A heartbeat, seen in light.
Follow a pulse from your wrist to the number on your watch.
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More blood. Less returned light.
The detector’s dips repeat at the pulse rate. Inverting the clean optical change makes the rhythm easier to see.
Scrub to pause at any point in this four-second window.
Lower trace: clean teaching reference, not a claim that noise can always be removed. Vertical line: current inspection time.
Optical pulse ≠ ECG
PPG tracks blood-volume changes at the wrist. The ECG feature on supported watches measures electrical activity using the back electrode and a finger on the Digital Crown. This optical experiment does not simulate ECG or blood oxygen.
What this model leaves out
Exterior proportions follow the 42 mm Series 10 (36 × 42 × 9.7 mm): the aluminium case, Digital Crown, side button, domed front glass and Sport Band are reconstructed from Apple’s product photographs, without logos. The rear layout follows Apple’s representative third-generation sensor diagram. The skin cutaway is a histology-style teaching section with depths enlarged several times; vessel size, ray paths, signal amplitudes and noise are illustrative, and the gap under the watch is an inspection aid. The noise examples deliberately withhold an estimate; this is not a validated quality threshold. No live health data is collected, and Apple’s proprietary estimator is not reproduced.