APPARATUS AND METHOD FOR RECORDING FRESNEL HOLOGRAMS

Abstract

An apparatus for producing a hologram of an object includes a light source that emits an incoherent electromagnetic wave toward the object, and a masking device configured to display a mask, receive the incoherent electromagnetic wave emitted toward the object, mask the received incoherent electromagnetic wave according to the displayed mask, and produce a masked electromagnetic wave. The apparatus also includes an image recording device configured to capture an image of the masked electromagnetic wave, and a processing device configured to convert the image of the masked electromagnetic wave into the hologram of the object. A method for producing a hologram of an object is also described.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus and method for recording information, such as Fresnel holograms, and in particular, to recording lensless digital incoherent Fresnel holograms using a mask, such as an absorption-only mask.

2. Discussion of the Background

Holograms recorded by incoherent light open many new applications like outdoor and astronomical holography (J. B. Breckinridge, “Two-Dimensional White Light Coherence Interferometer,” Appl. Opt. 13, 2760 (1974)) and fluorescence holographic microscopy (G. Indebetouw, A. El Maghnouji, R. Foster, “Scanning holographic microscopy with transverse resolution exceeding the Rayleigh limit and extended depth of focus,” J. Opt. Soc. Am. A 22, 892-898 (2005)). The oldest methods of recording incoherent holograms have made use of the property that every incoherent object is composed of many source points each of which is self spatial coherent and therefore can create an interference pattern with light coming from the point's mirrored image. Under this general principle there are various types of holograms (J. B. Breckinridge, “Two-Dimensional White Light Coherence Interferometer,” Appl. Opt. 13, 2760 (1974)) (A. W. Lohmann, “Wavefront Reconstruction for Incoherent Objects,” J. Opt. Soc. Am. 55, 1555-1556 (1965)) (G. Sirat, D. Psaltis, “Conoscopic holography,” Optics Letters, 10, 4-6 (1985)) including Fourier (J. B. Breckinridge, “Two-Dimensional White Light Coherence Interferometer,” Appl. Opt. 13, 2760 (1974)) (G. W. Stroke and R. C. Restrick, “Holography with Spatially Incoherent Light,” Appl. Phys. Lett. 7, 229 (1965)) and Fresnel holograms (G. Cochran, “New method of making Fresnel transforms,” J. Opt. Soc. Am. 56, 1513-1517 (1966)) (P. J. Peters, “Incoherent holography with mercury light source,” Appl. Phys. Lett. 8, 209-210 (1966)). The process of beam interfering demands high levels of light intensity, extreme stability of the optical setup and a relatively narrow bandwidth light source. These limitations have prevented holograms from becoming widely used for many practical applications.

More recently two groups of researchers have proposed to compute holograms of 3-D incoherently illuminated objects from a set of images taken from different points of view. (Y. Li, D. Abookasis and J. Rosen, “Computer-generated holograms of three-dimensional realistic objects recorded without wave interference,” Appl. Opt. 40, 2864-2870 (2001)) (Y. Sando, M. Itoh, and T. Yatagai, “Holographic three-dimensional display synthesized from three-dimensional Fourier spectra of real existing objects,” Opt. Lett 28, 2518-2520 (2003)) This method, although it shows promising prospects, is relatively slow since it is based on capturing tens of images of the subject scene from different view angles.

Another method is called scanning holography (G. Indebetouw, A. El Maghnouji, R. Foster, “Scanning holographic microscopy with transverse resolution exceeding the Rayleigh limit and extended depth of focus,” J. Opt. Soc. Am. A 22, 892-898 (2005)) (Poon T.-C., “Three-dimensional image processing and optical scanning holography,” Adv. in Imag. & Elec. Phys. 126, 329-350 (2003)) in which a Fresnel Zone Plate (FZP) pattern is scanned across the object such that at each and every scanning position the light intensity is integrated by a point detector. The overall process yields a Fresnel hologram obtained as a correlation between the object and FZP patterns. However the scanning process is a relatively slow and is done by mechanical movements. A similar correlation is actually done also in the present work; however, unlike the case of scanning holography, we propose here a correlation without movement.

Mertz and Young (L. Mertz and N. O. Young, “Fresnel transformations of images,” in Proceedings of Conference on Optical Instruments and Techniques, K. J. Habell, ed. (Chapman and Hall, London 1961) p. 305) already proposed holographic photography based on correlation without movement between object and FZPs. However, their process relies on geometrical optics, which cannot yield good imaging results in the optical regime. On the contrary, our suggested correlator for implementing the holographic recording is valid in the optical regime, since its operation principle is based on the diffraction theory (J. Goodman, Introduction to Fourier Optics, 2^nd ed., McGraw-Hill, New York, 1996, pp. 63-95 (Chapter 4)).

SUMMARY OF THE INVENTION

Accordingly, one object of this invention is to provide a novel apparatus for producing a hologram of an object, the apparatus comprising: a light source that emits an incoherent electromagnetic wave toward the object; a masking device configured to display a mask, receive the incoherent electromagnetic wave emitted toward the object, mask the received incoherent electromagnetic wave according to the displayed mask, and produce a masked electromagnetic wave; an image recording device configured to capture an image of the masked electromagnetic wave; and a processing device configured to convert the image of the masked electromagnetic wave into the hologram of the object.

Another object of this invention is to provide a novel method for producing a hologram of an object, the method comprising: emitting an incoherent electromagnetic wave toward the object; displaying a displayed mask; receiving the incoherent electromagnetic wave emitted toward the object; masking the received incoherent electromagnetic wave according to the displayed mask to produce a masked electromagnetic wave; capturing an image of the masked electromagnetic wave; converting the image of the masked electromagnetic wave into the hologram of the object.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic representation of an embodiment of the present invention; and

FIG. 2 is a schematic representation of another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention includes a new method of recording digital holograms under incoherent illumination, and corresponding apparatuses that may be configured to record digital holograms under incoherent illumination. According to an embodiment of the invention, light is transmitted through, or reflected from a three-dimensional (3-D) object, propagated through an absorption Fresnel zone plate (FZP), and recorded by a digital camera. In one embodiment, five holograms are recorded sequentially, each for a different phase factor of the FZP, and the five recorded holograms are superposed such that the result is a complex valued Fresnel hologram. When the hologram is reconstructed, 3-D properties of the object are revealed.

Thus, the present invention includes a lensless method of recording digital Fresnel holograms under incoherent illumination. The present invention is an extension of the FINCH holographic technique which we have previously described (J. Rosen, and G. Brooker “Digital spatially incoherent Fresnel holography,” Accepted for publication in Opt. Lett) (J. Rosen, and G. Brooker “Fluorescence incoherent color holography,” Submitted for publication in Opt. Exp. (2007)) (Patents pending). In an embodiment of a method or apparatus according to the present invention, incoherent light transmitted through, or reflected from, a 3-D object propagates through an absorption-only FZP and is recorded by a digital camera. Five holograms may be recorded sequentially (or simultaneously under certain conditions) each with a different phase factor of the FZP. Information representing the five recorded holograms are superposed using a superposing process (for example using dedicated hardware or using a general purpose computing device such as a computer) such that the result is a complex valued Fresnel hologram. As we show in the following, the 3-D properties of the object are revealed by reconstructing this hologram using a digital reconstructing process.

The setup is lensless and contains absorption-only FZPs. These properties make the system suitable for operating with waves other than electromagnetic waves, or with electromagnetic waves in spectral regimes other than the visible light regime, in which it is impossible or extremely difficult to implement lenses and other phase elements. For electromagnetic waves the present system can be applied to x-rays, and therefore the method can be an important tool for medical imaging. The present invention may also be applied to 3D x-ray imaging, and elements of the inventive apparatus or method may be configured such that the radiation is transferred from the source through the semi-absorption object, rather than reflected from the object as we have shown in previous demonstrations of FINCH. (J. Rosen, and G. Brooker “Digital spatially incoherent Fresnel holography,” Accepted for publication in Opt. Lett) (J. Rosen, and G. Brooker “Fluorescence incoherent color holography,” Submitted for publication in Opt. Exp. (2007))

FIG. 1 shows a schematic view of a first embodiment of a lensless FINCH apparatus according to the present invention. The lensless FINCH apparatus includes an incoherent source 10 that illuminates a 3-D object 12 and the transmitted light 14 is captured by a CCD camera 16 after passing through the object 12 and reflecting from one or more reflective FZPs 18 displayed by a spatial light modulator (SLM) 20, or on another device that can be configured to change from one FZP to another.

Following is analysis of the inventive apparatus to demonstrate that the apparatus may produce a hologram of a 3D object. The apparatus can be analyzed as an incoherent correlator, where the FZP function is considered as a part of the system's transfer function. Alternatively, it may be more useful to analyze the apparatus as an incoherent interferometer, where the grating displayed on the SLM (i.e., the FZP) is considered as a beam splitter. In particular, the apparatus may be analyzed by following its response to an input object of a single infinitesimal point. Based on the point spread function (PSF) of the apparatus, the operation of the apparatus may be analyzed for any general object. Analysis of a beam originated from narrow band infinitesimal point source is done using Fresnel diffraction theory (J. Goodman, Introduction to Fourier Optics, 2^nd ed., McGraw-Hill, New York, 1996, pp. 63-95 (Chapter 4)) since such a source is coherent by definition.

Referring to the embodiment of FIG. 1, a Fresnel hologram of a point object is obtained on a plane of the CCD 16 that is distance d from the SLM 20, when a reflection function R(x_D,y_D) is real and positive in the form of,

$\begin{array}{cc}R\left(x_D,y_D\right)=\frac{1}{2}+\frac{1}{4}\exp \left[\frac{\pi }{\lambda f}\left(x_D^2+y_D^2\right)+\theta \right]+\frac{1}{4}\exp \left[-\frac{\pi }{\mathrm{\lambda f}}\left(x_D^2+y_D^2\right)-\theta \right]=\frac{1}{2}+\frac{1}{4}Q\left(\frac{1}{f}\right)\exp \left(\theta \right)+\frac{1}{4}Q\left(-\frac{1}{f}\right)\exp \left(-\theta \right),& \left(1\right)\end{array}$

Where λ is the central wavelength, and for reasons of increased clarity, the quadratic phase function is designated by the function Q such that Q(b)=exp[iπb/λ(x²+y²)].

The angle θ plays an important role later in the computation process in order to get rid of the twin images and the bias terms.

A point source located at the point (0,0,z_s) a distance L=2f−z_s from the SLM 20 induces on the SLM plane a diverging spherical wave of the form of Q(1/L). Right after the SLM 20, which has a reflection (or transmission) function given in Eq. (1), the complex amplitude of the wave is related to Q{1/(L)}[0.5+0.25·Q(−1/f)exp(iθ)+0.25·Q(−1/f)exp(iθ)].

Finally, in the CCD plane a distance d from the FZP, the intensity of the recorded hologram of a point is,

$\begin{array}{cc}{H}_P\left(x,y\right)=A{\frac{1}{2}Q\left[\frac{1}{L}\right]+\frac{1}{4}Q\left[\frac{f+L}{\mathrm{Lf}+\mathrm{df}+\mathrm{Ld}}\right]\exp \left(\theta \right)+\frac{1}{4}Q\left[\frac{f-L}{\mathrm{Lf}+\mathrm{df}-\mathrm{Ld}}\right]\exp \left(-\theta \right)}^2=A\frac{1}{2}Q\left[{\left(2f-z+d\right)}^{-1}\right]+\frac{1}{4}Q\left[{\left(\frac{f\left(2f-z\right)}{3f-z}+d\right)}^{-1}\right]\exp \left(\theta \right)+{\hspace{1em}\frac{1}{4}Q\left[{\left(\frac{-f\left(2f-z\right)}{f-z}+d\right)}^{-1}\right]\exp \left(-\theta \right)}^2\propto 6+2Q\left[S_1\right]\exp \left(-\mathrm{\theta }\right)+2Q\left[-S_1\right]\exp \left(\theta \right)+2Q\left[S_2\right]\exp \left(\theta \right)+2Q\left[-S_2\right]\exp \left(-\theta \right)+Q\left[S_3\right]\exp \left(2\theta \right)+Q\left[-S_3\right]\exp \left(-2\theta \right)& \left(2\right)\end{array}$

where

$\begin{array}{cc}{S}_1=\frac{-L^2}{\left(L+d\right)\left(\mathrm{Lf}+\mathrm{df}+\mathrm{Ld}\right)},S_2=\frac{L^2}{\left(L+d\right)\left(\mathrm{Lf}+\mathrm{df}-\mathrm{Ld}\right)},S_3=\frac{2L^2f}{{\left(\mathrm{Lf}+\mathrm{df}\right)}^2-L^2d^2}.& \left(3\right)\end{array}$

Eq. (2) has seven different terms where for a good holographic recording only one term of quadratic function (Q function) should remain after the superposition. Otherwise, reconstructing the hologram given in Eq. (2) with its all seven terms will affect an overlap between all seven different images. Looking closely in Eq. (2), there are two terms with the constant 2exp(iθ) and two terms with 2exp(−iθ). Therefore, we conclude that at least five holograms with different values of θ's angles are needed to be recorded, and to be superposed together, in order to get a single term out of the seven. Furthermore, the superposition can yield only one of the two terms Q[S₃] or Q[−S₃].

In order to remain with a single correlation term out of the seven terms given in Eq. (4), we follow the usual procedure of phase stepping (Patents pending), but with five holograms. Five holograms of the same object are recorded each of which with a different phase constant θ. The final hologram H_F is a superposition according to the following,

$\begin{array}{cc}{H}_F\left(x,y\right)=\frac{\left[H_1\left(x,y\right)\exp \left({\theta }_1\right)-H_5\left(x,y\right)\exp \left({\theta }_5\right)\right]\left(A_3B_2-A_2B_3\right)}{\left[\exp \left(2{\theta }_1\right)-\exp \left(2{\theta }_5\right)\right]}-\frac{\left[H_2\left(x,y\right)\exp \left({\theta }_2\right)-H_5\left(x,y\right)\exp \left({\theta }_5\right)\right]\left(A_3B_1-A_1B_3\right)}{\left[\exp \left(2{\theta }_2\right)-\exp \left(2{\theta }_5\right)\right]}-\frac{\left[H_3\left(x,y\right)\exp \left({\theta }_3\right)-H_5\left(x,y\right)\exp \left({\theta }_5\right)\right]\left(A_3B_2-A_2B_1\right)}{\left[\exp \left(2{\theta }_3\right)-\exp \left(2{\theta }_5\right)\right]}-\frac{\left[H_4\left(x,y\right)\exp \left({\theta }_4\right)-H_5\left(x,y\right)\exp \left({\theta }_5\right)\right]\left(A_2B_1-A_2B_3-A_3B_1+A_1B_3\right)}{\left[\exp \left(2{\theta }_4\right)-\exp \left(2{\theta }_5\right)\right]}& \left(4\right)\end{array}$

where H_k is the kth recorded hologram with the phase constant θ_k, and the constants are,

$\begin{array}{cc}{A}_k=\frac{\exp \left({\theta }_k\right)-\exp \left({\theta }_5\right)}{\exp \left(2{\theta }_k\right)-\exp \left(2{\theta }_5\right)}-\frac{\exp \left({\theta }_4\right)-\exp \left({\theta }_5\right)}{\exp \left(2{\theta }_4\right)-\exp \left(2{\theta }_5\right)}B_k=\frac{\exp \left(-{\theta }_k\right)-\exp \left(-{\theta }_5\right)}{\exp \left(2{\theta }_k\right)-\exp \left(2{\theta }_5\right)}-\frac{\exp \left(-{\theta }_4\right)-\exp \left(-{\theta }_5\right)}{\exp \left(2{\theta }_4\right)-\exp \left(2{\theta }_5\right)}& \left(5\right)\end{array}$

The intensities of the five recorded holograms of an object are superposed according to Eq. (5). The result is an integral of the PSF, one quadratic phase function resulting from Eq. (5), over all object intensity g(x_s,y_s,z_s), as follows

$\begin{array}{cc}H\left(x,y\right)\cong A\int \int \int g\left(x_s,y_s,z_s\right)\exp \left\{\frac{\pi S_3\left(z\right)}{\lambda }\left[{\left(x+\frac{ax_s}{f}\right)}^2+{\left(y+\frac{ay_s}{f}\right)}^2\right]+\theta \right\}x_sy_sz_s& \left(6\right)\end{array}$

Eq. (4) is a correlation between an object and a quadratic phase, z-dependent, function, which means that the recorded hologram is indeed a Fresnel hologram.

A 3-D image s(x,y,z) can be reconstructed from H_F(x,y) by calculating the Fresnel propagation formula, as follows,

$\begin{array}{cc}s\left(x,y,z\right)=H_F\left(x,y\right)*\exp \left[\frac{\pi }{\lambda z}\left(x^2+y^2\right)\right],& \left(7\right)\end{array}$

where the asterisk denotes a 2-D convolution.

There are numerous modifications based upon the lensless holographic concept and proof described above which are possible and are included in the present invention. For example, there are numerous other embodiments based on this invention that may speed up the image capture. The invention also includes related methods to reduce the number of holographic images which are needed to create a single hologram. Furthermore, the present invention also includes simultaneously capturing plural images and holograms, as described above, by splitting the captured image beam into five beams and simultaneously capturing the 5 described holograms.

Thus, the apparatus as described above, and a corresponding method, may record incoherent holograms of realistic 3-D objects using only absorption masks. Since the lensless FINCH system has only a single channel it does not demand complicated alignment. However, the invention also applies to operation using plural channels. The advantages of the present invention may be applied to the design of a portable and low cost holographic camera for electromagnetic waves other than just the visible light, which might be useful for various applications in medical imaging. (Patents pending)

FIG. 2 is a schematic diagram of an alternative embodiment of the lensless FINCH apparatus. The embodiment of FIG. 2 is similar to that of FIG. 1. However, in FIG. 2, and an SLM 24 configured to produce transmissive FZPs 22 replaces the SLM 20 configured to produce reflective FZPs 18.

Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

1. An apparatus for producing a hologram of an object, the apparatus comprising: a light source that emits an incoherent electromagnetic wave toward the object;a masking device configured to display a mask, receive the incoherent electromagnetic wave emitted toward the object, mask the received incoherent electromagnetic wave according to the displayed mask, and produce a masked electromagnetic wave;an image recording device configured to capture an image of the masked electromagnetic wave;and a processing device configured to convert the image of the masked electromagnetic wave into the hologram of the object.

2. The apparatus of claim 1, wherein the electromagnetic wave is a visible light.

3. The apparatus of claim 1, wherein the displayed mask includes an absorption mask.

4. The apparatus of claim 1, wherein the masking device is configured to display the displayed mask including plural different absorption masks over a predetermined time period.

5. The apparatus of claim 1, wherein the displayed mask includes a reflective absorption mask.

6. The apparatus of claim 1, wherein the displayed mask includes a transmissive absorption mask.

7. The apparatus of claim 1, wherein the masking device includes a spatial light modulator (SLM).

8. The apparatus of claim 1, wherein the displayed mask includes an absorption mask based on a Fresnel Zone Pattern (FZP).

9. A method for producing a hologram of an object, the method comprising: emitting an incoherent electromagnetic wave toward the object;displaying a displayed mask;receiving the incoherent electromagnetic wave emitted toward the object;masking the received incoherent electromagnetic wave according to the displayed mask to produce a masked electromagnetic wave;capturing an image of the masked electromagnetic wave;converting the image of the masked electromagnetic wave into the hologram of the object.

10. The method of claim 9, wherein the electromagnetic wave is a visible light.

11. The method of claim 9, wherein the displayed mask includes an absorption mask.

12. The method of claim 9, wherein the masking includes displaying plural different absorption masks over a predetermined time period.

13. The method of claim 9, wherein the displayed mask includes a reflective absorption mask.

14. The method of claim 9, wherein the displayed mask includes a transmissive absorption mask.

15. The method of claim 9, wherein the masking device includes a spatial light modulator (SLM).

16. The method of claim 9, wherein the displayed mask includes an absorption mask based on a Fresnel Zone Pattern (FZP).