LEDs & photodiodes
Diodes that emit light, and diodes that detect it — paired across a fingertip, exactly how a pulse oximeter reads SpO2.
In one line
An LED is a diode that emits light when current flows; a photodiode produces current when light hits it. Point one at the other across living tissue and you can measure what flows through.
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- An LED releases a photon each time an electron crosses the junction; the semiconductor sets the colour.
- A photodiode does the reverse, producing a tiny current proportional to brightness, boosted by an op-amp.
Where it shows up — pulse oximetry
A pulse oximeter shines red and infrared LEDs through a fingertip onto a photodiode. Oxygenated and deoxygenated haemoglobin absorb the two differently, so the ratio reveals SpO2; the pulsing signal (the photoplethysmogram) gives heart rate — the basis of optical wearables.
Watch for
Optical readings are fooled by motion, poor perfusion, and skin-tone calibration — a known equity issue in pulse oximetry. Good devices flag a weak signal rather than report a confident wrong number.