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Conventional transmission pulse oximetry is a noninvasive technique for the continuous monitoring of arterial oxygen saturation (SaO 2) from peripheral vascular beds such as the finger tip or earlobe. It is proposed to exploit the unique transparency of the ocular media to make reflectance pulse oximetry measurements on the retinal fundus. This technique potentially offers significant advantages over conventional pulse oximetry, primarily in the ability to monitor cerebral, as opposed to peripheral, oxygen saturation. An in vitro system has been developed to simulate the retinal circulation and ocular optics. This system consists of a flexible cuvette located in a model eye and an extracorporeal blood circuit to simulate arterial blood flow. The system was used to investigate the relationship between SaO 2 and the R/IR ratio in reflectance pulse oximetry. To enable in vivo measurements to be made, a standard haptic contact lens was modified to hold the pulse oximeter probe in front of the pupil. In a preliminary study, the lens was fitted to an awake volunteer and cardiac-synchronous signals were detected by the retinal pulse oximeter.