Carbon oscillator

A carbon oscillator was a powerful laser beam.

In the 1900s, the Fourth Doctor turned Fang Rock lighthouse into an amplified carbon oscillator. Leela suggested the use of the lighthouse and he used a diamond to convert the carbon arc beam. The laser destroyed a Rutan mothership. The light caused pigmentation dispersal in Leela, changing the colour of her eyes. (TV: Horror of Fang Rock)