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.