Pulse oximetry is based on a relatively new concept, using the pulsatile variations in optical density of tissues in the red and infrared wavelengths to compute arterial oxygen saturation without need for calibration. The method was invented in 1972 by Takuo Aoyagi, a bioengineer, while he was working on an ear densitometer for recording dye dilution curves. Susumu Nakajima, a surgeon, and his associates first tested the device in patients, reporting it in 1975. A competing device was introduced and also tested and described in Japan. William New and Jack Lloyd recognized the potential importance of pulse oximetry and developed interest among anesthesiologists and others concerned with critical care in the United States. Success brought patent litigation and much competition.