  A stable reference light source based on an LED (Light Emission Diode) is
presented for stabilizing the conversion gain of the opto-electronic system of
a gamma- and fast-neutron radiographic and tomographic imaging device. A
constant fraction of the LED light is transported to the image plane of the
camera and provides a stable reference exposure. This is used to normalize the
images during off-line image processing. We have investigated parameters
influencing the stability of LEDs and developed procedures and criteria to
prepare and select LEDs suitable for delivering stable light outputs for
several 100 h of operation.
