medical field where comparison is an usual interpretation
method.
Looking at the symmetry of the pattern will often give good
results but one should remember that the fringes do not form
on the object but in the space in front of it. Ihis gives
always asymetrical fringes except when the vector is normal
to the mean plane of the test object. This requires an
adapted set-up.
It becomes clear that qualitative interpretation is merely
based on experience but the study of some practical examples
can largely help to start (ref.6,7,8,89).
c. Time lapse. As mentioned in the paragraph on the working
principles, there exists a time lapse between the trigger
pulse and the first laser flash of between 800 us and 1,5 ms,
depending on the preset pulse interval. It must be kept in
mind while interpreting displacement patterns at what moment
the recorded action has occured with respect to the moment of
triggering. That time lapse can easily be measured with the
aid of a storage oscilloscope.
To make things clear we will use the test on the safety
helmet as an example.
In this case the time lapse (t, or t. on Fig.22). turned out to
be 1,38 ms; and moment of impact and triggering were equal.
This means that the first picture (Fig.8a) is the situation
1,36 ms after impact superimposed on one 5 ys later.
If measurements have to be done closer to the moment of
impact, other trigger techniques would be required, e.g. the
electromagnetic release of a weight dropping on the helmet
could be used. The release signal could be converted to the
trigger signal and retardation arranged by changing the
height. For equal impact energy the weight would have to be
adapted to the height. ;
This example. shows the necessity for inventive reflections
on the trigger system, that must be adapted for every
particular application.
d. Image quality. It can be seen from the pictures that the
fringe sharpness is not so high as might be expected. The
scatter plate reduces the spatial coherence of the laser
beam. This is of less importance for the object beam, but
when the reference beam has reduced spatial coherence, the
sharpness of the hologram and hence of the fringes decreases.
If sharp fringes are required - which is not strictly
necessary for this kind of investigation - a more elaborated
set-up will ne needed, as shown in Fig.10. The reference
beam is deflected directly outward onto the hologram plate
via additional mirrors M8 and MS. The reference beam lens L3
is dismantled from its original place and mounted in the
reference beam path. Eventually other focal length lenses
can be substituted as the divergence angle has to be adapted
to the intensity required on the hologram plate H. The
difference in path length must be kept within the coherence:
length,approximately im for this laser.
105