The CLEO II calorimeter consists of 7800 thallium-doped Cesium Iodide (CsI) crystals like the one shown here, each CsI block approximately 5 cm square by 30 cm long. The barrel portion of the calorimeter contains 6144 arranged so that all the crystals point at or near the interaction vertex, covering 71% of the solid angle. Charged particles entering the detector ionize atoms in the scintillator, which deexcite by emitting light. High energy photons are detected by the showers of secondary charged particles they emit in the detector. Four silicon photodiodes mounted on a lucite window on the rear face of each crystal convert the scintillation light into electrical signals which are then shaped, amplified, digitized and analyzed by computer. Light from several adjacent crystals must be analyzed using clustering algorithms to account for all the energy deposited by a given particle.