Three-beam technique superimposes a number of patterns in the same plane of a hologram and then uniquely identifies each pattern by a suitable readout process. The developed readout process does not require any movement of parts

Hallock, J. N.

English

NASA Technical Reports Server

September 1968
	 Brief 68-10347 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Improvement in Recording and Reading Holograms 
1
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FIGURE 2 FIGURE I fl A three-beam technique has been developed to superimpose a number of patterns in the same plane 
of a hologram and then to uniquely identify each 
pattern by a suitable readout process. In contrast to 
prior techniques, the developed readout process does 
not require any movement of parts. 
Figure 1 illustrates both the prior art and the tech-
nique developed. Coherent collimated light L is inci-
dent upon a Rayleigh lens R and upon a pattern I to 
be holographically recorded. The Rayleigh lens 
focuses the incident light L to a point F which should 
appear in the same plane as the input pattern I (plane 
normal to the 0-0' axis). The light diverges from F 
to the transform lens T, which is located one focal dis-
tance from the plane of pattern I. The light amplitude 
in the back focal plane of the transform lens T is 
equal to the Fourier transform of the input pattern I. 
The interference between the Fourier transform and 
the beam of light emanating from F is recorded on a 
photographic plate P placed at the back focal plane of 
lens T. 
The above description is common to both the prior 
art and the new three-beam method; however, the 
method for storing additional patterns is different. 
In the prior art, one would rotate the photographic
plate P about an axis perpendicular to axis 0-0' by 
some finite angle and then repeat the steps outlined 
above. The three-beam technique introduces another 
lens B which comes to a focus at a point G in front 
of the photographic plate P. 
When a new pattern is to be stored, the location of 
lens B is changed in such a manner to keep constant 
the distance from the center of lens B to the axis 
0-0' as well as the distances from the center of the 
plate P to point G and from point G to the center of 
lens B (i.e., a rotation about the 0-0' axis). Now 
there are three beams interfering at the photographic 
plate P. 
The distinct advantage of the three-beam technique 
can be seen in Figure 2, which displays the readout 
process of the images stored on the photographic plate 
P. An unknown pattern is placed in a window formed 
by the stops S; the collimated coherent light L will 
pass only through the pattern I. The transform lens T 
performs the Fourier transform of the pattern, and in 
the plane of the photographic plate P the Fourier 
transform is multiplied with the previously recorded 
Fourier transforms. Lens A then performs another 
Fourier transform and exhibits the cross correlation of 
the input pattern I with all the stored patterns as a 
(continued overleaf) 
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Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
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point of light F. Using just the prior art, one would 
have to rotate the photographic plate P in order to 
search for the formation of point F', and the angle of 
the plate P will indicate which of the stored patterns 
is matched by the input pattern I. In the three-beam 
technique, point F' will be present without any move-
ment of plate P; there will also be another point of 
light G' which is the image of point G of Figure 1. The 
position of point G' indicates which of the stored 
patterns matches the input pattern I (if any). (As 
noted in Figure 2, there is also an image 1' of the 
input pattern I.) F' and G' come to a point focus in 
different planes which is convenient for readout pur-
poses. Thus, the new three-beam technique allows one 
to interrogate the stored patterns for a matching pat-
tern in one operation, without the previously required 
need for moving the photographic plate P. 
Note: 
Details may be obtained from: 
Technology Utilization Officer 
Electronics Research Center 
575 Technology Square 
Cambridge, Massachusetts 02139 
Reference: B68-10347 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D. C. 20546
Source: J. N. Hallock 
(ERC- 10 15 1) 
Brief 68-10347	 Category 02


Improvement in recording and reading holograms

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