
457Spaceborne Applications
Chapter 16
and Riede 2005). e signal to noise ratio of the collinear scatter probing can be greatly
improved with lock-in detection, necessitating chopping of the HeNe laser beam. For
optimum sensitivity, the HeNe laser is focused down to an diameter on the sample,
which will cover at least the assumed damage area. In practice, an HeNe beam diame-
ter of half of the diameter of the Nd:YAG beam is recommended. An increase in ampli-
tude of scattered HeNe light measured with a photodiode aer passing a lter stack
indicates damage having occurred either on the sample front or back surface, or in the
bulk. As soon as t