Improvement in sensitivity of holography, the technique of lensless interferometry, is obtained by enhancing the higher-order structure in the interferogram. By using the light diffracted into higher orders than the first, phase sensitivity is increased over the first order sensitivity by a factor equal to the order number used

Heflinger, L. O.

English

NASA Technical Reports Server

November 1969	 Brief 69-10663 
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. 
Fine-Line Sensitivity for Holographic Interferograms 
.
Holography - the technique of lensless interferom-
etry - has proven to be an extremely useful tool in an 
expanding number of scientific and engineering disci-
plines. And, as in most technologies, there are contin-
ual efforts to improve the sensitivity of the technique. 
It is intuitively apparent that since holography is a 
light interference phenomenon, greater sensitivity can 
be obtained by enhancing the higher-order structure 
in the interferogram. 
By using the light diffracted into higher orders than 
the first, the phase sensitivity of holographic interfero-
grams can be increased over the first order sensitivity 
by a factor equal to the order number used. In other 
words, if the fourth order is used, the sensitivity is 
increased by a factor of four. 
The experimental arrangement for the formation of 
the holograms consists of two beams from the same 
laser source impinging at a small angle on the holo-
gram so that moderately coarse fringes are produced. 
The Michelson arrangement is convenient for this 
purpose since it permits the subject to be placed easily 
in one of the beams while avoiding long paths between
subject and hologram. Interferometric accuracy is not 
required for the components nor the adjustment. The 
general procedure for constructing the holographic 
interferogram consists of the following operations: 
step one - make a heavily exposed hologram of the 
empty scene; step two - on a second photographic 
plate, make a heavily exposed hologram with the phase 
subject in place in one of the beams; step three - on a 
third photographic plate, make a double-exposed 
contact print of the holograms made in steps one and 
two, one exposure from the hologram of step one and 
the other exposure from the hologram of step two; 
step four - reconstruct the contact print of step three 
with a laser and view the light diffracted into the nth 
order. A lens and aperture near the focal point are use-
ful in isolating the light of a particular order, as shown 
in the figure. The intensity variations viewed by the 
observer correspond to the intensity variations of 
conventional interferograms of the subject except that 
the phase sensitivity is now n times as great. A number 
of experimental tests of the two exposure double-
contact print process have been carried out with a 
(continued oerIeafl 
., 
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He-Ne gas laser. The best performance achieved so 
far is a factor of four, obtained by readout in the fourth 
order. Since each fringe in the interferogram results 
from an average taken over a great many fringes on 
the contact print, the lines are straighter and more 
uniform. Also, the new technique makes small phase 
shifts evident by visual inspection. 
Further enhancement can be obtained by displacing 
the original holograms during the contact printing 
step. For instance, if each original exposure is exposed 
n times during the contact printing step, with a dis-
placement of one nth the fringe spacing between each 
exposure, the resulting contact print will diffract n-
squared times as much light as the singly exposed 
contact print. The one limitation is that the thickness 
of the line must not cause an overlap of lines during 
the multiple printing.
Note: 
No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Headquarters 
National Aeronautics 
and Space Administration 
Washington, D.C. 20546 
Reference: 1369-1660 
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: Lee Opert Heflinger of

TRW Systems Group

under contract to

NASA Headquarters 
(HQN- 10348)
0/1 
Brief 69-10663	 Category 02


Fine-line sensitivity for holographic interferograms

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