Design and performance of LED calibration system prototype for the lead   tungstate crystal calorimeter
  A highly stable monitoring system based on blue and red light emitting diodes coupled to a distribution network comprised of optical fibers has been developed for an electromagnetic calorimeter that uses lead tungstate crystals readout with photomultiplier tubes. We report of the system prototype design and on the results of laboratory tests. Stability better than 0.1% (r.m.s.) has been achieved during one week of prototype operation. 
V.A.Batarin, J.Butler, A.M.Davidenko, A.A.Derevschikov,   Y.M.Goncharenko, V.N.Grishin, V.A.Kachanov, V.Y.Khodyrev, A.S.Kons...