LEDs & photodiodes

In one line

An LED is a diode that emits light when current flows; a photodiode is a diode that produces current when light hits it. Point one at the other across living tissue and you can measure what's flowing through.

How it works

• An LED releases a photon each time an electron crosses the junction. The colour (wavelength) is set by the semiconductor — red, infrared, the white in a screen backlight, or the UV in a disinfection lamp.
• A photodiode does the reverse: incoming light frees charge carriers, producing a tiny current proportional to brightness. That faint signal is boosted by an op-amp front-end.

Where it shows up in digital health — pulse oximetry

A pulse oximeter shines two LEDs — red and infrared — through a fingertip onto a photodiode. Oxygenated and deoxygenated haemoglobin absorb red and infrared light differently, so the ratio of the two transmitted signals reveals SpO₂. The signal also pulses with each heartbeat (the photoplethysmogram), which gives the pulse rate — and is the basis of optical heart-rate sensors in wearables.

Other uses: bilirubin phototherapy lamps (blue LEDs), fluorescence in lab analysers, optical isolation that protects patient-connected circuits, and the indicator lights on every device.

Watch for

Optical readings are fooled by motion, poor perfusion, nail polish and skin tone calibration — a known equity issue in pulse oximetry. Good devices detect a weak or noisy signal and say so rather than reporting a confident wrong number.

Key takeaways

• An LED emits light from current; a photodiode produces current from light — paired, they measure transmission.
• Pulse oximetry: red + infrared LEDs through tissue → photodiode → the absorption ratio gives SpO₂; the pulsating signal gives heart rate.
• Also: phototherapy lamps, fluorescence analysers, optical isolation, indicator lights.
• Optical readings are fooled by motion, poor perfusion, nail polish, and skin-tone calibration (a real equity issue) — good devices flag low confidence.