Hyperbilirubinemia in the newborn (neonatal jaundice) may cause irreversible brain damage if plasma concentrations of bilirubin exceed the number of binding sites on albumin and other blood components. Phototherapy or exchange transfusions to prevent the excessive rise in concentration of the pigment should be instituted in appropriate clinical situations. In phototherapy, the jaundiced infant is exposed to visible light containing the wavelength (about 450 nm) bilirubin will absorb. Because bilirubin is quite photolabile and will readily isomerize in vivo, it is rapidly converted to excretable forms. The effectiveness of this therapy, however, depends upon the maintenance of adequate radiant flux in the required wavelength. Energy output and spectral distributions of phototherapy lamps must be measured. The long-term effects of irradiation of newborn infants over several days are not yet known.