Green-LED background sensing

The hardware mechanism behind the Series 12 / Ultra 4's headline jump in sampling rate — the detail that the announcement coverage left as a black box.

Older Apple Watches ran 24/7 background heart rate off low-power infrared LEDs, and only when the wearer was relatively still, yielding roughly one reading every 5 minutes. Exercise mode used the green PPG LEDs plus the accelerometer at ~5-second intervals. The new sensor rearranges the LED array so the diodes are clustered by color — four green, four red, four infrared — where previously they couldn't be grouped. That regrouping is what lets Apple drive background sensing from the green LEDs instead of infrared, extending exercise-grade 5-second sampling to passive all-day monitoring (apple-watch).

Why it matters: the generational story is availability, not per-reading accuracy. Workout-mode sampling is unchanged; only the background path improved. Every derived feature downstream — readiness-score, daytime HRV, the Vitals HRV card — exists because this measurement stream got dense enough to support them. dc-rainmaker surfaced the LED-grouping detail in the Ultra 4 hands-on; the Series 12 video and rob-ter-horst's analysis both described the outcome ("60x more frequent") without the cause.

Source: DC Rainmaker Ultra 4 hands-on