An observation by an English nurse in 1956 led to the discovery that visible light could lower serum bilirubin levels in newborn infants, and subsequent research showed how photons of light energy are absorbed by the bilirubin molecule converting it into isomers that are readily excreted by the liver and the kidney. Understanding the dose-response effect and other factors that influence the way light works to lower bilirubin levels has led to the effective use of phototherapy and has eliminated the need for exchange transfusion in almost all jaundiced infants.