Possibility of Optical Cooling of Ruby

The possibility of cooling by an optical process of ruby is pointed out. When C^ ions in ruby are optically pumped by photons with a given energy and emit photons with higher energies, the lattice energy can be extracted as light, so that a cooling effect appears. If some possible heating effects did not overcome this cooling effect, the temperature would be actually reduced. Cooling rates and cooling capacities are given. If we could have light input powers 10^ Watt or 10^ Watt for optical pumping, it might be possible to cool from 80°K or 20°K to temperatures below 1°K, respectively. Technical problems and possible heating effects are discussed