Abstract: The disclosed device provides illumination for a radiation dosimeter or other tubular instrument which is held directly in front of the eye for viewing. It can also be used as a compact flashlight for inconspicuous use such as for reading a map in a car or a program in a theater, and it can be carried by medical personnel to illuminate the eye of a patient. The illuminator comprises a light-emitting diode (LED) protruding through an annular spacer. The leads from the LED are bent around a battery comprising one or more button cells. The inner assembly comprising the LED, spacer and battery is held together by a plastic sleeve of shrink-wrap material, such as polyolefin. The inner assembly is enclosed in a thimble-shaped flexible plastic casing, which is adapted to slide onto the end of a dosimeter for permanent mounting. When the user squeezes the casing, the leads are brought into contact with the batteries, thereby completing the circuit, lighting the LED, and illuminating the dosimeter.