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## Summary Design spec for **ambient-light (ALS) adaptive brightness for external monitors** in Power Display, as discussed and agreed in #49038. The feature continuously maps the device''s ambient light sensor reading (the same ALS **Light Switch** already reads) to each external monitor''s brightness over **DDC/CI** (the path **Power Display** already uses) — i.e. auto-brightness for external monitors, the continuous/sensor-driven counterpart to the schedule-based #47480. This PR adds only the spec doc (`doc/specs/power-display-adaptive-brightness.md`), per the repo''s spec-first process (`doc/specs/readme.md`). No code. ## What the spec covers - **Core model** — a single per-monitor formula: `target = clamp(curve(lux) + offset, min, max)`. - **Per-monitor calibration curve** — the mechanism that keeps differently-behaving panels visually matched across the whole ambient range. - **Per-monitor offset** — live, phone-style personalization; manual/CLI nudges become an offset (default) with an optional `pause` behaviour. - **Live slider UX** — when adaptive is on, the slider reflects the computed value and dragging it sets the offset. - **Sensor trust & lifecycle** — lid-closed / clamshell detection (`GUID_LIDSWITCH_STATE_CHANGE` + `QueryDisplayConfig`), hold-last-good instead of dimming to black, and honest fallbacks. - Smoothing/hysteresis/rate-limiting for flicker- and wear-safe DDC/CI writes, settings persistence, telemetry, a11y/localization, risks, and phased delivery. ## Discussion / sign-off Behaviour was reviewed with @moooyo on #49038 (configurability, CLI/manual +/- while adaptive, slider behaviour) — agreed to proceed. Closes nothing yet; tracks #49038. Related: #47480, #42566, #35564. --------- Co-authored-by: Rishabh Jain <14334305+MrRishabhJain@users.noreply.github.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>