The bright colour of haemoglobin has, from the very beginning, played a significant role in both the investigation of this compound as well as in the study of blood oxygen transport. Numerous optical methods have been developed for measuring haemoglobin concentration, oxygen saturation, and the principal dyshaemoglobins in vitro as well as in vivo. Modern applications include pulse oximeters, fibre optic oximeters, multiwavelength haemoglobin photometers ('co-oximeters') and instruments for near infrared spectroscopy in vivo. Knowledge of the light absorption spectra of the common haemoglobin...