Precise Measurement of the Absolute Yield of Fluorescence Photons in   Atmospheric Gases
  We have performed a measurement of the absolute yield of fluorescence photons at the Fermilab Test Beam. A systematic uncertainty at 5% level was achieved by the use of Cherenkov radiation as a reference calibration light source. A cross-check was performed by an independent calibration using a laser light source. A significant improvement on the energy scale uncertainty of Ultra-High Energy Cosmic Rays is expected. 
AIRFLY Collaboration: M. Ave, M. Boh\'a\v{c}ov\'a, K. Daumiller, P. Di   Carlo, C. Di Giulio, P. Facal San Luis, D. Gonzales...
astro-ph.IM astro-ph.HE