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 darksider. The illumination was so faint that it was only visible to the naked eye in darkness.
The Emperor 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 remaining Jedi of the galaxy, force-detectors were far from perfect. They could only be used to scan a single creature at a time, had a range of mere centimeters, and were not always accurate.