Impact testing was often used for trouble-
shooting vibration problems in the field. The ability to visualize
mode shapes quickly and conveniently is especially important in
the field. Initially, the mode shapes were plotted by hand, which
was time consuming. Several plotting routines were developed
in which the mode shapes could be plotted in the field. Some
of these plotting programs would generate 3D images that could
be viewed with special training (crossed-eye 3D images). In gen-
eral, this was not completely satisfactory, since many people had
difficultly viewing these images; as a result, this technique was
quickly abandoned as a functional method. Later, there was the
development of an animated display for the HP 5451B system, in
which the mode shapes could be viewed directly on the display.
This was revolutionary.