Opto-aud
The opto-aud was used by Theela on Planet Two of the Taurean system to find suitable men from whom to drain their vital energy. It was activated by tonal control and revealed anything that it was asked. 

The opto-aud was a very advanced device that once, precisely, every 27.345 star years sent out a powerful subspace radio signal to transmit an audio-visual suggestion to those who intercepted it, at a range of no-less than twenty light years. ()

de:Opto-aud
Sensor technology
Communications technology