  Currently the standard light sensors for imaging atmospheric Cherenkov
telescopes are the classical photo multiplier tubes that are using bialkali
photo cathodes. About eight years ago we initiated an improvement program with
the Photo Multiplier Tube (PMT) manufacturers Hamamatsu (Japan), Electron Tubes
Enterprises (England) and Photonis (France) for the needs of imaging
atmospheric Cherenkov telescopes. As a result, after about 40 years of
stagnation of the peak Quantum Efficiency (QE) on the level of 25-27%, new PMTs
appeared with a peak QE of 35%. These have got the name super-bialkali. The
second significant upgrade has happened very recently, as a result of a
dedicated improvement program for the candidate PMT for Cherenkov Telescope
Array. The latter is going to be the next generation major instrument in the
field of very high energy gamma astrophysics and will consist of over 100
telescopes of three different sizes of 23m, 12m and 4-7m, located both in
southern and northern hemispheres. Now PMTs with average peak QE of
approximately 40% became available. Also, the photo electron collection
efficiency of the previous generation PMTs of 80- 90% has been enhanced towards
95-98% for the new ones. The after-pulsing of novel PMTs has been reduced
towards the level of 0.02% for the set threshold of 4 photo electrons. We will
report on the PMT development work by the companies Electron Tubes Enterprises
and Hamamatsu Photonics K.K. show the achieved results and the current status.
