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