Source : Webster's Revised Unabridged Dictionary (1913)

Iris diaphragm \I"ris di"a*phragm\
   An adjustable diaphragm, suggesting the iris of the eye in
   its action, for regulating the aperture of a lens, consisting
   of a number of thin pieces fastened to a ring. It is used in
   cameras and microscopes.

Source : WordNet®

iris diaphragm
     n : diaphragm consisting of thin overlapping plates that can be
         adjusted to change the diameter of a central opening
         [syn: {iris}]