Noise elimination technique in holography P. K. Katti and P. C. Mehta A method has been described for reducing speckling and the noise arising due to cosmetic defects present in the elements of the hologram recording geometry by using a multifrequency grating-diffuser-lens com- bination for object illumination at the time of recording the hologram. The averaging of the speckling is simulated at the plane of the object. The experimental results show that this method of object illumina- tion is quite effective for noise reduction and may be applied to recording noise-free holograms of short- lived events. It may also be useful in holographic microscopy. Introduction The quality of reconstructed images from a holo- gram is highly affected by dust particles, scratches, and various scattering and reflecting points present in the elements of the hologram recording optics. These cosmetic defects can be minimized by placing a diffuser with the object transparency at the time of recording the hologram. In such a type of hologram, the SNR can be improved by increasing the size of the hologram aperture, but very large apertures are not suitable for holographic memories, where the ho- logram area is usually kept much smaller than the area of the object transparency. Moreover, the dif- fuse illumination of the object, however, reduces sys- tem noise and introduces a new noise known as spec- kling, which is not desirable in many applications of holography. Many techniques have been proposed for speckle reduction. The techniques include rotation of optic elements,", 2 diffuse wavefronts, 3 incoherent addition of diffuse wavefronts, 4 - 6 phase modulation, 78 multi- ple coherent and incoherent wave illumination, 9 -' 4 spatial filtering, 15 etc. Firester et al.," Gerritsen et al., 9 and Upatnieks 7 have recorded speckle-free holo- grams by illuminating the object from different di- rections by multiple beams obtained by a set of mir- rors or by a collection of amplitude and/or phase gratings. By this technique, the coarseness of the in- terference pattern produced by the interference be- tween different beams, if not eliminated properly, will appear in the reconstructed image. The authors are with the Instruments Research & Development Establishment, Dehra Dun, India 248008. Received 19 December 1974. Martienssen and Spiller 4 and Yokozeki et al. 6 have demonstrated that the speckle noise can be sup- pressed considerably by constructing a composite ho- logram consisting of a number of Fourier transform holograms of the same object but with different dif- fusers. For reconstruction, all the holograms are si- multaneously illuminated by the reference beam. The reconstructed image contains low speckle noise due to averaging of the noise. This method, how- ever, is not convenient for short-lived events as it re- quires a large number of exposures. We report here a simple technique to record noise-free holograms with a high degree of redundancy, and it requires only a single exposure. Experimental In the present technique the averaging of the spec- kle pattern is simulated at the plane of the object. The cosmetic defects and the speckle noise are fur- ther suppressed by recording a highly redundant sin- gle exposure hologram. Figure 1 shows the schemat- ic diagram for recording the hologram. For illumi- nating the object a large number of channels are pro- duced by putting a multifrequency diffraction grat- ing G in the object beam. A ground glass D is placed in front of the grating so that its different parts are illuminated by the beams. The scattered beams from the diffuser are collected by a wide aperture lens L and are brought at the plane of the object 0. This simulates the effect of a rotating diffuser with- out creating any stability problems. In actual exper- iments, the ground glass was placed at the wider end of a metallic conical cylinder, the other end of which carried an adjustable lens. Three sets of holograms were recorded for compar- ison purposes: (1) conventional holograms by putting the ground glass in contact with the object transparency and without placing the grating at G; 530 APPLIED OPTICS / Vol. 15, No. 2 / February 1976
Transcript
Noise elimination technique in holography
P. K. Katti and P. C. Mehta
A method has been described for reducing speckling and the noise arising due to cosmetic defects presentin the elements of the hologram recording geometry by using a multifrequency grating-diffuser-lens com-bination for object illumination at the time of recording the hologram. The averaging of the speckling issimulated at the plane of the object. The experimental results show that this method of object illumina-tion is quite effective for noise reduction and may be applied to recording noise-free holograms of short-lived events. It may also be useful in holographic microscopy.
Introduction
The quality of reconstructed images from a holo-gram is highly affected by dust particles, scratches,and various scattering and reflecting points presentin the elements of the hologram recording optics.These cosmetic defects can be minimized by placinga diffuser with the object transparency at the time ofrecording the hologram. In such a type of hologram,the SNR can be improved by increasing the size ofthe hologram aperture, but very large apertures arenot suitable for holographic memories, where the ho-logram area is usually kept much smaller than thearea of the object transparency. Moreover, the dif-fuse illumination of the object, however, reduces sys-tem noise and introduces a new noise known as spec-kling, which is not desirable in many applications ofholography.
Many techniques have been proposed for specklereduction. The techniques include rotation of opticelements,",2 diffuse wavefronts,3 incoherent additionof diffuse wavefronts,4-6 phase modulation,7 8 multi-ple coherent and incoherent wave illumination,9-'4
spatial filtering,15 etc. Firester et al.," Gerritsen etal.,9 and Upatnieks7 have recorded speckle-free holo-grams by illuminating the object from different di-rections by multiple beams obtained by a set of mir-rors or by a collection of amplitude and/or phasegratings. By this technique, the coarseness of the in-terference pattern produced by the interference be-tween different beams, if not eliminated properly,will appear in the reconstructed image.
The authors are with the Instruments Research & DevelopmentEstablishment, Dehra Dun, India 248008.
Received 19 December 1974.
Martienssen and Spiller4 and Yokozeki et al. 6 havedemonstrated that the speckle noise can be sup-pressed considerably by constructing a composite ho-logram consisting of a number of Fourier transformholograms of the same object but with different dif-fusers. For reconstruction, all the holograms are si-multaneously illuminated by the reference beam.The reconstructed image contains low speckle noisedue to averaging of the noise. This method, how-ever, is not convenient for short-lived events as it re-quires a large number of exposures. We report herea simple technique to record noise-free hologramswith a high degree of redundancy, and it requiresonly a single exposure.
Experimental
In the present technique the averaging of the spec-kle pattern is simulated at the plane of the object.The cosmetic defects and the speckle noise are fur-ther suppressed by recording a highly redundant sin-gle exposure hologram. Figure 1 shows the schemat-ic diagram for recording the hologram. For illumi-nating the object a large number of channels are pro-duced by putting a multifrequency diffraction grat-ing G in the object beam. A ground glass D is placedin front of the grating so that its different parts areilluminated by the beams. The scattered beamsfrom the diffuser are collected by a wide aperturelens L and are brought at the plane of the object 0.This simulates the effect of a rotating diffuser with-out creating any stability problems. In actual exper-iments, the ground glass was placed at the wider endof a metallic conical cylinder, the other end of whichcarried an adjustable lens.
Three sets of holograms were recorded for compar-ison purposes:
(1) conventional holograms by putting theground glass in contact with the object transparencyand without placing the grating at G;
All the beams meet at the plane of the object and in-terfere with each other, thus simulating a large num-ber of coherent sources. However, this reconstructsJ spurious background noise as observed in Figs. 2(d)and 2(g). This can be suppressed by destroying thecoherence between the individual channels, since co-
PH herence among the channels is not required.
A12
Fig. 1. Schematic diagram for recording a noise-free hologram:S-laser; BS-beam splitter; M-mirror; SF-spatial filter; G-grating; D-ground glass; L-lens; 0-object; PH-photographic
plate.
(2) multiple coherent beam illumination of theobject by inserting the grating at G but with noground glass at D; and
(3) diffuse illumination of the object by insertingboth the grating and the ground glass at G and D, re-spectively.
In each case only small aperture holograms wererecorded on Agfa-Gevaert Scientia film 10 E 75 anddeveloped in Kodak D-19 developer under identicalconditions. No care was taken to clean the mirrors,lens, beam splitter, etc., and, in fact, the lens L, se-lected in order to study the noise suppression capa-bilities of different methods, was of poor quality hav-ing finger prints, dust particles, and scratches.
Two types of gratings, one giving nine beams andthe other multiple beams (about 100), were used.The multifrequency gratings were produced hologra-phically on a single photographic plate. For this, arecording was made of the interference pattern be-tween two plane waves interfering at an angle ofabout 100, and then three more exposures were madeon the same plate by rotating it through 450 aftereach successive exposure, about an axis that is per-pendicular to the plane of the photographic plate andpassing through its center. The second grating wasproduced by simply recording the interference pat-tern between the beams emerging from the first grat-ing. The holograms were bleached by a ferric chlo-ride bleach bath. The angle between the beams inthe first grating was kept small (10°) so that the en-tire light distributed in various channels includingthe zero order could be used for object illumination.Figure 2(a) shows the multiple beams generated bythe multifrequency grating.
Results and Discussion
Figure 2 shows the results obtained by all threemethods. Figure 2(c) is the reconstructed imagefrom the conventional hologram [type (1)]. It wasobserved that poor optics yields considerable noise inthe image. Figures 2(d) and 2(g) are the reconstruct-ed images obtained by the holograms of type (2) withthe multifrequency grating giving nine beams andmultiple beams [Fig. 2(a)], respectively. The noisebecomes smaller as the number of beams increases.
With the introduction of ground glass in the planeD in the setup (Fig. 1) [hologram type (3)], the quali-ty of the reconstructed images improves greatly.Figures 2(e) and 2(h) show the reconstructed imagesfrom holograms of type (3) with nine-beam and mul-tiple-beam gratings, respectively. Again, the im-provement is better with a larger number of beams.It can be observed that the spurious backgroundpresent in Figs. 2(d) and 2(g) is now absent. It hasbeen observed that if the object is placed with a sec-ond diffuser, a further reduction in the noise takesplace [Figs. 2(f) and 2(i)]. A rapid scanning of Figs.2(b)-2(i) shows that the system noise and specklenoise have been eliminated almost completely in Fig.2(i). The improvement in the resolution is also evi-dent. It has also been observed that the hologramsproduced are not sensitive to scratches, dust parti-cles, etc. Only a small portion of the holograms wereused to reconstruct the images, showing the redun-dancy present in them. In fact, the redundancy isdirectly proportional to the number of object illumi-nating beams. The present method of illuminationof the object is also suitable for making Fouriertransform holograms. This method of illuminationof the object may be useful for holographic microsco-py-
Conclusions
The noise suppression capabilities of two-hologramrecording techniques have been studied experimen-tally. In the first technique, multiple coherentbeams were used for object illumination, while in thesecond technique the effect of a rotating ground glasshas been simulated by using a multifrequency grat-ing-diffuser-lens combination. The results showthat the latter technique efficiently suppresses thesystem noise and the speckling.
One of the authors (PCM) is thankful to V. V.Rampal for taking interest in the work. Thanks arealso due to Sarita Swami and T. S. Jassal for techni-cal assistance.
Fig. 2. (a) Multiple beams generated by multifrequency grating, (b) object. Reconstructed real images by holograms produced by (c)
conventional method, poor optics, (d) nine object beams without diffuser at D, (e) nine object beams with diffuser at D, (f) as (e) but objectwith a second diffuser, (g) multiple object beams without diffuser at D, (h) multiple object beams with diffuser at D, (i) as (h) but object
1. P. Kirkpatrik and H. M. A. El-Sum, J. Opt. Soc. Am. 46, 825(1956).
2. R. Pawluczyk, Opt. Commun. 7, 366 (1973).3. E. N. Leith and J. Upatnieks, J. Opt. Soc. Am. 54, 1295
(1964).4. W. Martienssen and S. Spiller, Phys. Lett. 24A, 126 (1967).5. V. Ichioka, M. Izumi, and T. Suzuki, Appl. Opt. 10, 403 (1971).6. S. Yokozeki, H. Okuyama, and S. Banda, Opt. Commun. 8,358
(1973).
7. J. Upatnieks, Appl. Opt. 6, 1905 (1967).8. E. N. Leith and J. Upatnieks, Appl. Opt. 7, 2085 (1968).9. H. J. Gerritsen, W. J. Hannan, and E. G. Ramberg, Appl. Opt.
7, 2301 (1968).10. D. Gabor, IBM J. Res. Dev. 14, 509 (1970).11. A. H. Firester, E. C. Fox, W. J. Hannan, and M. Lurie, RCA
Rev. 33, 131 (1972).12. R. F. Van Ligten, Appl. Opt. 12, 255 (1973).13. J. Upatnieks and R. W. Lewis, Appl. Opt. 12, 2161 (1973).14. G. B. Brand, Appl. Opt. 12, 368 (1973).15. P. K. Katti and M. Singh, Opt. Commun. 8, 345 (1973).
