Measurement of absolute fluorescence yield in air by AIRFLY

Fluorescence detection of UHECR air showers is a widely used technique. Precise knowledge of the air fluorescence yield in dependence on the atmospheric conditions in the region of production propagates into the primary cosmic ray energy reconstruction. Relative measurements, such as dependence on pressure, temperature, humidity, energy of the secondary particles and nitrogen emission spectrum have already been studied in the previous measurement campaigns of the experiment AIRFLY. A precise measurement of the absolute fluorescence yield of the 337 nm line - the most prominent line in the nitrogen spectrum - has been recently performed at the Fermilab MTest beam facility. The experimental method and first results are presented.