[Source] A Force detector or Jedi reader was a device used by Emperor Palpatine's Jedi hunters to identify Force-sensitive individuals. It consisted of two sheet crystal paddles connected to a control pack powered by rare thaissen crystals found only on Mimban. When an individual was placed between the paddles, the device would produce a hologram of the person in question. If the individual was Force sensitive, a blue corona would surround the hologram. If the individual was tainted by the dark side then the hologram would have red marks in the blue corona.