Positioned with the diaphragm blades in an image plane, these enable the experimenter to accurately define the area of interest reaching the detection device(s). Twin levers provide independent adjustment in X and Y planes, allowing definition of a rectangle of arbitrary size and proportions. The holder allows movement and rotation of the aperture defined by the diaphragm. The diaphragm should be tested in conjunction with the infrared viewing system as described later in this section.
