Black light

Black light was an unstable form of light. It was presumably dark in nature, hence its name, very unstable and difficult to contain. It may be found naturally in the universe but ultraviolet radiation can be converted into black light. The Doctor, who said it was not his area of expertise and called it "tricky stuff," said that some believed that if a black light explosion occurred the ensuing effects could cause a chain reaction that would roll on until all matter in the universe was exhausted, and dimensional transference that would threaten the stability of the universe.

Black light's only known use is as a power source. When it is used this way, it requires an extensive and extremely sophisticated system connected to an aerial for the conversion of ultraviolet radiation to Black light.

An un-named race from the Andromeda Galaxy used it to power their technology. It is not known whether its usage originates with this race or not. After the three Sleepers retreated to Earth, they established an underground hiding place maintained by the L3 robot Drathro and put themselves into suspended animation, both of which were powered by a Black light power system. A few years before the Doctor arrived the system had begun to breakdown and Drathro, only trained in installation and maintenance was unable to correct the problem, nor were his two assistances able to either. Despite Drathro’s stubbornness to have it shutdown plus other difficulties, the Doctor was able to prevent the first black light explosion from occurring. (DW: The Mysterious Planet)