When light strikes a photocathode inside the tube, an electron is emitted, which accelerates toward the first anode in a multiplier unit.
Interactive Flash Tutorial Photomultiplier Tubes Discover how photomultipliers work by amplifying the electrons generated by a photocathode exposed to a photon flux.
Any available light from the outside world enters the lens and strikes a special device called a photocathode.
The photocathode converts the few photons that strike it into a corresponding pattern of electrical energy called electrons.
The next evolution of the technology involved a more advanced photocathode that could further improve the amplification of light.