WO2012150472A1

WO2012150472A1 - Apparatus for producing a three-dimensional colour image

Info

Publication number: WO2012150472A1
Application number: PCT/HU2012/000031
Authority: WO
Inventors: Szabolcs TŐKÉS; László ORZÓ; Zoltán GÖRÖCS; Márton KISS; István SZATMÁRI
Original assignee: MTA Számítástechnikai és Automatizálási Kutató Intézet
Priority date: 2011-05-03
Filing date: 2012-05-02
Publication date: 2012-11-08
Also published as: HU229591B1; HUP1100228A2

Abstract

The invention is an apparatus for producing a three-dimensional colour image, said apparatus comprising at least two feeding light sources (40a, 40b, 40c) generating coherent light beams of different colours, at least two optical fibres (14a, 14b, 14c) having input end and light emitting ends (16a, 16b, 16c), the input ends of the optical fibres (14a, 14b, 14c) are connected to the feeding light sources (40a, 40b, 40c), respectively, the light emitting ends (16a, 16b, 16c) of the optical fibres (14a, 14b, 14c) are placed closely side by side and constitute an illuminating light source (10), an object space (22) suitable for locating an object (18) to be illuminated by the illuminating light source (10), at least one digital image-sensing device (20) for recording an interference images of reference light beams and object light beams scattered on or reflected by the object (18) as a hologram, and a digital image-processing device (42) for producing the three-dimensional colour image of the object (18) from the hologram recorded by the at least one digital image-sensing device (20) with a correction of distortions resulting from placing side by side the light emitting ends (16a, 16b, 16c) of the optical fibres (14a, 14b, 14c).

Description

APPARATUS FOR PRODUCING A THREE-DIMENSIONAL COLOUR IMAGE TECHNICAL FIELD The invention relates to an apparatus for producing a three-dimensional

(3D) colour image.

BACKGROUND ART Digital holography has gained ground after the development of digital image sensors - i.e. camera sensors organised into semiconductor matrix based for example on CCD, CMOS and other configurations. When making digital holograms, contrary to holograms exposed on conventional photo materials, there is no need to treat the photo material physically or chemically. A further advantage of digital holography as against conventional holography is that the recorded hologram can be stored in digital memory, and that the stored hologram can be improved by appropriate algorithms on the basis of digital methods only. The spatial image can be reconstructed from the hologram numerically, i.e. there is no need for an illuminating light source and other optical means for the reconstruction.

In colour holography, the object is illuminated with more than one colour, i.e. with at least two colours, characteristically with the three basic colours, contrary to the monochromatic illumination of a monochrome hologram. It is an advantage of colour holography that colour information can also be obtained from the object which is recorded in the hologram. From a static, i.e. still object, a colour holographic image can be also obtained by means of monochromatic camera or cameras and several shots with different colour illumination in each shot, followed by the combination of the images. However, by means of high resolution colour image sensors, the holograms of different colour can be recorded simultaneously. The advantage of this is that the object of the shot does not have to be stationary to see it in similar status in the holographic shots of different colour. This advantage can be utilised in the creation of colour holographic videos.

A study (A. Khmaladze, Myung Kim and Chun-Min Lo, Phase imaging of cells by simultaneous dual-wavelength reflection digital holography, Optics Express, Vol. 16, No. 15 (2008), pp. 16900-16911 ) describes a method and an apparatus for producing a two-colour hologram. In this study, the lights of two different colour lasers are combined by various optical elements for illuminating the object as well as recording the hologram. A method and an apparatus suitable also for producing a two-colour hologram are presented in another study (S. Yeom, B. Javidi, P. Ferraro, D. Alfieri, S. DeNicola and A. Finizio, Three- dimensional color object visualization and recognition using multi-wavelength computational holography, Optics Express, Vol. 15, No. 15 (2007), pp. 9394- 9402).

A method and apparatus appropriate for producing a hologram using three colours are described in many studies (K. Choi, H. Kim and B. Lee, Full-color autostereoscopic 3D display system using color-dispersion-compensated synthetic phase holograms, Optics Express, Vol. 12, No. 21 (2004), pp. 5229-5236; L. Chen and D. Zhao, Optical color image encryption by wavelength multiplexing and lensless Fresnel transform holograms, Optics Express, Vol. 14, No. 19 (2006), pp. 8552-8560; J.-M. Desse, P. Picart and P. Tankam, Digital three-color holographic interferometry for flow analysis, Optics Express, Vol. 16, No. 8 (2008), pp. 5471- 5480; M. Makowski, M. Sypek and A. Kolodziejczyk, Colorful reconstructions from a thin multi-plane phase hologram, Optics Express, Vol. 16, No. 15 (2008), pp. 1 1618-1 1623; A. Shiraki, N. Takada, M._ Niwa, Y. Ichihashi, T. Shimobaba, N. Masuda and T. Ito, Optics Express, Vol. 17, No. 18 (2009), pp. 16038-16045; H. Toge, H. Fujiwara and K. Sato, One-shot digital holography for recording color 3-D images, Proc. of SPIE, Vol. 6912 (2009), 69120-U; K. Maejima and K. Sato, Simplified electroholographic color reconstruction system using graphics processing unit and liquid crystal display projector, Proc. of SPIE, Vol. 7233 (2009), 72330-U), as well as in US 2005/004 271 A1 , US 2007/0103757 A1 and US 2009/0147334 A1 .

In a study (T. Ito and K. Okano, Color electroholography by three-colored reference lights simultaneously incident upon one hologram panel, Optics Express, Vol. 12, No. 18 (2004), pp. 4320-4325) such a method and apparatus for producing a three-colour hologram are described, where the illumination comes from three LEDs placed side by side, the coherence characteristics of them are much weaker than those of a laser. In a study (X. Mo, B. Kemper, P. Langehanenberg, A. Vollmer, J. Xie and G. von Bally, Application of color digital holographic microscopy, DGaO Proc. (2009), ISSN: 1614-8436) such a method and apparatus for producing a three- colour hologram is described, where optical fibres are used for leading the light of the light sources to the appropriate points.

Digital holographic microscopes are described in further studies (L. Orzo, Z. Gorocs, I. Szatmari and Sz. Tokes, GPU implementation of volume reconstruction and object detection in digital holographic microscopy, 12th International Workshop on Cellular Nanoscale Networks and Their Applications (CNN A), IEEE, Berkeley, U.S.A., pp. 1-4; D. Parshall and M. K. Kim, Digital holographic microscopy with dual-wavelength phase unwrapping, Applied Optics, Vol. 45, No. 3 (2006), pp. 451 -459; D. Kim, J. W. You and S. Kim, White light on-axis digital holographic microscopy based on spectral phase shifting, Optics Express, Vol. 14, No. 1 (2006), pp. 229-234).

A common disadvantage of the above approaches is that they require an extensive optical apparatus, the use of which is not only costly, but in some cases a substantial light loss and a reduction of efficiency may also be expected.

In further studies (Z. Gorocs, L. Orzo, M. Kiss, V. Toth and Sz. Tokes, Inline color digital holographic microscope for water quality measurements, Proc. of SPIE, Vol. 7376 (2010), 737614; Z. Gorocs, M. Kiss, V. Toth, L. Orzo and Sz. Tokes, Multi-color digital holographic microscope (DHM) for biological purposes, Proc. of SPIE, Vol. 7568 (2010), 75681 P-10) such a method and apparatus for producing a three-colour hologram is presented, where the three different colours are coupled to the same optical fibre by fibre couplers, in order to make sure that there is a dot-like light source at the end of the optical fibre. In the apparatuses described in the studies, digital image sensor device is applied which is provided with Foveon-type or Bayer pattern filter. Furthermore, in the course of the process, numerical methods are used to improve the colour cross-talk of the hologram recorded on a digital image sensing device. However, the disadvantage of the approach described in the studies is that the colours can only be coupled to a single optical fibre with low efficiency. A further disadvantage is that the optical fibre, to which the three different colours are coupled, can only be sized to the wavelength of one colour, even in the best case. DESCRIPTION OF THE INVENTION

The idea behind the invention is the requirement to make available such an apparatus suitable for producing three-dimensional colour images based on holography, which is of high efficiency, can be implemented with a simple optical apparatus, and is exempt from the disadvantages of prior art approaches. The object of the invention is to meet this requirement.

The invention is an apparatus for producing a three-dimensional colour image, said apparatus comprising at least two feeding light sources generating coherent light beams of different colour, at least two optical fibres having input ends and light emitting ends, the input ends of the optical fibres are connected to the feeding light sources, respectively, the light emitting ends of the optical fibres are placed closely side by side and constitute an illuminating light source, an object space suitable for locating an object to be illuminated by the illuminating light source, at least one digital image-sensing device for recording an interference image of reference light beams and object light beams scattered on or reflected by the object as a hologram, and a digital image-processing device for producing the three-dimensional colour image of the object from the hologram recorded by the at least one digital image-sensing device with a correction of distortions resulting from placing side by side the light emitting ends of optical fibres.

By means of the apparatus according to the invention, preferably a three- dimensional holographic microscope can be provided, which produces a digital three-dimensional colour image of an object.

In an embodiment of the apparatus according to the invention, each of the optical fibres is laid out according to the wavelength of the light beam to be guided therein. By this solution, light losses can be minimised, and the light of feeding light sources can be almost ideally transferred to an appropriate point, from which the object is illuminated by the illuminating light source. Therefore, in this embodiment, so-called single-mode optical fibres are preferably used, which - as an optical wave guide - only pass through a single-frequency light beam, and higher order, different frequency and other propagation direction modes are filtered out. In another embodiment of the apparatus according to the invention, the object space and the at least one digital image-sensing device are located in the optical axes of the light beams emitted by the light emitting ends of the optical fibres.

In a further embodiment of the apparatus according to the invention, the apparatus comprises optical elements splitting the light beams emitted by the light emitting ends of the optical fibres into an object light beam and a reference light beam. In this embodiment, preferably optical elements may be applied to make sure that the object light beams and the reference light beams are separated in space.

In an embodiment of the apparatus according to the invention, the feeding light sources are laser light sources or LED light sources emitting light of different wavelength.

In a further embodiment of the apparatus according to the invention, the digital image-processing device comprises a wave-front propagating device producing reconstruction images of different colours by means of digital wave-front propagation from the hologram using reconstructing light beams equivalent to the reference light beams applied for different colour light beams, a correction device performing a numerical correction in the reconstruction images to ensure a correction of distortions resulting from placing the light emitting ends of the optical fibres side by side, and a digital image-matching device producing the colour three-dimensional image from the corrected reconstruction images of different colour.

In an embodiment of the apparatus according to the invention, the reconstructing light beams are plane wave light beams or spherical wave light beams in the wave-front propagating device. By the application of plane wave light beams or spherical wave light beams, a magnification can preferably be achieved.

In an embodiment of the apparatus according to the invention, the correction device corrects the distortions by modifying parameters of the plane wave light beams or the spherical wave light beams.

In another embodiment of the apparatus according to the invention, the correction device subtracts the average amplitude of the hologram from the hologram for the correction of distortions, and adds to the hologram the average amplitude obtained by digital wave-front propagation.

In a further embodiment of the apparatus according to the invention, the digital image-processing device further comprises a digital image recognition/classification device for recognizing and classifying the images produced by the digital image-matching device.

It is a further advantage of the apparatus according to the invention that by means of applying separate optical fibres for each colour, the light loss is low or negligible, the efficiency is good, and the other than dot-like character of the applied illumination does not represent a disadvantage, due to its numerical correction.

By means of the holographic system provided by the apparatus according to the invention, colour image information can be obtained at least by micron resolution from at least one cubic millimetre volume. This resolution is achieved with an illuminating light source according to the invention, applying a good projecting system and a numerical correction according to the invention.

BRIEF DESCRIPTION OF DRAWINGS The invention will be further described on the basis of preferred embodiments by way of example depicted in drawings, where

Fig. 1 is a schematic drawing showing an embodiment of the apparatus according to the invention,

Fig 2 is a schematic drawing which shows the fixing of optical fibres to one another in the apparatus according to the invention,

Fig. 3 is a schematic drawing which shows in one embodiment of the invention the arrangement of the light emitting ends of the optical fibres, with the said ends pressed to each other,

Fig. 4 is a schematic drawing which shows the optical arrangement of one embodiment of the apparatus according to the invention,

Fig. 5 is a schematic drawing which shows the optical arrangement of another embodiment of the apparatus, in which preferably an afocal lens system is applied, Fig. 6 is a schematic drawing which shows the optical arrangement of a further embodiment of the apparatus according to the invention applying an afocal lens system, and

Fig. 7 is a schematic drawing showing an optical arrangement of a yet further embodiment of the apparatus according to the invention.

MODES FOR CARRYING OUT THE INVENTION

Fig. 1 shows an embodiment of the apparatus according to the invention, wherein feeding light sources 40a, 40b, 40c produce coherent light beams of different colour. A light beam can be considered practically coherent, if the path length travelled by this light beam in the apparatus is not longer than the coherence length of the light beam. The feeding light sources 40a, 40b, 40c can be laser or LED light sources emitting different wavelength lights. The coherence characteristics of laser light sources are much more favourable, but LED light sources can also be applied, because their light is made coherent by the optical fibres 14a, 14b, 14c connected to them. The input ends of the three optical fibres 14a, 14b, 14c are connected to the feeding light sources 40a, 40b, 40c, respectively. Light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c are placed closely side by side, and constitute an illuminating light source 10. When it is said that the light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c are placed closely side by side, this means that the ends of the optical fibres 14a, 14b, 14c are pressed side by side in a way that the light emitting ends 16a, 16b, 16c are as close to one another as possible. The light source 10 illuminates an object 18 located in an object space 22. Furthermore, a digital light- sensing device 20 is shown, which is suitable to record an interference image of reference light beams and object light beams scattered on or reflected by the object 18 as a hologram. After the light beams are split into object light beam and reference light beam, when designing the optical arrangement guiding them to the digital image-sensing device 20, it is to be considered that the object light beams and the reference light beams will be coherent with each other and thereby suitable for producing a hologram, if the difference of paths covered by them is not longer than the coherence length of the light beams. From the hologram recorded digitally by the digital image-sensing device 20, a digital image-processing device 42 generates the three-dimensional colour image of the object 18, and at the same time it corrects the distortions resulting from placing the light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c side by side. The distortions stemming from placing the light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c side by side are understood as follows. On the one hand it is to be corrected that as a result of the configuration of the light source 10 described above, three dot-like unaligned light sources are produced. On the other hand, the given angular and position situation of the so produced dot-like light sources may also lead to errors. The digital image- processing device 42 comprises a digital wave-front propagating device 44, which uses digital wave-front propagation algorithm to produce reconstruction images of different colours for different colour light beams from the hologram with reconstructing ligh beams equivalent to the reference light beams applied for producing. The digital image-processing device 42 furthermore comprises a correction device 46, which performs a numerical correction in the reconstruction images. The numerical correction ensures the correction of distortions resulting from placing the light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c side by side. In addition, the digital image-processing device 42 also comprises a digital image-matching device 48, which produces a colour three- dimensional image from the different colour reconstruction images.

The digital image-processing device 42 preferably also comprises a digital image recognition/classification device 50 as observable in Fig. 1 , which - after the comparison of image features with reference image features stored in a database - recognizes and classifies the images produced by the digital image- matching device 48. For object identification in the digital image recognition/classification device 50 a software component is applied. In the learning process of this software component preferably supervised learning is used. During the learning process the software component extracts features from the elements of a learning set, which contains reconstructed images labeled with the appropriate group-names, i.e. the relevant features to be observed are marked in the images of the learning set. Furthermore, the software component of digital image recognition/classification device 50 chooses a subset of these relevant features with strong discriminative ability and feeds the chosen feature-set to a classification method. For an input image these methods give probabilities of the possible classes as the output. Exemplary feature selection methods and classification methods are described in Theodoridis, Pikrakis, Koutroumbas, and Cavouras, Introduction to Pattern Recognition: A Matlab Approach, 1^st Edition, Academic Press, 30 Apr 2010, and in C. M. Bishop, Pattern Recognition and Machine Learning (Information Science and Statistics), Springer, 2006. Preferably, combinations of feature selection methods and classification methods called voters can be used. The final classification result for an image is obtained from summing up all the given voters. This result can be stored and displayed. For example, when the number of different types of microorganisms is searched for in a volume of liquid, the number of them can be evaluated by the digital image/recognition device 50, and it can be determined whether the number of given microorganisms is greater than an alarm level.

In accordance with the invention "colour image" is meant to be an image with more than one colours. Colour information can be obtained from an object in another embodiment of the invention, where the apparatus comprises two feeding light sources generating coherent light beams of different colours and two optical fibres, the input ends of said optical fibres are connected to the feeding light sources, respectively. In this embodiment the colour information is minimal, but when one of the colours is characteristic for the objects to be investigated in a volume, these objects are emphasized in the reconstructed image. E.g. the characteristic colour is green - connected top chlorophyll - when organisms present in water are under investigation. The embodiment which uses only two different colours can be produced by lower costs compared to the other more complicated embodiments. In the above case of two colours such a picture can be obtained from the reconstruction which comprises only a characteristic colour on the top of an ordinary non-colour picture.

In another embodiment of the apparatus according to the invention, it comprises more than three feeding light sources generating coherent light beams of different colours and more than three optical fibres, the input ends of said optical fibres are connected to the feeding light sources, respectively. The number of the feeding light sources is preferably four or five. By the application of three different colours a general colour picture can be obtained by the reconstruction. When one or more other characteristic colours is/are added, specific spectral information can be obtained from an object. Consequently, additional colours which are characteristic to an investigated object can be applied preferably.

In Fig. 2 another embodiment of the invention is depicted wherein the light emitting ends of the optical fibres 14a, 14b, 14c secured by the fixing 12 pressed to each other. The three light emitting ends of the optical fibres 14a, 14b, 14c are pressed to one another and are roughly positioned in one plane to provide illuminating light source 10. The input ends of the optical fibres 14a, 14b, 14c are coupled with the connections 13a, 13b, 13c to feeding light sources not shown in the figure. According to an embodiment of the invention, three different colour lights are guided into the light conducting core of each of the optical fibres 14a, 14b, 14c, and these three different colours are preferably the red, green, blue colour triple.

The light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c are preferably fixed side by side in an arrangement according to Fig. 3. The internal structure of the optical fibres 14a, 14b, 14c is shown in the figure, i.e. shells 15a, 15b and 15c surround light conducting cores, and the ends of these cores representing the light emitting ends 16a, 16b, 16c. Each of the ends 16a, 16b, 16c emits different colour light. In the apparatus according to the invention, the light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c guiding the light of feeding light sources 40a, 40b, 40c thereto are applied as an illuminating light source 10. The light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c are preferably placed side by side so that the optical fibres 14a, 14b, 14c can be surrounded by a periphery as short as possible, i.e. they should represent an order of a symmetrical and regular triangle. The space between the optical fibres 14a, 14b, 14c and the fixing 12 is filled up by a plastic bed 11. Due to the establishment of a standard connection resisting breakage are the very thin, characteristically approx. 125 micrometer outer diameter optical fibres 14a, 14b, 14c embedded into the plastic bed 11 made of a flexible material. The diameter of the light conducting cores of the optical fibres 14a, 14b, 14c is characteristically approx. 2 microns.

In the apparatus according to the invention, the light beams produced by each feeding light source are coherent, but with one another they are mutually incoherent, because the feeding light sources produce different colour, i.e. different wavelength light beams. By the application of feeding light sources 40a, 40b, 40c emitting such a light, the speckle noise, i.e. the grain of the image can be favourably reduced. Furthermore, in this way, each incoherent hologram is superimposed, hence, the holograms obtained by the different colours can simply be separated. When reconstructing the holograms obtained for the different colours, i.e. when producing the three-dimensional colour image, they can be brought into alignment with the application of correction propagation algorithms, i.e. the components resulting from each colour can be matched in a way that the obtained colour image corresponds to the colour distribution of the object serving as a basis of the hologram.

It is an advantage of applying the optical fibres 14a, 14b, 14c in some embodiments of the invention, that by means of these the light can be simply guided to a specified point from the feeding light sources 40a, 40b and 40c. Each optical fibre 4a, 14b, 14c is of single-mode, i.e. it is sized to the wavelength of the light beam guided in the optical fibres 14a, 14b, 14c. In the case of applying single-mode optical fibres 14a, 14b, 14c, the wave-front of the light beam leaving the optical fibre 14a, 14b, 14c can be modelled well by a Gaussian beam having large coherence length.

In the case of an illuminating light source created by prior art optical fibres, fibre optic beam unifiers are used. In this approach, the required various wavelength light beams are coupled to a single large numerical aperture optical fibre, and the end of the core of this optical fibre is the dot-like light source, i.e. the different colour light beams originate from one point. By this solution, illumination from a single direction is ensured. However, a disadvantage of this prior art solution is that in the case of applying beam unifiers, the light loss born is high, and therefore the efficiency of this prior art approach is below 8%. A further disadvantage of this approach is the complexity which stems from the intercoupling of optical fibres. Another disadvantage of this prior art solution is that the single-mode optical fibres applied therein are sized for one wavelength, and therefore an optical fibre in which three light beams are propagating at the same time, is not ideal for the other two different wavelength light beams propagating therein. According to the invention, we do not unify the lights of the feeding light sources 40a, 40b, 40c in a single optical fibre, but the light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c are pressed to each other as closely as possible by means of the fixing 12, thereby providing the illuminating light source 10. Hence, by the approach according to the invention, the light losses are minimised. Each light emitting end 16a, 16b, 16c making up the illuminating light source 10 according to the invention radiates a slightly different reference light beam according to the design, to produce each different colour hologram. For perfect reconstruction of the holograms, i.e. for the generating the three- dimensional image of the object 18, a reconstructing light beam identical with the applied reference light beam or one which can be obtained by appropriate transformation from the reference light beam have to be used. This applies to the direction, colour and shape of the numerically simulated reconstructing light beam used in the reconstruction of the hologram. If the spherical wave light beam applied as a reference light beam by the recording of the hologram is replaced by a plane wave reconstructing light beam, i.e. the reconstructing light beams of the wave-front propagating device 44 are plane wave light beams, this leads to magnification, i.e. the magnified image of the object 18 is obtained by using the apparatus according to the invention. When the hence eventually arising distortions are measured, they can be numerically compensated.

In the apparatus according to the invention, a digital image-sensing device 20 is used for recording the hologram. The prior art colour digital image sensors are produced by way of example in a way that the black and white sensors are made selectively colour sensitive by means of light filters. In this case the colour filters are located in front of each pixel in accordance with a kind of pattern - like for example the so-called Bayer pattern. Another method of implementation of prior art colour digital image sensors by way of example is that the pixels are sensitive to various colours in various depths, for example such units are the Foveon-type sensor cameras. However, the applied colour filters and colour sensitive layers are not perfectly colour selective. As a result, the neighbouring pixels sensitive theoretically to the various colours or the layers one below the other display a colour cross-talk. Colour cross-talk means that e.g. the blue- sensitive sensors, pixels illuminated e.g. by blue coherent light also sense the red and green lights arriving together with the blue light. While the hologram is actually an interferogram, it is extremely sensitive to the wavelength. In the course of numerical reconstruction, for the reconstruction of each different colour hologram, a reconstructing light beam of a wavelength identical with that of the reference light beam used in the recording is to be applied. When a colour cross-talk is present, only noisy image can be reconstructed. The digital image-sensing device can also be created by using dichroic mirrors and/or filters, by which the object light beams and the reference light beams are guided to different cameras tuned to the different colours of the feeding light sources, and the hologram is hence recorded by the different cameras after splitting to colours.

In the apparatus according to the invention, the colour cross-talks generated by the application of an appropriate type of digital image-sensing devices are corrected by a prior art method described in two of our studies (Z. Gorocs, L. Orzo, M. Kiss, V. Toth and Sz. Tokes, In-line color digital holographic microscope for water quality measurements, Proc. of SPIE, Vol. 7376 (2010), 737614; Z. Gorocs, M. Kiss, V. Toth, L. Orzo and Sz. Tokes, Multi-color digital holographic microscope (DHM) for biological purposes, Proc. of SPIE, Vol. 7568 (2010), 75681 P-10).

In the apparatus according to present invention, the positional and eventual directional discrepancies of non-aligned light emitting ends 16a, 16b, 16c radiating the light beams are numerically corrected. Regarding the direction and position of the so-called central rays of the light beams as well as the shape of the wave- front, the critical place is the plane of the digital image-sensing device 20 which records the digital hologram.

The arrangement may be of the so-called in-line or off-axis type. In-line embodiments are shown in Figs. 1 , 4, 5, 6 and 7. In the in-line arrangement depicted by Figs. 1 , 4, 5 and 6, the hologram is recorded in a way that the object 18 is transilluminated, and the object light beams scattered by the said object and the original light beams mostly passing through unobstructed as reference light beams interfere on the surface of the digital image-sensing device 20. The interference image produced on the surface of the digital image-sensing device 20 is the hologram itself. It follows from the method of recording that in the case of such an in-line arrangement, only an overall recording can be made. Fig. 7 shows such an in-line arrangement, where the hologram is created by an object light beam reflected from the object 18. This arrangement can easily be tuned similarly to an off-axis one.

Further on, various embodiments of the apparatus according to the invention are shown in Figs. 4, 5, 6 and 7. In the embodiments depicted, the arrangements of the components of the apparatus are different. In each embodiment, even in the case of the same arrangement, various correction parameters may be necessary for reconstructing the hologram, for example due to the difference of distances and inclinations among the ends 16a, 16b, 16c of the cores of the optical fibres 14a, 14b, 14c and that of the distance between the illuminating light source 10 and the object 18. The necessary correction parameters can be generated by calculation or measurements. In the apparatus according to an embodiment of the invention, resulting from the various positions of the ends 16a, 16b, 16c of the cores of the optical fibres 14a, 14b, 14c and from its distance from the other elements - for example, from the object 18 or the digital image-sensing device 20 - the hologram must be reconstructed with light beams of various inclinations by means of numerical wave propagation.

Various embodiments of the projecting optical system of the apparatus according to the invention are described in details below. In the embodiments, in the case of the light beams coming from the illuminating light source 10 generating the holograms, the optical paths of reference light beams and object light beams are different from one another.

The illuminating light source 10 creates more than one, preferably three, different colour holograms about the same object simultaneously. These holograms are magnified by the optical system of the apparatus according to the invention and they are projected on the digital image-sensing device 20. By means of a computer program, the digitally recorded three different colour holograms are numerically reconstructed separately, i.e. a three-dimensional image is compiled through the use of wave-front propagation, and then the obtained different colour monochrome images are assembled into a colour image. In an ideal case, the images of the more than one holograms projected to an identical distance by means of digital wave-front propagation all show the same object plane sharply and they are aligned also. However, in reality this is not achieved in most cases, but the digital image-processing device 42 is able to compensate the imaging errors and the colour aberrations by an appropriate computer program.

In an embodiment of the apparatus according to the invention shown in Fig. 4, the so-called lens-free in-line arrangement is applied. In the figure, the characteristically some micron diameter ends 16a, 16b, 16c of the cores of the optical fibres 14a, 14b, 14c are not shown to scale, and the quasi-spherical wave light beams emerging from the ends 16a, 16b, 16c are also depicted. The figure shows that the object 18 is located in the object space 22, and the spherical wave light beams originating from and deflected by the object 18 are also depicted. In the digital image-sensing device 20, the reference light beams originating and propagating unobstructed from the ends 16a, 16b, 16c and the object light beams originating from the object 18 interfere.

Fig. 5 shows another embodiment of the apparatus according to the invention, where a single-lens in-line arrangement is applied, and where the imaging paths are also depicted, and the reconstruction process can result in an afocal or telecentric imaging.

The advantage of lens arrangements against a no-lens arrangements is that by the application of lenses, larger magnification and a larger effective numerical aperture can be achieved. Therefore, in the lens arrangement, compared to the no-lens arrangement, the resolution of the holographic microscope can be enhanced, i.e. the resolution of the colour three-dimensional image of the object obtained by reconstruction will be higher.

In the embodiment shown in Fig. 5, the object space is transilluminated by a quasi plane wave light beam shown with horizontal dotted lines in the figure, i.e. by a Gaussian beam coming from a distance or by a light beam paralleled with collimator. In the case of a sparse object 18, most of the light beam proceeds through the object space unobstructed, and a lens 24 generates a small dot from the light beams passing through unobstructed in the vicinity of its rear focal plane 26, therefore, a quasi spherical wave light beam illuminates the digital image- sensing device 20. The so generated light beam plays the role of the reference light beam. The light beams scattered on the object 18, i.e. the object light beams are also projected by the lens 24. Therefore, a magnified but distorted three- dimensional image 28 of the three-dimensional object 18 is produced, and the light beams proceeding further are also incident on the digital image-sensing device 20. The reference light beam and the light beams of the image 28 generated about the object 18 by the scattered and magnified light beams are in interference as object light beams in a holographic sense with the reference light beam, and they produce a hologram on the surface of the digital image-sensing device 20. Digital reconstruction with a plain wave results in an afocal type imaging.

Fig. 6 shows a two-lens in-line embodiment of the apparatus according to the invention. The two-lens in-line arrangement can be afocal, i.e. with an infinitely large focal distance, as well as non-afocal.

The advantage of an afocal system as against a non-afocal system is that it only distorts in a longitudinal but not in a transversal direction. Hence, for example, a cube-shaped object is distorted in an afocal system into a square based prism, however, such a type of distortion can be easily corrected numerically.

In the embodiment shown in Fig. 5, the light beams falls on the object 18 and the lens 24 in parallel, and as a result, the digital image-sensing device 20 records an image distorted in the longitudinal direction according to the afocal arrangements, i.e. in the digital image-processing device 42, a longitudinal correction similar to the afocal systems must be carried out similarly to the embodiment presented in Fig. 6.

A non-afocal system can be considered just like a single-lens embodiment, and only the equivalent, finite focal distance lens of the two lenses and its main planes and focal planes must be calculated and taken into consideration.

Furthermore, Fig. 6 shows the main beam paths and the image 16' of the end 16b representing part of the illuminating light source 10 and the image 22' of the object space 22, so that the distortion of the arrangement is demonstrated. Two lenses 30 and 32 making up the projecting optical system are also shown.

Fig. 7 depicts a further embodiment of the invention. The figure shows that the light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c are not aligned with the object space 22 of the object 18 and the digital image-sensing device 20. The light beams coming from the optical fibres 14a, 14b, 14c fall on the digital image-sensing device 20 directly as reference light beams by means of a semipermeable mirror 36 and on a mirror 34 as object light beams. In this arrangement, the reference light beam does not proceed through the object space 22, and only the object light beam reflected by the mirror 34 falls on the object 18. The light reflected from the object 18 falls on the digital image-sensing device 20 through the semipermeable mirror 36, and by interfering with the reference light beam, it produces an interference image on the digital image-sensing device 20. In Fig. 7 the main beam paths are depicted, and in this way a light beam coming from the end 6b of the optical fibre 14b is also depicted. The end 16b of the optical fibre 14b has a virtual image 38 from which the light beam indicated with a dotted line in Fig. 7 originates. The virtual image 38 is a mirror image of the end 16b of the optical fibre 14b according to the semipermeable mirror 36. Fig. 7 also shows that as a result of illuminating the object with a light beam, a light beam originates from the object 18, and when this interferes with the reference light beam, a hologram is produced on the digital image-sensing device 20.

The advantage of this embodiment compared to the above described embodiments is that the reference light beam does not go through the object space 22, and therefore the modifying effect of the object space 22 on the wave- front of the reference light beam does not deteriorate the quality of the hologram. Therefore, not only sparse samples - for example micro-organisms and cells floating in water or blood serum - can be examined, but also the contents of dense fluids or any reflecting type objects. In addition, the apparatus according to this embodiment is also suitable for examining solid surfaces. Furthermore, a lens version of this embodiment can be implemented, where generation of object light beams and reference light beams is carried out in the same way as in this embodiment.

In the embodiment shown in Fig. 7, the object light beam and the reference light beam are coaxially directed to the digital image-sensing device 20, by means of the semipermeable mirror 36, consequently this is actually an in-line arrangement. The advantage of in-line arrangement is that a hologram without any loss of information can be obtained. If the semipermeable mirror 36 and the illuminating ends 16a, 16b, 16c are adjusted obliquely, i.e. the optical axis of the light coming from the ends 16a, 16b, 16c is at an angle other than 45° when it falls on the semipermeable mirror 36, the arrangement is of off-axis type. The light beams of the zero order and twin image produced as a result of applying the semipermeable mirror 36 are also in alignment with the object light beams in the in-line arrangement and, therefore, in the case of holograms obtained by an in-line arrangement, the twin image is to be removed from the hologram by a separate algorithm, if a low noise reconstruction is required. However, in the off-axis arrangement, the light beams of the twin image are not aligned with the object light beams and, therefore, in an off-axis arrangement it is not necessary to apply a separate algorithm aimed at removing the twin image. In addition, the off-axis arrangement entails a loss of information in comparison with the in-line arrangement.

In the off-axis arrangement, recording of the hologram is performed similarly as above. The light beam exiting from the coherent light source is split into two parts, i.e. two separate light beams by means of a suitable beam splitter, for example by using a semipermeable mirror or a fibre optic beam splitter. One light beam is used for illuminating the object as an object light beam, either in an overall or in perspective, i.e. reflection mode, that is a light or dark field illumination can be created. And, when the other light beam is converted into a plane wave light beam or a spherical wave light beam, it is projected unobstructed, i.e. without scattering to the digital image-sensing device, and this light beam is the actual reference light beam.

In embodiments of the invention where the object light beam scattered by the object is also projected to the digital image-sensing device, the interference image generated between the reference beam and the object light beam is recorded as a hologram. In the case of a photoelectric type digital image-sensing device, this image is fed into the memory of the digital processor.

The apparatus according to the invention may have further preferred embodiments. As an example, an off-axis arrangement can be implemented in a way (not shown) that the light of the feeding light source coupled to each optical fibre is guided to two separate optical fibres each, by fibre optic beam splitters, respectively. One optical fibre of the so created optical fibre pairs of identical colour and providing a coherent light beam with each other is used for illuminating the object, and the other optical fibre illuminates the digital image-sensing device, which records a hologram by the interference of the object light beams reflected by the object and the reference light beams. To produce a hologram, the length of optical fibres have to be determined in a way that the optical path length calculated from the feeding light source ends of the optical fibre to the digital image-sensing device is within the coherence length in the case of both the reference light beam and the object light beam. The reference optical fibre ends are fixed closely side by side similarly to the in-line case, and the hologram reconstruction shifts resulting from the transversal positional difference of the so obtained unaligned quasi-dot sources are numerically corrected.

Consequently, the apparatus according to the invention may have such embodiments where the object space 22 and the digital image-sensing device 20 are located in the optical axes of the light beams emitted by the light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c. The optical axes of the light beams emitted by the light emitting ends of the optical fibres 14a, 14b, 14c are understood to be the three straight lines originating from the centre of each of the light emitting ends 16a, 16b, 16c, perpendicular to the surface of the light emitting ends 16a, 16b, 16c.

In addition, various embodiments can be distinguished according to whether the apparatus according to the invention comprises such optical elements which split the light beams emitted by the light emitting ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c into an object light beam and a reference light beam.

The digital wave-front propagating device 44 in the apparatus according to the invention performs the reconstruction of the hologram, i.e. the creation of a three-dimensional colour image of the object 18 by a digital wave-front propagating algorithm with the numerical correction of distortions emerging during the recording of the hologram. If the hologram had been created by an oblique plane wave light beam, an appropriately parameterised plane wave light beam is to be applied for a positionally correct digital reconstruction. On the hologram a phase shifting corresponding to the oblique plane wave light beam used in producing the hologram has to be applied by the digital image-processing device 42.

Digital wave-front propagation means that for the description and propagation of light beams, some kind of known physical-mathematical model is applied and numerically implemented. By way of example, this can be the so- called Fresnel diffraction and propagation model or the plane wave propagation model applying Fourier transformation. In the latter case the situation is that each Fourier component of the three-dimensional object 18 can be considered to be a sinusoidal grid, on which the simulated incidental plane wave light beam is subjected to a light diffraction, therefore, plane wave light beams subjected to diffraction according to the spatial frequency of the component emerge from the object 18. If these plane wave light beam components are propagated to a desired propagation distance z,, then only the phase changes, i.e. a phase member depending on the distance z, is added to the complex Fourier transformation. Then, by an inverse Fourier transformation, the complex amplitude of the distance Zi wave-front is obtained, which comprises point by point the amplitude as well as the phase of the wave-front. If the square of the absolute value of complex amplitude is taken, the intensity distribution for distance z, is obtained. Digital wave-front propagation performed by the digital wave-front propagating device 44 and its numerical implementation are per se known from the literature.

For a reconstruction from digital holograms, the digital wave-front propagation is applied in such a way that the matrix representing the recorded hologram, i.e. the intensity of the interference fringe arrangement and comprising real values are multiplied point by point with the matrix comprising the complex values to simulate the reconstructing light beam. Then the complex Fourier transformation of this result matrix is taken, which is multiplied by point multiplication with the natural exponential function comprising the direction dependent k_z(x,y) wavenumber vector matrix representing the diffracted plane wave light beams and the propagation distance. The inverse Fourier transformation of this matrix yields the matrix which represents the distance z, complex wave-front.

A concise mathematical description of the digital wave-front propagation is the following:

Wave(x, y,

{F [ Holo(x, y, z = 0)-Wave(x, y, z = 0)] -exp{ik_z-Zj} } ;

DiffractionIntensityImage(x, y, z = Zj) = \ Wave(x, y, z = ζ,)\ ; where

• the hologram: Holo(x,y, z = 0) ;

· the reconstructing light beam illuminating the hologram: Wave(x, y, z = 0);

2π

• the absolute value of the wavenumber vector: \k\ =— , where λ is the wavelength in the actual propagation medium; 2]

• the transfer function carrying over the space frequencies: ^z ' _;

• z-component of the wave number vector: ^k _z =

;

• the i^th propagation distance: z,;

• the matrix operator of the Fourier transformation: F ;

· the matrix operator of the inverse Fourier transformation: ^"1 ;

• the complex wave-front propagated over the distance z,: Wave(x, y, z = z_t).

In the course of the image processing performed by the digital image- processing device 42, however, due to a phase jump appearing at the edges of the image, diffractions may emerge which may substantially deteriorate the final quality.

The error resulting from the phase jump can be removed by many methods according to the invention, using the correction device 46. On the one hand, by changing the parameters of the reconstructing plane wave until the phase jump disappears at the edges of the image, i.e. the correction device 46 corrects the distortions by modifying the parameters of plane wave light beams. This can be done generally, and the error caused by the applied approximations is negligible in case the illumination is done by optical fibres.

On the other hand, the error above can be eliminated by subtracting the average amplitude of the hologram from the hologram, because in this case the phase jumps detectable at the edges of the image also disappear. However, the reconstruction is not accurate without the average of the hologram, i.e. its DC component. But, if a theoretical value of the DC component propagated by the oblique plane wave light beam is used and this is added as an average value to the propagated average value compensated member, then the reconstruction will be correct and the diffraction caused by the phase jump of the edges also disappears. Now then, the correction device 46 subtracts the average amplitude of the hologram from the hologram for the correction of distortions, and adds the average amplitude obtained by digital wave-front propagation to the hologram. The latter method has a slightly larger operational requirement than the former, but it is more accurate and there is no approximation error which would distort the result.

The purpose of the invention is to create a three-dimensional image of the object 18 placed into the object space 22. The apparatus according to the invention can be a three-dimensional microscope which is able with a single exposure to record colour information about the whole - characteristically about 1 mm³ - volume, and to produce the image of the objects in the volume by means of the wave-front propagating device 44, proceeding layer by layer preferably with a resolution of approx. 1 micrometer. The layers can be blended into three- dimensional, colour and even numerically rotatable images by means of the digital image-matching device 48. The digital image-matching device 48 can be implemented by means of prior art algorithms, and ready to use programme packages are also available for this purpose.

In an embodiment of the apparatus according to the invention, three different colour holograms are recorded with the different colour sensitivity pixels of the digital image-sensing device 20. Because the three different colour coherently illuminating ends 16a, 16b, 16c of the optical fibres 14a, 14b, 14c are not aligned if from all the three holograms with a simulated plane wave light beam (or even with a spherical wave light beam) propagating in the same direction are used for producing the three-dimensional image, it is true for any object point that its reconstructed points are not aligned in the three-dimensional image. Therefore, for an aligning reconstruction, the real light beams propagating in various directions and actually illuminating the digital image-sensing device 20 applied for recording the hologram must be numerically simulated. The so obtained colour images can then be added point by point and displayed.

Of course, the invention is not limited to the presented preferred embodiments, but further versions, modifications and developments are possible within the scope of protection defined by the claims.

Claims

1. An apparatus for producing a three-dimensional colour image, said apparatus comprising

- at least two feeding light sources (40a, 40b, 40c) generating coherent light beams of different colours,

- at least two optical fibres (14a, 14b, 14c) having input ends and light emitting ends (16a, 16b, 16c), the input ends of the optical fibres (14a, 14b, 14c) are connected to the feeding light sources (40a, 40b, 40c), respectively, the light emitting ends (16a, 16b, 16c) of the optical fibres (14a, 14b, 14c) are placed closely side by side and constitute an illuminating light source (10),

- an object space (22) suitable for locating an object (18) to be illuminated by the illuminating light source (10),

- at least one digital image-sensing device (20) for recording an interference image of reference light beams and object light beams scattered on or reflected by the object (18) as a hologram, and

- a digital image-processing device (42) for producing the three- dimensional colour image of the object (18) from the hologram recorded by the at least one digital image-sensing device (20) with a correction of distortions resulting from placing side by side the light emitting ends (16a, 16b, 16c) of the optical fibres (14a, 14b, 14c).

2. The apparatus according to claim 1 , characterised in that each of the optical fibres (14a, 14b, 14c) is laid out according to the wavelength of the light beam to be guided therein.

3. The apparatus according to claim 1 or claim 2, characterised in that the object space (22) and the at least one digital image-sensing device (20) are located in the optical axes of the light beams emitted by the light emitting ends (16a, 16b, 16c) of the optical fibres (14a, 14b, 14c).

4. The apparatus according to any of claims 1 to 3, characterised in that the apparatus comprises optical elements splitting the light beams emitted by the light emitting ends (16a, 16b, 16c) of the optical fibres (14a, 14b, 14c) into an object light beam and a reference light beam.

5. The apparatus according to any of claims 1 to 4, characterised in that the feeding light sources (40a, 40b, 40c) are laser light sources emitting light of different wavelength.

6. The apparatus according to any of claims 1 to 4, characterised in that the feeding light sources (40a, 40b, 40c) are LED light sources emitting light of different wavelength.

7. The apparatus according to any of claims 1 to 6, characterised in that the digital image-processing device (42) comprises a wave-front propagating device (44) producing reconstruction images of different colours by means of digital wave-front propagation from the hologram using reconstructing light beams equivalent to the reference light beams applied for different colour light beams, a correction device (46) performing a numerical correction in the reconstruction images to ensure a correction of distortions resulting from placing the light emitting ends (16a, 16b, 16c) of the optical fibres (14a, 14b, 14c) side by side, and a digital image-matching device (48) producing the colour three-dimensional image from the corrected reconstruction images of different colour.

8. The apparatus according to claim 7, characterised in that the reconstructing light beams are plane wave light beams or spherical wave light beams in the wave-front propagating device (44).

9. The apparatus according to claim 8, characterised in that the correction device (46) corrects the distortions by modifying parameters of the plane wave light beams or the spherical wave light beams.

10. The apparatus according to claim 7 or claim 8, characterised in that the correction device (46) subtracts the average amplitude of the hologram from the hologram for the correction of distortions, and adds to the hologram the average amplitude obtained by digital wave-front propagation.

11. The apparatus according to any of claims 7 to 10, characterised in that the digital image-processing device (42) further comprises a digital image recognition/classification device (50) for recognizing and classifying the images produced by the digital image-matching device (48).