The photomultiplier tube(PMT) is the most sensitive photoelectric sensor available at this time, capable of detecting the lowest light intencity possible, a single photon. It consists of a vaccum tube with a light sensitive cathode at one end. When a photon hits this cathode, it will emit an electron. This electron try to fly in a straight line through the tube, but the dynode(Dy1)'s lower electron charge(more positive) will attract it. When the electron strikes the dynode, more secondary electrons are released. These electrons fly off at an angle towards the anode end of the tube. They will be attracted by Dy2, and a new electron multiplication will happen. This continues until the electrons finally hits the anode. By measuring the bursts of electrons an indication of the number of photons can be computed.
Simple schematic of a PMT with four dynodes.

      |                                  |
      #Cathode                   anode | |
      #                                |-+-.
      #                                | | |
      #                                | || |R6
      #    Dy1   Dy2   Dy3   Dy4         || |
      #    ___   ___   ___   ___         | T
      |     T     T     T     T          | |
      |-----+-----+-----+-----+----------' |
      |  _  |  _  |  _  |  _  |  _         |
      |  R1    R2    R3    R4    R5        |
      |                                   ---
    High                                  GND

A normal PMT have ten or more dynodes.

An example of the multiplication:
If we have a tube with 12 dynodes, where each have a four times multiplication. We will have:
This means that one photon triggers the flow of over four million electrons.

The tube must be cooled or you might get false readings (due to photons from heat(infrared light)). The tubes accuracy may also be affected by magnetic fields, since magnetic fields deflects the electrons, and may cause them to miss the dynodes. Also the high voltage supply must be very stable, since fluctuation might affect the results.

The PMT is expencive and relatively fragile but is much used in applications where high sensitivity is needed.The PMTs are available for wavelengths between 200nm and 1100nm(from UV to IR).