Optics: measuring and testing – By light interference
Reexamination Certificate
1999-08-16
2001-06-19
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
C359S370000
Reexamination Certificate
active
06249349
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
PCT/FR97/01695
26th of september, 1997
FR 96/11773
27th of september, 1996
FR 96/15255
12th of december, 1996
FR 97/07469
17th of june, 1997
BACKGROUND-FIELD OF THE INVENTION
The present invention relates to an optical microscope, which generates a three-dimensional representation of the observed object, using a method derived from digital holography.
BACKGROUND-DESCRIPTION OF PRIOR ART
One method for fast recording of a 3D image of an object is holographic microscopy. This method is described, for example, in the article “Holographic Microscopy”, by M.Pluta, published in 1987 in volume 10 of “Advances in Optical and Electron Microscopy”, Academic Press, London.
Microscopes have been constructed that use CCD photodetectors instead of a special holographic paper to detect the interference pattern, and render possible the digital reconstruction of an image by simulating the analog process of holographic microscopy. Such microscopes are described in: “Three dimensional imaging of cells through digital holographic microscopy”, Beltrame F. & al., Proceedings of the ISMII '84: IEEE computer society international symposium on medical images and icons, 24-27 juillet 84, Arlington, U.S., pages 232-235, XP002056887
. “Fourier
-
transform holographic microscope
”, Waleed S. Haddad & al., applied optics vol. 31 no 24 p.4973, 20/8/92
“Study of Biological Samples with a Laser Fourier Holographic Microscope
”, Karpov V. B., Laser Physics vol.4 no 3 juin 1994.
These microscopes do not use a microscope objective to generate the interference pattern. The wave scattered by the object travels through empty space to a receiving surface where it interferes with the reference wave. The interference pattern produced that way can eventually be enlargened by a microscope objective like in Beltrame's article. Because the light scattered by the object does not pass through a microscope objective before it interferes with the reference wave, the digital holographic reconstruction of the object can be based on reverse light propagation in free space.
Microscopes in which ones the wave scattered by the object passes through a lens system element before it interferes with the reference wave have also been designed. Examples of such microscopes are: “
Interferometric computed
-
microtomography of
3
D phase objects
”, Gennady N. Vishnyakov & Gennady G. Levin, SPIE proceedings vol.2984 p.64, mars 95
“The Synthetic Aperture Microscope, experimental results
”, Terry Turpin, Paul Woodford & al., SPIE proceedings vol.2751 p.230, juin 96.
In these two documents, there is an intermediate step of the image acquisition which consists in measuring the wave scattered by the observed sample and reaching the receiving surface, for a given light beam illuminating the object. This yields a bidimensional image. However, in these two documents, no three-dimensional representation is directly obtained from the value of the wave on the receiving surface. Instead, a three-dimensional representation can only be obtained from a series of bidimensional images, each of these being obtained for a different direction of the illuminating beam.
The microscopes described by Vishnyakov and by Turpin differ from the microscopes described by Karpov, by Beltrame and by Haddad, by the fact that they use a microscope objective or an equivalent device, which modifies the wave originating from the object before it interferes with the reference wave. Such a configuration makes the digital reconstruction difficult, because the propagation of the optical wave is no more described by the simple law of propagation in empty space. The specific properties of the microscope objective must be taken into account to generate the representation of the light wave present in the object. This implies that a precise knowledge must exist of the correspondance between the wave as it is recorded on the receiving surface, and the wave as it is when it leaves the observed sample, before entering the microscope objective. This presents some notable difficulties. For example Turpin writes that the samples are equally spaced angularly on the surface, which is not true.
This is why the microscopes described by Vishnyakov and by Turpin only generate bidimensional representations from each wave measured on the receiving surface. In the case of Turpin's microscope, this is equivalent to assimilating a portion of a sphere to a portion of a plane. These microscopes can compute three-dimensional images only by recombining several of these bidimensional representations.
This is also why Karpov, Beltrame, and Haddad, who achieved numerical three-dimensional reconstructions of the wave present in the object for a given illumination beam, always avoided the use of a microscope objective to collect the scattered wave before it interferes with the reference wave.
A further insight into the difficulties of three-dimensional reconstruction of the wave present in the object for a given illumination beam, when a microscope objective is used to collect the scattered wave before it interferes with the reference wave, is given by: “
Three
-
dimensional microscopy with phase-shifting digital holography
”, Tong Zhang & Ichirou Yamaguchi, optics letters vol.23 no 15 p.1221, Aug. 1, 1998.
In this document, the authors generate a three-dimensional representation, and uses a microscope objective to collect the scattered wave before it interferes with the reference wave. The three-dimensional representation generated is a set of two-dimensional images that are optically formed in the image space of the microscope objective, in a series of planes including the receiving surface. Between the receiving surface and any of these planes, the light travels through free space. Reconstruction can therefore be made according to the laws of propagation in free space. However, the three-dimensional reconstruction obtained is not a three-dimensional reconstruction of the optical wave in the object, but a three-dimensional reconstruction of the optical wave in the image space of the microscope objective. It is known, from the theory of light propagation (incompatibility of Abbe and Herschel's conditions), that only one plane (the image plane for which the objective is designed, usually 160 mm from the objective shoulder) in the image space of the microscope objective is the exact enlarged image of a plane of the observed object. The other planes in the image space of the microscope objective are not exact images of object space planes, and therefore there is no simple relation between the three-dimensional image reconstructed by this microscope and a three-dimensional representation of the light wave in the object. This author, who clearly attempted to reconstruct the object from a single record of the optical wave scattered by it, and who used a microscope objective to collect this wave before letting it it interfere with a reference wave, failed in generating a valid three-dimensional representation of the object, because he did not consider the possibility to reconstruct the image taking into account the properties of the objective rather than using propagation in free space.
Other interferometric microscopes exist, that use a microscope objective to collect a wave scattered by the object, and let it later interfere with a reference wave. With these devices, the image is observed directly in a plane, which is the geometric image of the object, with at least one of the beams passing through the object. These devices produce two-dimensional images similar to those produced with standard microscopes, but the variations of light intensity on the image do not represent the sample's absorptivity, but the interference between the two beams in a plane which is an image of the object. These devices do not allow the generation of a three-dimensional image. They are described, for example, in chapter III of the book “
Progress in Microscopy
”, by M. Francon, published in 1961 by Pergamon Press, Oxford, Great Britain.
A specific application of interfe
Font Frank G.
Natividad Phil
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