pulse oximetry
A pulse oximeter is the little clip on your fingertip that reads blood-oxygen by shining light through you — a beautiful piece of optoelectronics that needs no needle. Oxygen-rich blood (bright red) and oxygen-poor blood (dark red) absorb red and infrared light by different amounts, so the trick is to shine two LEDs (one ~660 nm red, one ~940 nm infrared) through the finger and measure how much survives with a single photodiode on the far side. Hold a flashlight behind your fingers in a dark room and you see the same physics: living tissue is translucent, and how it tints the light reports on the blood inside.
The clever part is isolating the blood that pulses. Every heartbeat the arteries swell, briefly absorbing more light, so the photodiode current carries a small throbbing AC component riding on a big steady DC background of skin, bone and venous blood. The circuit measures the ratio of the AC-to-DC pulsation at the two wavelengths (the 'R ratio') and maps it to SpO2 — because it watches only the pulsing part, it automatically ignores everything that isn't flowing arterial blood. The same red/IR LED-plus-photodiode signal, taken on its own, is the photoplethysmogram (PPG) that smartwatches use to read your heart rate from the wrist.
Because it tracks ratio not absolute brightness, a pulse oximeter is wonderfully tolerant of finger thickness and skin tone — but it can be fooled: it cannot tell oxygen from carbon monoxide (both make blood bright red), so it reads dangerously normal during CO poisoning.