divergence ao through the master diffusor D'. Several plates 
were exposed with increasing energy to obtain densities: 
between 0,4 and 1,2 when developed in a 1:1 mixture of 5 gr. 
pyrogallol/l water and 60 gr. Na C04/1 water. Afterwards, 
bleaching is performed in a solution of 1,5 cc sulfuric acid 
and 3 gr. ammoniumbichromate/1 water. The same processing was 
used for the holographic interferograms. 
After processing, the plate giving minimal attenuation was 
selected and mounted in the pulsed laser beam, in the same 
position as it had in the He-Ne laser beam. 
The equipment is provided with a low power He-Ne laser (0,3..mW) 
for internal alignment purposes; it can hardly be used for 
external alignment. For easy positioning of the external opti- 
cal parts in our set-up, we used a white light projector moun- 
ted close to the output beam axis, with a divergence comparable 
to that of the laser beam. 
With the aid of an additional integrating photocell, the inten- 
sities of reflected object beam and reference beam were 
equalised. The total energy impinging on the plate was measured 
and the amplifier gains were adjusted until the required energy 
level was reached. 
4) EXPERIMENTS. 
8. Impact on a worn safety helmet. Figure 6 shows holographic 
interferograms of a shock wave propagating through a safety 
helmet and the face of the wearer, due to an impact of a 
300 gr. hammer dropped from a height of 10 cm. The laser was 
triggered by a current flowing via the hammer to a brass 
strip sticked on the helmet, at the moment of impact. 
Fig.B8a is taken at 5 ls interval and shows the propagation of 
the shock wave in the helmet; Fig.6b at 50 us interval ,shows 
severe increase in helmet displacement and deformation 
concentration around the ventilation holes and the attachment 
points of the inner structure; some displacement around the 
eyes is already visible. Finally, in Fig.Bc, taken at 250 Us 
interval, the helmet displacement is too large to leave 
fringes visible, while the shock wave passing over the face 
becomes clear. ; 
From these pictures one can conclude, from the engineering 
viewpoint, that the ventilation holes create spots of strain 
concentration in the helmet and that principally the region 
of the eyes are submitted to large movements long before the 
shock wave enters the lower skull. 
b. Firing an air gun. An air gun can be a cheap tool to create 
reliable impact loads in a mechanical laboratory (ref.14). 
The triggering of the laser occurs when the pellet breaks 
a 0,05 mm thick wire mounted over the barrel mouth. Fig.7a 
shows the displacement of the hand (interval 1 us). Note the 
rather homogeneous displacement of the fist and the slight 
displacement of fore finger and wrist. In Fig.7b interval 
one can see the displacement of the weapon substructure due 
to the release of the compression spring. The hand has moved 
further although the displacement of the wrist is still low. 
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