[2] During his doctorate studies in Cambridge he built a dual-wavelength colorimeter for blood oxygen level measurements.
[4] Unable to return to Cambridge, he accepted an unpaid laboratory appointment at the University of Pennsylvania and concentrated on bioluminescence research.
[4] According to Adrian, pilots regularly lost consciousness during high-altitude dogfights, and needed "an oxygen delivery system with a demand valve responsive to altitude and activity".
The oxygen supply system, developed by Bendix Corporation, relied on the oximeter as the primary sensor in its feedback loop.
[4] The earpiece of Millikan's oximeter (an open photocell-lamp assembly manufactured commercially by Coleman Electric) contained an incandescent light bulb, a set of red and green filters, and a selenium barrier level photocell.
[8] In 1938, Millikan married Frances Clare Leigh-Mallory, daughter of George Mallory, the mountaineer who died while climbing Mount Everest.