A photoelectric device applied externally to a body site, e.g., a fingertip, ear lobe, the bridge of nose, a toe, or the bridge of the foot of a paediatric or adult patient for the transcutaneous measurement of haemoglobin oxygen saturation (SpO2) in arterial blood using light detection. The signals, typically produced by light-emitting diodes (LEDs) and a receiving detector, are received by an oximeter to which this device is connected. The oximeter displays the SpO2 values and may calculate/display other parameters, e.g., electrocardiogram (ECG). This is a reusable device.