An apparatus (100) senses a degree of cleanliness of a plasma reactor having a chamber (102) containing a plasma (103) that emits light (104) during a process conducted in the chamber (102). The apparatus (100) also has a light sensing element (180), configured to sense an intensity of the light (104) emitted by the plasma (103) after the light (104) passes through a film (135) that accrues in the chamber (102) during the process, and to provide a light intensity indication signal, and an electronics assembly (170) configured to receive the light intensity indication signal and to provide an indication of the degree of cleanliness of the plasma reactor.