[1] Aoyagi received an undergraduate degree in electrical engineering from Niigata University in 1958.
Similar ideas were developed slightly later by Masaichiro Konishi and Akio Yamanishi of Minolta.
[2] Nihon Kohden submitted an application for a patent on the resulting device in 1974, which named Aoyagi and his colleague Michio Kishi (who helped create a pilot model) as co-inventors.
[2] In 2007, World Health Organization listed pulse oximeter as an essential device for Surgical Safety Checklist for Patient.
[3][4] Nihon Kohden moved Aoyagi to a desk job in 1975, and only brought him back into their research group ten years later.
He developed a device, which he called a "pulse spectrophotometer", and which used these ideas to measure the diffusion of a dye injection in the bloodstream.