'Iris diaphragm' definitions:

Definition of 'iris diaphragm'

noun

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

From: GCIDE

Iris diaphragm \I"ris di"a*phragm\ An adjustable diaphragm, reminiscent of the iris of the eye in its action, by which the diameter of an approximately circular opening may be controlled, as for regulating the aperture of a lens; it consists of a number of movable thin curved plates fastened at regular intervals around the inside of a ring, the positions of which are simultaneously adjusted by a single knob on the outside of the lens. It is used in cameras and microscopes. [Webster 1913 Suppl. +PJC]