Thaissen crystal

Thaissen crystals were found only on the planet Mimban. The crystals glowed lightly in close proximity of a force-sensitive individual--Blue normally, but red when in close proximity of a dark-sider. The illumination was so faint that it was only visible to the naked eye in darkness. The emporer saw that these crystals could be used to create a force detector so that common imperial troops could detect and hunt jedi as well. Over ten thousand force detectors were created to help hunt down and kill force-sensitives. Fortunately for the future of the galaxy, force detectors are far from perfect. They can only be used to scan a single creature at a time, have a range of mere centimeters, and are not always accurate.

SOURCE: Star Wars - Power of the Jedi Sourcebook