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Pseudocolour images with volume reflection holograms
Volume 35, number 1
PSEUDOCOLOUR
OPTICS COMMUNICATIONS
October 1980
IMAGES WITH VOLUME REFLECTION HOLOGRAMS
P. HARIHARAN CSIRO Division of Applied Physics, Sydney, Australia 2070 Received 16 June 1980
A technique is described that can be used to produce pseudocolour images of threedimensional objects, using volume phase reflection holograms. These are recorded on two photographic plates \trith a HeNe laser. The thickness of the emulsion is controlled during processing, so that the finished holograms reconstruct at different wavelengths in white light. When these holograms are superposed, a multicolqur image is obtained.
1. Introduction
Pseudocolour techniques based on the use of superposed rainbow holograms, recorded with a single laser wavelength, but with different interbeam angles have been described in recent publications [l-3]. These have the advantage that, within certain limitations, a single He-Ne laser can be used to produce holograms that reconstruct multicolour images of three-dimensional objects when illuminated with white light. However, rainbow holograms have two disadvantages: vertical parallax is lost, and the colours of the image change if the observer moves his head in the vertical plane. These disadvantages do not exist with volume reflection holograms. Because of their high wavelength selectivity, they can reconstruct an image in a single colour when illuminated with white light [4,5]. If two or three holograms are recorded on the same plate with suitably chosen laser wavelengths, multicolour images can be obtained [6,7]. This paper described a technique that exploits the wavelength selectivity of volume reflection holograms to produce pseudocolour images with a single laser wavelength.
2. Pseudocolour
techniques
With volume reflection holograms, changing the angle between the reference and object beams has 42
little effect on the colour of the reconstructed image. However, the colour is affected readily by changes in the thickness of the recording medium. In fact, even normal chemical processing of photographic emulsion layers results in a reduction in their thickness, as a result of which, typically, a reflection hologram recorded with red light from a He-Ne laser (X0 = 633 nm) reconstructs a green image (Xc = 530 nm) [8]. Normally this shift is corrected with a swelling agent [9,10] ; however, it can also be used to produce pseudocolour images. To record in the same emulsion layer two holograms that reconstruct images of different colours, the first exposure is made, say, with red light, with the emulsion in its normal condition. The emulsion is then soaked in a 3% solution of triethanolamine (which also increases its sensitivity) and dried in darkness. The second exposure is made on the swollen emulsion with the same laser. Normal processing eliminates the swelling produced by the triethanolamine and produces the usual shrinkage. Accordingly, the first exposure yields a green reconstructed image, while the second produces an image at an even shorter wavelength, that is to say, a blue image. If the normal loss of thickness of the emulsion is corrected, red and green images are obtained. Alternatively, with bleached volume reflection holograms [ 111, it is possible to record the component holograms on separate plates which are processed to obtain reconstructed images of appropriate colours
Volume 35, number 1
October 1980
OPTICS COMMUNICATIONS
and then stacked to produce a multicoloui image. This is because volume phase reflection holograms are effectively transparent at wavelengths outside the relatively narrow band which is diffracted [ 121. Both the above techniques lend themselves readily to the production of two-colour images. For threecolour images, it is most convenient to use a combination of the two techniques.
3. Image shifts With any pseudocolour technique, a reconstructed image point is liable to undergo a shift in position. This image shift is proportional to the wavelength shift and is a potential source of problems in registration of the different component images [ 1,2]. To analyse this image shift, it is convenient to use the coordinate system shown in fig. 1, in which the hologram plate is in the x-y plane. In the recording set-up, the spherical wavefront (wavelength ho) emerging from a point P (x0, yo, zo) on the object is incident on the hologram plate from the left, while a coherent reference wavefront is incident on the hologram plate from a point R (xr, z,) on the right. To reconstruct the image, the hologram is illuminated with a white light source located at a point C (x,., z,) which need not coincide with R. If, after processing, the hologram reconstructs an image at a wavelength Xc (h, # ho), the lateral mag-
Fig. 1. Coordinate system used for analysis of the image shifts.
nification
of the image is given by the relation
Mht = l/ [l + (ZO@J
-
[ 13,141
(z&)1 P
0)
where p = h,/A,. To avoid lateral misregistration, iQ, must be independent of p; this is possible if the hologram is reconstructed with a parallel beam (zc = m), so that Mlat = z,/(z, - ZO) * On the other hand, the longitudinal of the image is Mlong = (l/L&
*
(2) magnification
(3)
To eliminate longitudinal misregistration, Maong must be independent of 12,while, to eliminate longitudinal distortion [ 151, it must be equal to Mlat. These conditions cannot be satisfied simultaneously with eq. (2). However, the images formed at different wavelengths coincide for the plane z. = 0, for which Mht = 1, and Mlons = (l/p). Since the eye is much more tolerant of longitudinal misregistration and longitudinal distortion than it is of lateral misregistration, acceptable results can be obtained over a limited depth centred around this plane.
4. Experimental Reflection holograms were recorded with the optical set-up shown in fig. 2. In this, a spherical concave mirror formed a real image of the object at unit magnification close to the plane of the hologram plate All the component holograms were recorded with a He-Ne laser (A = 633 nm) on Holotest 8E75 plates; these were developed for 4 min in Kodak X-ray developer and then bleached in a dichromate-iodide bath [ 111. The red component hologram was recorded on a plate exposed with the emulsion side towards the reference beam. To correct the shrinkage in thickness of the emulsion, the plate was soaked in a solution (-6%) of D(-) sorbitol, to which a few drops of a wetting agent had been added, before it was dried. Triethanolamine cannot be used to correct emulsion shrinkage in bleached holograms because of the rapid formation of printout silver [ 121. The green and blue component holograms were recorded on another plate exposed with the emulsion side towards the object beam. The green component hologram was exposed 43
OPTICS COMMUNICATIONS
Volume 35, number 1
I
-
He Ne laser
I
October 1980
the holograms. Finally, the plates were cemented together with Kodak Optical Cement HE-80 and the rear surface of the sandwich was sprayed with a flat back acrylic paint.
=’
5. Conclusions
t
-_
References
L
[ll P.N. Tamura, Proc. SPIE 126, Clever optics (SPIE, Bellingham, 1977) pp. 59-66.
Fig. 2. Optical set-up. with the emulsion
in its normal
condition,
121P.N. Tamura, AppL Optics 17 (1978) 2532. while the
blue component hologram was exposed after swelling the emulsion with triethanolamine. This plate was processes without any correction for emulsion shrink. age. To compensate for the thickness of the glass, the plate holder was mounted on a micrometer slide and, between the two exposures, was moved normal to its plane through a distance equal to d[ 1 - (l/n)], where d is the thickness of the plates, and n is the refractive index of the glass. After drying, the plates were assembled with the emulsion layers in contact, and the reconstructed images were viewed with the hologram reconstructing the green and blue images in front, and the hologram reconstructing the red image behind. This arrangement helps to equalize the diffraction efficiencies of
44
Pseudocolour reflection holograms made by this technique gave bright multicolour images when illuminated with a 12 V, 50 W halogen lamp. With selected subjects of limited depth, problems of longitudinal misregistration and distortion were not noticeable. While bleached photographic materials and a He-Ne laser have been used in these experiments, the same technique could also be applied to volume phase reflection holograms recorded in dichromated gelatin with an argon laser.
131 C. Yan-Song, W. Yu-Tang and D. Bi-Zhen, Acta Phys. Sin. 27 (1978) 723. 141 Yu.N. Denisyuk, Opt. Spectry. 15 (1963) 279. [51 G.W. Stroke and A.E. Labeyrie, Phys. Lett. 20 (1966) 368. 161 L.H. Lin, K.S. Pennington, G.W. Stroke and A.E. Labeyrie, Bell Syst. Tech. .I. 45 (1966) 659. [71 J. Upatnieks, J. Marks and J. Federowicz, AppL Phys. Lett. 8 (1966) 286. 181 R.J. Collier, C.B. Burckhardt and L.H. Lin, Optical holography (Academic Press, New York, 1971) p. 520. [91 L.H. Lin and C.V. Lo Bianco, Appl. Optics 6 (1967) 1255. [lOI N. Nishida, AppL Optics 9 (1970) 238. [ill P. Hariharan, G.S. Kaushik and C.S. Ramanathan, Optics Comm. 6 (1972) 75. 1121 P. Hariharan, J. Optics (Paris) 11 (1980) 53. [131 E.N. Leith, J. Upatnieks and K.A. Haines, J. Opt. Sot. Am. 55 (1965) 981. [141 R.W. Meier, J. Opt. Sot. Am. 55 (1965) 987. [151 P. Hariharan, Optics Comm. 17 (1976) 52.
PSEUDOCOLOUR
OPTICS COMMUNICATIONS
October 1980
IMAGES WITH VOLUME REFLECTION HOLOGRAMS
P. HARIHARAN CSIRO Division of Applied Physics, Sydney, Australia 2070 Received 16 June 1980
A technique is described that can be used to produce pseudocolour images of threedimensional objects, using volume phase reflection holograms. These are recorded on two photographic plates \trith a HeNe laser. The thickness of the emulsion is controlled during processing, so that the finished holograms reconstruct at different wavelengths in white light. When these holograms are superposed, a multicolqur image is obtained.
1. Introduction
Pseudocolour techniques based on the use of superposed rainbow holograms, recorded with a single laser wavelength, but with different interbeam angles have been described in recent publications [l-3]. These have the advantage that, within certain limitations, a single He-Ne laser can be used to produce holograms that reconstruct multicolour images of three-dimensional objects when illuminated with white light. However, rainbow holograms have two disadvantages: vertical parallax is lost, and the colours of the image change if the observer moves his head in the vertical plane. These disadvantages do not exist with volume reflection holograms. Because of their high wavelength selectivity, they can reconstruct an image in a single colour when illuminated with white light [4,5]. If two or three holograms are recorded on the same plate with suitably chosen laser wavelengths, multicolour images can be obtained [6,7]. This paper described a technique that exploits the wavelength selectivity of volume reflection holograms to produce pseudocolour images with a single laser wavelength.
2. Pseudocolour
techniques
With volume reflection holograms, changing the angle between the reference and object beams has 42
little effect on the colour of the reconstructed image. However, the colour is affected readily by changes in the thickness of the recording medium. In fact, even normal chemical processing of photographic emulsion layers results in a reduction in their thickness, as a result of which, typically, a reflection hologram recorded with red light from a He-Ne laser (X0 = 633 nm) reconstructs a green image (Xc = 530 nm) [8]. Normally this shift is corrected with a swelling agent [9,10] ; however, it can also be used to produce pseudocolour images. To record in the same emulsion layer two holograms that reconstruct images of different colours, the first exposure is made, say, with red light, with the emulsion in its normal condition. The emulsion is then soaked in a 3% solution of triethanolamine (which also increases its sensitivity) and dried in darkness. The second exposure is made on the swollen emulsion with the same laser. Normal processing eliminates the swelling produced by the triethanolamine and produces the usual shrinkage. Accordingly, the first exposure yields a green reconstructed image, while the second produces an image at an even shorter wavelength, that is to say, a blue image. If the normal loss of thickness of the emulsion is corrected, red and green images are obtained. Alternatively, with bleached volume reflection holograms [ 111, it is possible to record the component holograms on separate plates which are processed to obtain reconstructed images of appropriate colours
Volume 35, number 1
October 1980
OPTICS COMMUNICATIONS
and then stacked to produce a multicoloui image. This is because volume phase reflection holograms are effectively transparent at wavelengths outside the relatively narrow band which is diffracted [ 121. Both the above techniques lend themselves readily to the production of two-colour images. For threecolour images, it is most convenient to use a combination of the two techniques.
3. Image shifts With any pseudocolour technique, a reconstructed image point is liable to undergo a shift in position. This image shift is proportional to the wavelength shift and is a potential source of problems in registration of the different component images [ 1,2]. To analyse this image shift, it is convenient to use the coordinate system shown in fig. 1, in which the hologram plate is in the x-y plane. In the recording set-up, the spherical wavefront (wavelength ho) emerging from a point P (x0, yo, zo) on the object is incident on the hologram plate from the left, while a coherent reference wavefront is incident on the hologram plate from a point R (xr, z,) on the right. To reconstruct the image, the hologram is illuminated with a white light source located at a point C (x,., z,) which need not coincide with R. If, after processing, the hologram reconstructs an image at a wavelength Xc (h, # ho), the lateral mag-
Fig. 1. Coordinate system used for analysis of the image shifts.
nification
of the image is given by the relation
Mht = l/ [l + (ZO@J
-
[ 13,141
(z&)1 P
0)
where p = h,/A,. To avoid lateral misregistration, iQ, must be independent of p; this is possible if the hologram is reconstructed with a parallel beam (zc = m), so that Mlat = z,/(z, - ZO) * On the other hand, the longitudinal of the image is Mlong = (l/L&
*
(2) magnification
(3)
To eliminate longitudinal misregistration, Maong must be independent of 12,while, to eliminate longitudinal distortion [ 151, it must be equal to Mlat. These conditions cannot be satisfied simultaneously with eq. (2). However, the images formed at different wavelengths coincide for the plane z. = 0, for which Mht = 1, and Mlons = (l/p). Since the eye is much more tolerant of longitudinal misregistration and longitudinal distortion than it is of lateral misregistration, acceptable results can be obtained over a limited depth centred around this plane.
4. Experimental Reflection holograms were recorded with the optical set-up shown in fig. 2. In this, a spherical concave mirror formed a real image of the object at unit magnification close to the plane of the hologram plate All the component holograms were recorded with a He-Ne laser (A = 633 nm) on Holotest 8E75 plates; these were developed for 4 min in Kodak X-ray developer and then bleached in a dichromate-iodide bath [ 111. The red component hologram was recorded on a plate exposed with the emulsion side towards the reference beam. To correct the shrinkage in thickness of the emulsion, the plate was soaked in a solution (-6%) of D(-) sorbitol, to which a few drops of a wetting agent had been added, before it was dried. Triethanolamine cannot be used to correct emulsion shrinkage in bleached holograms because of the rapid formation of printout silver [ 121. The green and blue component holograms were recorded on another plate exposed with the emulsion side towards the object beam. The green component hologram was exposed 43
OPTICS COMMUNICATIONS
Volume 35, number 1
I
-
He Ne laser
I
October 1980
the holograms. Finally, the plates were cemented together with Kodak Optical Cement HE-80 and the rear surface of the sandwich was sprayed with a flat back acrylic paint.
=’
5. Conclusions
t
-_
References
L
[ll P.N. Tamura, Proc. SPIE 126, Clever optics (SPIE, Bellingham, 1977) pp. 59-66.
Fig. 2. Optical set-up. with the emulsion
in its normal
condition,
121P.N. Tamura, AppL Optics 17 (1978) 2532. while the
blue component hologram was exposed after swelling the emulsion with triethanolamine. This plate was processes without any correction for emulsion shrink. age. To compensate for the thickness of the glass, the plate holder was mounted on a micrometer slide and, between the two exposures, was moved normal to its plane through a distance equal to d[ 1 - (l/n)], where d is the thickness of the plates, and n is the refractive index of the glass. After drying, the plates were assembled with the emulsion layers in contact, and the reconstructed images were viewed with the hologram reconstructing the green and blue images in front, and the hologram reconstructing the red image behind. This arrangement helps to equalize the diffraction efficiencies of
44
Pseudocolour reflection holograms made by this technique gave bright multicolour images when illuminated with a 12 V, 50 W halogen lamp. With selected subjects of limited depth, problems of longitudinal misregistration and distortion were not noticeable. While bleached photographic materials and a He-Ne laser have been used in these experiments, the same technique could also be applied to volume phase reflection holograms recorded in dichromated gelatin with an argon laser.
131 C. Yan-Song, W. Yu-Tang and D. Bi-Zhen, Acta Phys. Sin. 27 (1978) 723. 141 Yu.N. Denisyuk, Opt. Spectry. 15 (1963) 279. [51 G.W. Stroke and A.E. Labeyrie, Phys. Lett. 20 (1966) 368. 161 L.H. Lin, K.S. Pennington, G.W. Stroke and A.E. Labeyrie, Bell Syst. Tech. .I. 45 (1966) 659. [71 J. Upatnieks, J. Marks and J. Federowicz, AppL Phys. Lett. 8 (1966) 286. 181 R.J. Collier, C.B. Burckhardt and L.H. Lin, Optical holography (Academic Press, New York, 1971) p. 520. [91 L.H. Lin and C.V. Lo Bianco, Appl. Optics 6 (1967) 1255. [lOI N. Nishida, AppL Optics 9 (1970) 238. [ill P. Hariharan, G.S. Kaushik and C.S. Ramanathan, Optics Comm. 6 (1972) 75. 1121 P. Hariharan, J. Optics (Paris) 11 (1980) 53. [131 E.N. Leith, J. Upatnieks and K.A. Haines, J. Opt. Sot. Am. 55 (1965) 981. [141 R.W. Meier, J. Opt. Sot. Am. 55 (1965) 987. [151 P. Hariharan, Optics Comm. 17 (1976) 52.