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The absolute photon yield of scintillation light produced by highly ionizing particles in pure CF4 has been measured using a photosensitive Gas Electron Multiplier (GEM) detector. The detector consists of two standard GEMs and a CsI coated GEM which acts as a photocathode that is sensitive to the 160 nm scintillation light produced in CF4. The light yield was determined in terms of the number of scintillation photons emitted into a 4π solid angle produced per MeV of energy deposited in the gas by a 5.5 MeV alpha particle and found to be 314 ±15 photons per MeV. The quantum yield was determined using a fitting method to determine the number of photoelectrons from the measured pulse height distribution, and by an independent method using the measured gain of the GEM detector. The effect of scintillation light in CF4 on the performance of Cherenkov detectors, such as the PHENIX Hadron Blind Detector (HBD) at RHIC, is also discussed.