The photon detector is then located at some distance (usually about 10 cm) behind the radiator, allowing the cone of light to expand and form the characteristic ring-shaped image. Such a proximity-focusing RICH is installed in the ALICE experiment. ALICE HMPID's momentum range is up to 3 GeV for pion/kaon discrimination and up to 5 GeV for kaon/proton discrimination. It is the world's largest caesium iodide RICH detector, with an active area of 11 m2. A prototype was successfully tested at CERN in 1997 and currently takes data at the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory in the US. Calorimeters Calorimeters measure the energy of particles, and determine whether they have electromagnetic or hadronic interactions. Particle Identification in a calorimeter is a destructive measurement. All particles except muons and neutrinos deposit all their energy in the calorimeter system by production of electromagnetic or hadronic showers. Photons, electrons and positrons deposit all their energy in an electromagnetic calorimeter. Their showers are indistinguishable, but a photon can be identified by the non-existence of a track in the tracking system that is associated to the shower. The photons (particles of light), like the light emitted from a hot object, tell us about the temperature of the system. To measure them, special detectors are necessary: the crystals of the PHOS, which are as dense as lead and as transparent as glass, will measure them with fantastic precision in a limited region, while the PMD and in particular the EMCal will measure them over a very wide area. The EMCal will also measure groups of close particles (called "jets") which have a memory of the early phases of the event. Photon spectrometer PHOS is a high-resolution electromagnetic calorimeter installed in ALICE to provide data to test the thermal and dynamical properties of the initial phase of the collision. This is done by measuring photons emerging directly from the collision. PHOS covers a limited acceptance domain at central rapidity. It is made of lead tungstate crystals, similar to the ones used by CMS, read out using Avalanche Photodiodes (APD). When high energy photons strike lead tungstate, they make it glow, or scintillate, and this glow can be measured. Lead tungstate is extremely dense (denser than iron), stopping most photons that reach it. The crystals are kept at a temperature of 248 K, which helps to minimize the deterioration of the energy resolution due