A method for non invasive determination of oxygen saturation of blood within a deep vascular structure of a human or animal patient comprising locating on skin of the patient in a vicinity of the deep vascular structure of interest emitter and receiver elements of a light oximeter device wherein optimal location of said elements is achieved through matching of a plethysmography trace obtained from the oximeter device to known plethysmography characteristics of the deep vascular structure of interest wherein the emitter element emits light at wavelengths of from about 1045nm to about 1055nm and from about 1085nm to about 1095nm and wherein oxygen saturation is determined from a ratio of light absorbed at these two wavelengths by haemoglobin in blood within the vascular structure of interest.
