SIPD stands for "self-indicating pocket dosimeter". SIPD's are issued to people prior to entering a high radiation area
. These are to supplement the TLD
normally worn on the belt, and is placed next to it, on the front of the body. A SIPD looks like a little black tube, about three inches long, and about as big around as one of those really thick writing pens.
A SIPD is used to measure the amount of radiation a person receives. From time to time, as you are being irradiated, you look through the little tube at a light, so you can see the little scale showing how many millirem you have received. Common scales are 0 to 200 millirem, and can be as much as 0 to 5 REM. If your reading gets close to your limit for allowed radiation exposure, you must leave the high radiation area.