A pulse oximeter is a medical device that clips onto your fingertip (or other body part) to measure the oxygen saturation of your blood (SpO2) and your pulse rate. It is a painless and non-invasive procedure.The pulse oximeter uses light waves to estimate the amount of oxygen-carrying hemoglobin in your blood. Hemoglobin is the protein in red blood cells that carries oxygen throughout your body. When hemoglobin is carrying oxygen, it is bright red. When it is not carrying oxygen, it is a darker shade of red. The oximeter shines two wavelengths of light through your fingertip: Red wavelength Infrared wavelength The amount of light that passes through your fingertip is determined by how much oxygen is attached to your hemoglobin. The oximeter measures the difference in how much of each wavelength of light is absorbed by your blood. Based on this difference, the oximeter can estimate the percentage of oxygen-saturated hemoglobin in your blood. This percentage is your SpO2.