ˈfōˌtän noun
( -s )
Etymology: phot- + -on
1. : a massless elementary particle with one quantum unit of spin that is the carrier of radiant energy (as light or X rays)
2. : a unit of intensity of light at the retina equal to the illumination received per square millimeter of a pupillary area from a surface having a brightness of one candle per square meter — called also troland