Thaissen crystals were exclusively located on the world of Mimban, a planet. When a Force-sensitive person was nearby, these crystals emitted a soft light; it was typically blue, but turned red when a darksider was close. The glow was so subtle that it could only be observed without assistance in the dark.
Recognizing their potential, the Emperor planned to utilize these crystals in the construction of a Force detector. This would enable ordinary Imperial soldiers to identify and pursue Jedi. To aid in the pursuit and elimination of Force-sensitive individuals, over ten thousand Force-detectors were manufactured. The remaining Jedi in the galaxy were lucky, because the force-detectors had significant limitations. Their scanning capability was limited to one individual at a time, their effective range was only centimeters, and their accuracy was questionable.
- Power of the Jedi Sourcebook
- Droids, Technology and the Force: A Clash of Phenomena on Hyperspace (article) (content obsolete and backup link not available)
- Jedi Academy Training Manual
- The Imperial Warlords: Despoilers of an Empire, Part 3 on StarWars.com (article) (content now obsolete; backup link)