Calorimeters measure the total energy of a particle passing through it. For it to do this, the particle has to be stopped completely, which then decays in the form of charged particles and photons that can then be detected in any kind of particle detector. They are made out of layers of absorbers, such as iron, lead or uranium as used in the ZEUS. This is where the active detector then detects the particles.

The Uranium scintillator calorimeter CAL in ZEUS has a linear and equal response for electrons and hadrons. The calorimeter has a response calibrated with an accuracy of about 1%. It weighs about 700 tons. It is subdivided into 80 modules that form the forward, central and back calorimeters. Absorbers are weaved in between the detector plates. The scintillators along with the absorbers form the calorimeter modules. The light generated by the charged particles passing through the scintillators is collected by the plastic guides transported to photo-multipliers which then converts them to electrical signals which the allows them to be measured.