The physics event, marked as a dot creates an electromagnetic shower inside the PbWO_4 crystal and light is produced. This light is detected by APDs (Avalanche Photodiodes). (Two APDs with a detection surface of 7×14 mm^2 are used per crystal.) The APDs create a charge proportional to the detected light. This charge is linearly converted to a voltage pulse by the so called APFEL (ASIC for PANDA Front End Electronics)