved even where the object is in motion. Thus the present apparatus has potential applications in medical diagnostic apparatus for recording images of internal tissues. It will be appreciated that the present apparatus allows the recordal of holograms in milliseconds, and enables the hologram to be viewed practically immediately, in real time. The photorefractive medium is indefinitely reusable. Conveniently the CCD camera allows a sequence of images obtained at different positions along the recording plane to be recorded. In this way a contour map of the object can quickly be obtained and stored for subsequent (three dimensional) image processing.

When used in conjunction with a frame grabber and computer, the apparatus permits averaging of a stored sequence of images to be routinely and rapidly carried out. This is useful when imaging through time varying diffuse media such as a living body which can randomly distort the wavefronts and produce significant laser speckle.

If the hologram is recorded on a time scale fast compared to the relaxation time of the speckle, then averaging of repeated exposures would remove much of the unwanted distortion.

While most applications of the present invention will require the provision of an electronic camera to electronically record the holographic imaging data for subsequent processing, some applications may not require an electronic camera, for example, where the image data is to be processed optically or the image is to be viewed by eye and directly reconstructed from the photorefractive medium.

To reduce the size of photorefractive medium it may prove advantageous to scan the field of view of the medium along the holographic image plane. This may be achieved either by moving the medium or, optically, by moving its field of view. Embodiments of apparatus constructed in accordance with the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: Figure 1 shows a first embodiment; Figure 2 shows a second embodiment; Figure 3 shows a third embodiment;

Figure 4 shows a fourth embodiment;

Figure 5 shows an alternative arrangement of the object to be imaged; Figure 6 shows a fifth embodiment;

Figures 7 and 8 show photorefractive semiconductor devices; and Figures 9, 10, 11 and 12 illustrate applications of the imaging apparatus.

The apparatus shown in Figure 1 includes a pulsed laser light source 1 , such an argon ion laser or Ti:Al₂O₃ laser, or semi-conductor diode pumped solid state layer or other suitable laser. These sources can be controlled in known manner to emit coherent pulsed light at any one of a variety of wavelengths. The source 1 generates a pulsed laser beam which is divided by a conventional beam splitter device 2 into an object beam 3 and a reference beam 4. The object beam 3 is diverged by a lens 5 to form a diverging object beam 31 to illuminate an object 7. Similarly, the reference beam 4 is diverged through a lens 6 to form a diverging reference beam 41 which is directed obliquely to illuminate a photorefractive recording medium 8 which may be provided by a photorefractive crystal. The photorefractive medium 8 is arranged to be exposed to light reflected from the object 7 so that the coincidence of the reflected light and diverging-reference beam 41 generates a pattern of interference fringes known as an interferogram, which is temporarily recorded by the medium 8. After the medium 8 is exposed the holographic image stored as the recorded interferogram is read out to a camera 9 which includes a charge coupled device (CCD) by illuminating the medium 8. The interferogram is read out using a reconstruction beam generated by a laser source 15 and directed onto the photorefractive medium via an optical assembly which may include mirrors and lenses 16. The reconstruction beam may illuminate the photorefractive medium at an angle of incidence different to that of the object beam, e.g., by Bragg-matching a different wavelength reconstruction beam in the photorefractive medium. The camera then converts the holographic image into electronic signals to be recorded in the memory of an image processing system 10.

This can then be processed to generate an image.

In the first embodiment of Figure 1 the medium 8 is of sufficient size to view the whole interferogram simultaneously. However, for most practical applications, the medium 8 cannot be made sufficiently large for this. To overcome this problem the medium 8 and the camera 9 can be mounted on a carriage 1 1 which scans the medium

8 through the plane of the interferogram I as shown in Figure 2. In this case the single interferogram is recorded as a sequence of image frames in the electronic memory of the data processing system 10. The reconstruction beam moves with the carriage 1 1 such that it continuously illuminates the photorefractive medium for the purpose of reconstructing the holographic image.

Figure 3 illustrates a third embodiment having a near colinear configuration. In the colinear apparatus the reference beam 4, 4' and object beam 3, 3' are split off of the source beam via a beam splitter device 2. The object beam 3' is directed onto the object via an object beam splitter 12. The object beam reflected from the object 7 is directed onto the holographic medium (photorefractive medium) through the object beam splitter 12 and a reference beam splitter 13. The reference beam splitter 13 directs the reference beam 4' onto the photorefractive medium 8 so that the reference beam falls onto photorefractive medium 8 close to colinear to the object beam. The reference beam 4, 4' is guided through an optical delay assembly 14 which allows the path length of the reference beam 4, 4' to be altered, by moving the guide mirrors 14B in the direction indicated by arrow C, towards or away from the mirrors 14A in order to view images of the object having different spatial depth and temporal relationships. Thus, this apparatus avoids the problems of having to scan across a large inteferogram generated by a reference beam which impinges obliquely on the object beam and also reduces compromising of the spatial resolution of the image. This configuration ameliorates the trade-off between spatial and temporal resolution which occurs for the highly oblique incidence configuration.

The apparatus presents the possibility of recording light-in-flight images of the object such that different depths of the object will be imaged separately, either at different positions on the holographic plate 8, or in different exposures corresponding to different settings of the optical delay assembly 14.

The apparatus shown in Figure 4 illustrates one potentially important application whereby an object 7 immersed in an obscuring scattering medium 17 can be imaged. In this case the object beam 3' is directed by a mirror 12' to be transmitted through the obscuring medium. An inteferogram is then constructed on the photorefractive medium using the reference beam 4' and the light transmitted through the obscuring medium 17.

Figure 5 illustrates an alternative application of the apparatus shown in Figure 3 in which the object is obscured behind the diffusing medium 17.

Figure 6 shows a fifth embodiment where the object beam and reference beam are made to be incident on the holographic recording medium from opposite directions. These beams may be anti-parallel or at an angle to each other.

The depth information of the 3-D image is obtained from the above embodiments by a time-of-flight measurement. A series of images is recorded with each image corresponding to a different arrival time of the light from the object at the holographic medium (e.g., photorefractive medium). This technique can also be used to image objects embedded in, or located behind, scattering media such as human tissue.

When an coherent image-bearing light signal propagates through a scattering medium, the light is mostly scattered and the coherence of the signal is destroyed. If the image bearing signal light is scattered in many directions, the image information will be lost.

In many practical situations, a very small fraction of the image bearing light does propagate without scattering directly through the scattering medium and this can be used to reconstruct the image provided that it can detected. Unfortunately, the ratio of the scattered light to the unscattered light is very large and usually the unscattered light swamps the remaining image-bearing signal light and makes detection and image reconstruction impossible.

The unscattered image-bearing light takes the most direct path through the scattering medium and can often be contrived to arrive at a suitable detector with a different arrival time compared to the scattered light. Using the apparatus described above, it is possible to make a holographic image using this coherent unscattered light. In this way an image can be reconstructed from the earliest arriving (and therefore image-bearing unscattered) light. Thus an alternative use of the invention described above is to view objects in real time or in single-shot which are obscured by scattering media. This is illustrated in Figure 4. The early arriving unscattered light is still coherent with the reference beam while the scattered light is not. This means that only the unscattered light contributes to the hologram. If this hologram is read out using a powerful reconstruction beam, even a low diffraction efficiency (caused by a very weak hologram) can diffract as a detectable amount of energy into the read out image.

The system can obtain depth information for 3-D images, in real-time or single shot mode, by time-gating the image bearing coherent light reflected from a 3-D object using a holographic medium such as a photorefractive medium. It can also use the time-gating in the holographic medium to discriminate in favour of unscattered image- bearing light, and form depth resolved 2-D images for 3-D image reconstruction of an object obscured by a scattering medium, against a background of scattered light which might otherwise swamp the signal light and make image reconstruction impossible.

It will be appreciated that the laser pulse duration and gating of the recording medium may be such that exposures of down to few microseconds may be achieved.

This coupled with the rapid image capture through operation of the CCD camera can result in a frame rate comparable to or better than a conventional video frame rate and in spatial resolutions of a few microns.

Figure 9 schematically illustrates application of the imaging (3-D scanning/profiling) apparatus described above to provide the equivalent of a 3-D facsimile machine. The object 7 to be imaged is irradiated with light using an interferometer 20, such as described above, and the image of a 2-D section of the object 7 at a predetermined depth X is captured on the recording medium 8. The recording medium 8 is then viewed with a CCD camera 15 to yield a digitised version of the 2-D section that is fed to a computer 22.

The process of capturing a 2-D image is repeated many times for differing depths X to build a 'stack' of such images. The spacing of such images captured has an effect upon the achievable spatial resolution in depth and must be balanced against the time taken to scan the complete object 7 over its full depth range. The computer 22 then takes this stack of 2-D images and interpolates a 3-D digital model of the object 7. The techniques that may be used in such interpolation are known from systems such as those produced by Voxel of Merit Circle, Laguna Hill, California, United States of America that produce computer generated holograms from 3-D models interpolated from a sequence of 2-D NMR or CAT scan images. As an alternative to the application illustrated in Figure 9, the 3-D model produced by the computer 22 could be transmitted elsewhere and used to produce a computer generated hologram at some remote location to yield a system that would give 3-D holographic television. The rapid capture time and real-time nature of the imaging system described above allows it to act as a 3-D real-time camera for such a system.

In the system illustrated in Figure 9, the 3-D model data produced by the computer 22 is transmitted over the telephone system 26 via modems 24, 28 to reach a remote 3-D rendering machine 30 (such as that produced by 3-D Systems of Hemel Hempstead. United Kingdom). This rendering machine 30 is responsive to the 3-D model data to reproduce in 3-D a copy of the object 7. There are several ways in which the rendering system 30 may operate. Typically, a laser beam is scanned through a photosensitive liquid 32 under control of the 3-D model data such that selected portions of the liquid 32 are removed. When the scan through the liquid 32 is complete the copy object 34 is formed. In Figure 9, the copy object 34 is shown partially formed.

Figure 10 illustrates another application of the imaging apparatus described above. In this case, coded information is embedded within a body 36 that is opaque to light of a visible wavelength and at least partially translucent to the light of the laser used to image the coded information. The coded information is in the form of an array of regions that reflect the laser light to a different extent and can be considered to be similar to an embedded barcode. In order to store more information and to increases the difficulty of unauthorised read or copying of such information, the region 38 that bears the data has a multilayer structure. The imaging system described above is able to produce 2-D images at differing depths (E, D, C, B and A) within the body 36 that may then be analyzed in turn to extract a digital data pattern therefrom. Examples of uses would be a highly secure entry pass or the forming of unique markings an object to combat theft or duplication.

Figure 1 1 illustrates another possible application of the imager described above to assist in combating counterfeit banknotes. In this case a banknote 40 has a security pattern 42 printed upon it that is then overprinted by a further layer of ink 44 that is opaque to visible light but partially translucent to the laser light of the imager. If a banknote is suspected of being counterfeit, then it may be viewed with the imager to a depth F in order to determine if the security pattern is present. Whilst the security pattern 42 is shown as a single layer, it could have a more complex multilayer form if desired.

Figure 12 illustrates a further possible application of the imager described above. A gemstone 46 will have a well defined external shape and a pattern a flaws

48 within its body. The combination of the shape and number and size of the flaws 48 will uniquely identify a gemstone 46. The imager can scan through the gemstone 46 making a collection of 2-D images that can then be used to form a 3-D model of the gemstone. This 3-D model can be electronically recorded and stored in a database to identify a particular gemstone. The high spatial resolution, rapid operation and ability to image within objects make the imager well suited to this use.

Claims

1. Apparatus for imaging an object comprising a light source to generate an object beam (3') and a reference beam (4') whereby an object (7) can be illuminated with the object beam (3'), a real time interferogram recording medium (8) arranged to be exposed to, and to record an interferogram formed by the coincidence of light reflected from the object (7) and the reference beam (4'), means for reconstructing an image from the interferogram and means for recording the image.

2. Apparatus according to claim 1 wherein the interferogram recording medium (8) is a medium exhibiting a non-linear optical response, such that incident light can interfere and write a refractive index interferogram or an absorption interferogram suitable for holography.

3. Apparatus according to any one of the preceding claims wherein the interferogram recording medium (8) is suitable for real time holography.

4. Apparatus according to any one of the preceding claims wherein the medium is photorefractive medium.

5. Apparatus according to claim 4 wherein the interferogram recording medium

(8) is a photorefractive crystal.

6. Apparatus according to claim 5 wherein the medium (8) is a bulk photorefractive crystal.

7. Apparatus according to claim 6 wherein the photorefractive crystal is Rhodium doped barium titanate (Rh-Ba:TiO₃).

8. Apparatus according to any one of claims 1 to 4 wherein the medium is a semiconductor device.

9. Apparatus according to claim 8 wherein the semiconductor device includes multiple quantum wells.

10. Apparatus according to claim 9 having means to apply an electric field perpendicular to the plane of the multiple quantum well barriers.

1 1. Apparatus according to claim 9 having means to apply an electric field parallel to the plane of the multiple quantum well barriers.

12. Apparatus according to any one of claims 8 to 11 wherein the semiconductor device has been made semi-insulating by the creation of defects to act as traps for charge carriers.

13. Apparatus according to claim 1 wherein the medium is an organic polymer film.

14. Apparatus according to claim 13 wherein the film is poled by applying an electric field normal or parallel to the plane of the film.

15. Apparatus according to claim 1 wherein the medium is a liquid crystal device.

16. Apparatus according to any one of the preceding claims wherein the recording means comprises an electronic camera (9) arranged to view the interferogram recording medium (8) and thus to electronically encode the image for recordal in a memory device.

17. Apparatus according to claim 16 wherein the image is reconstructed and the electronic camera (9) records the image, as it is recorded on the interferogram recording medium (8).

18. Apparatus according to any one of the preceding claims wherein the reference beam (4) impinges on the interferogram plane at a slanting angle so that the range and time of the image viewed correlate with position on the interferogram.

19. Apparatus according to anyone of the preceding claims wherein means is provided to scan the recording medium (8) through the interferogram.

20. Apparatus according to claim 19 wherein said means comprises a carriage (11) whereby the recording medium is moved to scan through the interferogram.

21. Apparatus according to claim 20 wherein the camera (9) is moved synchronously with the recording medium (8) by the carriage (1 1).

22. Apparatus according to claim 19 wherein the camera (9) is held stationary and the field of view of the camera (9) is scanned by an optical assembly to keep the image in view.

23. Apparatus according to any of claims 19 to 22 wherein the reconstruction beam is scanned to follow the recording medium (8).

24. Apparatus according to claim 19 wherein the interferogram is scanned by optical means across the recording medium (8).

25. Apparatus according to any one of the preceding claims wherein the object and reference beams (3, 3'), (4, 4') impinge upon the recording medium (8) and the reference beam (4, 4') passes through an adjustable delay line assembly (14) whereby the length of the path followed by the reference beam (4, 4') can be adjusted.

26. Apparatus according to claim 25 wherein the object and reference beams are close to parallel.

27. Apparatus according to any one of the preceding claims whereby a series of holographic images, each corresponding to a particular time-of-flight, or relative delay between the object and reference beams, can be combined to form a complex image of the object.

28. Apparatus according to any one of the preceding claims wherein a series of holographic images, each corresponding to a different time-of-flight, or relative delay between the object and reference beams, can be combined to form a 3-D image of the object.

29. Apparatus according to any one of the preceding claims having means whereby a series of holographic images, each corresponding to the same time-of-flight, or relative delay between the object and reference beams (3, 3', 4, 4'), can be combined to form a 2-D image of the object.

30. Apparatus according to any one of claims 27 to 29 wherein the series of holographic images are stored and processed electronically.

31. Apparatus according to any one of claims 17 to 30 wherein averaging of multiple holographic images is used to improve the quality of the reconstructed images.

32. Apparatus according to any one of the preceding claims wherein the object is obscured by a diffusing medium.

33. Apparatus according to claim 28 wherein the averaging of multiple holographic images is used to improve the quality of the reconstructed images by reducing the deleterious effects of speckle.

34. Apparatus according to claim 31 wherein the averaging of multiple holographic images is used to improve the quality of the reconstructed images by reducing the deleterious effects of random or quasi-random processes occurring on time scales faster than the time between holographic exposures but slower than the recording time of the holographic exposure.

35. Apparatus according to any one of the preceding claims wherein the object beam is relayed by an imaging system such that the object is imaged into the holographic recording medium.

36. Apparatus according to any one of the preceding claims wherein the holographic recording medium is located in the Fourier plane of the object beam.

37. Apparatus according to any one of the preceding claims wherein the holographic recording medium is located in a intermediate region between an image plane and a Fourier plane of the object beam.

38. Apparatus according to any one of the preceding claims wherein a spatial filter is employed in the holographic imaging system.

39. Apparatus according to any one of the preceding claims wherein a series of holographic images, each corresponding to a particular time-of-flight, or relative delay between the object and reference beams, can be viewed in real-time.

40. Apparatus according to claim 39 whereby a series of holographic images, each corresponding to a particular time-of-flight, or relative delay between the object and reference beams, can be viewed in real-time with the time-of-flight, or relative delay between the object and reference beams, being adjustable.

41. Apparatus according to claim 33 or 40 whereby the series of holographic images viewed in real-time benefit from the average or otherwise processing of multiple holographic exposures.

42. Apparatus according to any one of the preceding claims wherein a holographic exposure or series of holographic exposures, corresponding to a particular time-of- flight, or relative delay between the object and reference beams, is used to discriminate in favour of a particular temporal portion of an optical signal.

43. Apparatus according to claim 42 wherein the optical signal is one which carries information about the image of an object.

44. Apparatus according to claim 43 whereby the optical signal is one which originally carried information about the image of an object but which has propagated in or through an optically diffused medium and consequently some of the signal light has undergone scattering.

45. Apparatus according to claim 44 whereby a holographic exposure or series of holographic exposures, corresponding to a particular time-of-flight, or relative delay between the object and reference beams, is used to discriminate in favour of the temporal portion of the optical signal which carries the most useful image information.

46. Apparatus according to claim 45 whereby a holographic exposure or series of holographic exposures, corresponding to a particular time-of-flight, or relative delay between the object and reference beams, is used to discriminate in favour of the early-arriving temporal portion of the optical signal which corresponds to the unscattered part of the optical signal.

47. Apparatus according to any of claims 41 to 46 whereby the resulting holographic image or images benefit from the averaging or otherwise processing of multiple holographic exposures.

48. Apparatus according to any one of the preceding claims whereby the images are of objects embedded in or located behind optically diffuse media.

49. Apparatus according to claim 48 whereby the optically diffuse media is animal or human body tissue or skin.

50. Apparatus according to claim 49 whereby the objects are tumours or other objects of interest to medical diagnosticians.

51. Apparatus according to claim 48 wherein the optically diffuse media is human breast tissue.

52. Apparatus for generating a 3-D digital model of an object (7) comprising: an apparatus for imaging as claimed in any one of the preceding claims operating to produce a sequence of 2-D images corresponding to differing depths within the object (7); an image data processor (22) responsive to the sequence of 2-D images to form the 3-D digital model of the object.

53. Apparatus as claimed in claim 52 comprising a transmitter for transmitting the 3-D digital model to a remote site.

54. Apparatus as claimed in claim 53 comprising a receiver for receiving the 3-D digital model and a 3-D rendering system responsive to the 3-D digital model for reproducing a 3-D copy of the object.

55. An object bearing an embedded pattern for reading with the apparatus of any one of claims 1 to 50, said object comprising: an outer layer at least partially translucent to light from the light source; and at least one embedded pattern layer formed with regions that reflect light from the light source by different amounts such that an image of the embedded pattern layer may be formed.

56. A method of imaging an object using light with a predetermined time of flight, the method comprising the steps of: generating an object beam (3') and a reference beam (4'); illuminating an object (7) with the object beam (3'); exposing a real time interferogram recording medium (8) to, and recording an interferogram formed by the coincidence of light reflected from the object (7) and the reference beam (4'); reconstructing an image from the interferogram; and recording the image.