In 1976, Physicists from University of Utah were the first to detect fluorescence light from cosmic ray air showers. Three prototype modules were used in a test at Volcano Ranch near Albuquerque, New Mexico.
Each prototype module contained a 1.8m diameter mirror for light collection, with 14 PMT's at the focal plane. Each PMT covers a solid angle about 0.008 steradians (~5° by 5°) in the sky. The large mirrors provided a 20-fold increase in the light collection area over that of the lenses used in the Cornell detector. The clear desert air also provided much improved visibility over the Cornell experiment.
Volcano Ranch was chosen for the test because the site also hosted a large ground array which had been operated since 1958 by a group from MIT under the leadership of John Linsley and Livio Scarsi. On Thanksgiving Night in 1976, the prototype fluorescence detectors were able to observe air showers in coincidence with the ground array.
It should be noted that the Volcano Ranch Ground Array itself was one of the great pioneering efforts in UHE cosmic ray physics. It was known as the "Desert Queen" within MIT. In 1961, the Volcano Ranch Array reported the detection of a cosmic ray air shower with a measured energy of 1020 eV. The significance of such a cosmic ray shower was not realized until after the discovery of the Cosmic Microwave Background Radiation (CMBR) in 1965 by Penzias and Wilson.