A pulse oximeter is a meical device that indirectly measures the oxygen saturation of a patient's blood (as opposed to measuring oxygen saturation directly through a blood sample) and changes in blood volume in the skin, producing a photoplethysmograph. It is often attached to a medical monitor so staff can see a patient's oxygenation at all times. Most monitors also display the heart rate. Portable, battery operated pulse oximeters are also available for home blood oxygen monitoring. The original oximeter was made by Milliken in the 1940s. In the 1970s designs were based on Aoyagi's ability to relate arterial hemoglobin saturation to vessel bed pulsation; the prototype for future pulse oximeters was invented in 1978 by Dr. William New. The device did not see wide adoption in the United States until the 1980's.