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The Physics of Pulse Oximetry
Learning how light is used to measure oxygen levels in blood.
Light as a Sensor
Have you ever wondered how a small clip on your finger can tell how much oxygen is in your blood? This device is called a pulse oximeter. It relies on a branch of physics called spectrophotometry, which essentially means measuring how much light is absorbed by different substances. In this case, the substance is hemoglobin, the protein in red blood cells that carries oxygen.
Red vs. Infrared
Pulse oximeters use two specific wavelengths of light: red light and infrared light. Oxygenated hemoglobin absorbs more infrared light, while deoxygenated hemoglobin absorbs more red light. By shining both lights through your finger and measuring how much reaches a detector on the other side, the device calculates the ratio of oxygenated blood. It sounds simple, but the engineering challenge is significant because the blood volume in your finger changes with every heartbeat.
Filtering the Pulse
Engineers design these devices to look for the 'pulsatile' nature of the blood flow. Because your arteries expand slightly with each beat, the amount of light reaching the sensor fluctuates. By ignoring the static light absorbed by skin, bone, and fat, and focusing only on the light absorbed by the pulsing arterial blood, the device gives you an accurate reading of your oxygen saturation levels in real-time.
Try this at home
Create a 'Light Sensor' model. Take a flashlight and place it on one side of your hand in a dark room. Place a white piece of paper on the other side. Notice how the color of the light passing through your skin is reddish. This is because your skin and blood absorb blue and green wavelengths while allowing red wavelengths to pass through. To simulate the sensor, use your phone's camera to record your finger while holding it against a bright light source. Use a simple video editing app to slow the footage down and observe the rhythmic 'flickering' in brightness, which mimics the arterial pulse detection process used in professional oximeters.
