In photoplethysmography, which kind of light is detected by the sensor to quantify blood volume changes?

• A. Transmitted light
• B. Reflected light
• C. Absorbed light
• D. Ultrasound

Answer: (B)

Photoplethysmography detects how much light is available after it has interacted with tissue, and the heartbeat-opposed changes in blood volume alter light absorption. The sensor picks up light that returns from the tissue—that backscattered or reflected light carries the pulsatile information. As blood volume rises with each pulse, more light is absorbed and less returns to the detector; as it falls, more light is detected again. This is why the detected signal comes from reflected light. Transmitted light is relevant in some setups, but in common reflectance configurations the key signal arises from backscattered light. Absorbed light itself isn’t measured directly, and ultrasound isn’t part of photoplethysmography.