The photomultiplier tube (right) is used to measure the amount of light emitted by the CsI crystals when struck by a high energy particle. The light is converted to a tiny electrical pulse, which is amplified several thousand times by the tube. The digital board on the left (DPMT) turns the size of the pulse into numbers, which are read out to our computers. The number can then be related back to the energy of the particle that hit the calorimeter.
There are 3100 independent CsI crystals in the calorimeter, each with a photomultiplier and DPMT.