The “Air Fluorescence” technique uses the atmosphere as a calorimeter to detect charged particles showers. A brief description of detectors based on this technique and how they are used to detect cosmic rays will be given in this laboratory session. An array of photomultiplier tubes will be calibrated and a method will be developed to measure the photon flux in individual PMTs by counting photoelectrons read out by a charge digitizing system. As an example of the application of this technique, we reconstruct the longitudinal profile of a real particle showers detected by a fluorescence telescope. © 2003 American Institute of Physics