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PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1975 
Illumination | — 1 v 
c—À o 
Hologram 
2% recording 
BC ! 
| Reference 
1 reversed 
Fringe Camera Aperture Hologram 
pattern after processing 
photogr 
Fic. 5. Holographic recording for measurement. 
ferometer such as that just described. Direct 
measurement of a large number of points in 
the volume by this technique would be dif- 
ficult and tedious, however, and if the condi- 
tions of strain could not be maintained for the 
long time necessary, the relationship of meas- 
urements would be invalidated. It is, there- 
fore, necessary to consider a suitable method 
of recording the phase relationships in the 
wavefronts which give the coherence 
measuring procedure its advantage. 
HoLoGRAPHIC RECORDING FOR MEASUREMENT 
The essential in holographic recording is 
the provision of a reproducible reference 
wave which falls on the photosensitive mate- 
rial at the same time as the wave from the 
object to be recorded, and is coherent with it, 
so that interference occurs, and the interfer- 
ence pattern is recorded to make the holo- 
gram. After processing, illuminating the 
hologram with a replica ofthe reference wave 
will reconstruct the wave pattern from the 
object, making a reconstruction of the object 
visible or measurable. 
In the case ofthe spheres embedded in the 
transparent medium, the reference wave may 
be the unused part of the incident wave, and 
the recording is similar to the “in-line” re- 
cording first demonstrated by Gabor’. No 
focusing optical system is used, and usable 
records of all spheres in all planes of depth 
are obtained (Figure 5b). Reconstruction 
with exactly the same geometry and 
wavelength of reference beam, but incident 
in the reverse direction on the hologram, pro- 
vides the means of locating and measuring all 
the secondary point sources by suitable opti- 
cal means. 
An even more powerful technique is avail- 
able for measuring individual displacements. 
Two successive exposures are made on the 
same hologram plate at different loadings. If 
the reconstructed wavefronts are focused so 
as to produce real images of the two succes- 
sive positions of one of the secondary 
sources, the two reconstructions, being coher- 
ent, will produce an interference pattern of a 
simple form. The spacing and orientation of 
this pattern enables the displacement of the 
sphere to be calculated (Figure 5c). 
THREE-DIMENSIONAL DISPLACEMENTS IN THE 
SURFACE OF SOLID BoDIEs 
Very similar principles have been 
employed for measurement from holograms 
of solid bodies®. Figure 6 shows one form of 
  
Object illumination 
  
x shift 0.019 um 
z shift 0.5 mm 
x shift 0,019 mm z shift 0.5 mm 
(The intersectíon of the diaphragm - lens axis 
with the hologram is shown by cross lines). 
Fic. 6. Holographic measurement of dis- 
placements in a solid body.