Most of the Ultra High Energy Cosmic Ray ( UHECR ) experiments and projects ( HiRes , AUGER , TA , EUSO , TUS , … ) use air fluorescence to detect and measure extensive air showers ( EAS ) .
The precise knowledge of the Fluorescence Light Yield ( FLY ) is of paramount importance for the reconstruction of UHECR .
The MACFLY - Measurement of Air Cherenkov and Fluorescence Light Yield - experiment has been designed to perform such FLY measurements .
In this paper we will present the results of FLY in the 290-440 nm wavelength range for dry air and pure nitrogen , both excited by electrons with energy of 1.5 MeV , 20 GeV and 50 GeV .
The experiment uses a ^ { 90 } Sr radioactive source for low energy measurement and a CERN SPS e ^ { - } beam for high energy .
We find that the FLY is proportional to the deposited energy ( E _ { d } ) in the gas and we show that the air fluorescence properties remain constant independently of the electron energy .
At the reference point : atmospheric dry air at 1013 hPa and 23 ^ { \circ } C , the ratio FLY / E _ { d } =17.6 photon/MeV with a systematic error of 13.2 % .