A plasma processing chamber optical temperature sensor is disclosed. The plasma processing chamber optical temperature sensor includes a light source, a light detector, and a means for transmitting light through a wall of a plasma processing chamber. An optical temperature sensing element is thermally coupled to a plasma processing chamber component within the plasma processing chamber. The optical temperature sensing element includes a monolithic crystalline phosphor element configured to be excited by light from the light source and to emit light back to the light detector indicative of a temperature of the monolithic crystalline phosphor element.