In the earlier description of the energy conversion matrix, we saw that
we could go through the energy transformation process repeatedly until
we obtained the kind of energy we wanted. This is exemplified in a type
of nuclear battery which uses the so-called _double conversion_
approach. First, the high-velocity nuclear particles are absorbed in a
phosphor which emits visible light. The photons thus produced are then
absorbed in a group of strategically placed solar cells, which deliver
electrical power to the load. Although efficiency is lost at each energy
transformation, the double conversion technique still ends up with an
overall efficiency of from 1 to 5%, an acceptable value for power
supplies in the watt and milliwatt ranges.