Air Fluorescence Relevant for Cosmic-Ray Detection - Review of   Pioneering Measurements
  Cosmic rays with energies exceeding $10^{17}$ eV are frequently registered by measurements of the fluorescence light emitted by extensive air showers. The main uncertainty for the absolute energy scale of the measured air showers is coming from the fluorescence light yield of electrons in air. The fluorescence light yield has been studied in laboratory experiments. Pioneering measurements between 1954 and 2000 are reviewed. 
Fernando Arqueros, Joerg R. Hoerandel, Bianca Keilhauer