  For most of the Ultra-High-Energy-Cosmic-Ray (UHECR) experiments and projects
(HiRes, AUGER, TA, JEM-EUSO, TUS,...), the detection technique of Extensive Air
Showers is based, at least, on the measurement of the air-fluorescence-induced
signal. The knowledge of the Fluorescence-Light Yield (FLY) is of paramount
importance for the UHECR energy reconstruction. The MACFLY experiment was
designed to perform absolute measurements of the air FLY and to study its
properties. Here, we report the result of measurement of dry-air FLY induced by
50 GeV electromagnetic showers as a function of the shower age and as a
function of the pressure. The experiment was performed at CERN using a
SPS-electron-test-beam line. The result shows the air FLY is proportional to
the energy deposited in air (E_d). The ratio FLY/E_d and its pressure
dependence remain constant independently of shower age, and more generally,
independently of the excitation source used (single-electron track or air
shower).
