fringes that originally existed. From the angles that the
sandwich hologram had been tilted to transform Fig. 7
into Fig. 8, it was also possible to calculate magnitude,
direction, and sign of the knee tilt, in spite of the fringes
originally being so closely spaced that they could not be
properly counted. The speckles of Fig. 8 are larger than
those of Fig. 7 for the following reason. When the
sandwich was tilted, the fringes started to move when
the point of observation was changed. Therefore the
aperture of the camera that was used to make the pho-
tograph had to be so small that the whole used area of
the lens sees the fringes in the same position. Because
of the small aperture the speckles are large in Fig. 8.
There exist effective means to get around this problem,
but in our experiments we were satisfied if the quality
was such that we could count the fringes.
It was, of course, also possible to study the deforma-
tion of the head, but then it was necessary to tilt the
sandwich hologram in the opposite direction (Fig. 9).
The fringes on the reference surface could, however, not
be counted because of the large speckle size.
Trying to find the limits of the sandwich method we
doubled the force difference applied to the machine and
made a third sandwich hologram. This time we used
a thicker extra glass plate to separate the hologram
plates so that the distance between the emulsions was
some 9 mm. The quality of this hologram and the res-
olution of the fringes were not reduced by this large
separation. The result without any sandwich tilt is seen
in Fig. 10. No fringes are seen on the knee, but it was
still possible to eliminate the about 250 fringes that had
theoretically originally existed there to study the de-
formation which had resulted in about twenty fringes
(Fig. 11). In later experiments we have managed to
eliminate up to 500 fringes, and we have not yet found
the limit.
VI. Results of the Second Experiment
When I presented some of my holographic studies of
a milling machine at the 25th General Assembly of the
International Institution for Production Engineering
Research in Germany, there was doubt that holography
really could be used so simply directly on the floor of a
workshop. It was mentioned that these methods could
perhaps work but only on the stable Swedish granite
rock. To test the statements in my paper I was invited
by E. Matthias to repeat my experiments during a 1-
month stay at the Department of Machine Tools and
Production Engineering at the Swiss Federal Institute
of Technology (ETH) in Zürich, Switzerland. It was
a great pleasure for me to accept the invitation, and in
January 1976 I arrived in Zürich with just one piece of
holographic equipment, our special hologram holder.
For the experiment a Swiss plain horizontal kneetype
milling machine, Oerlikon model MN2H, was selected.
It was about 1.7 m high and stood directly on the ground
floor in a workshop which was only about 7 X 10 m.
Because of the limited space we could not without dif-
ficulty get further from the machine than 4 m, and the
spatial filter had to be placed within half a meter of a
window. ETH is situated in the middle of Zürich, and
the stability problems appeared to be rather severe
2528 APPLIED OPTICS / Vol. 16, No. 9 / September 1977
because of the heavy traffic of lorries, busses, and trams
at the street corner outside our working place. The
floor was covered with cork tiles, a material we found
rather difficult to work on because it retained previous
weight depressions and dimensions for a long time after
a load had been changed. Thus it was necessary to wait
a few hours after changes in the holographic setup be-
fore a hologram could be exposed successfully.
For the illumination we used a Spectra-Physics argon
laser model 165 which was kindly lent to us for 1 week
by Brown Boveri Research Center in Baden, Switzer-
land. We used an output of 0.5 W in the green
514.4-nm wavelength. After some adjustments of the
etalon we could prove with a simple interferometric test
that the coherence length was more than 2 m and
therefore did not put any tight restrictions on our ho-
lographic configuration. The laser was placed along the
wall on an ordinary table, and the beam was deflected
90° by a mirror and directed onto a spatial filter which
was fixed to a very stable, heavy steel table (weighing
some 300 kg) onto which we also fixed the plate holder.
The reference mirror was placed about half a meter
from the milling machine on a stable stand that weighed
around 100 kg. All the heavy equipment used were
such pieces that could be found in any well equipped
heavy duty machine shop.
The milling machine itself had a green shiny finish,
and a definite desire was that it not be repainted or
treated in any special way. Therefore that those parts
of the machine that produced a direct reflex got much
brighter in the hologram than other parts was un-
avoidable.
The holograms were exposed during the night be-
cause the activity in the institute was much lower and
most of the traffic had stopped. The force deforming
the machine was produced simply by turning the screw
Fig. 12. The deformation of a horizontal milling machine was studied
using double exposure holography. Temperature gradients in the
air produced fringes on the reference surface at the lower left.
