diff --git a/custom_components/adaptive_lighting/switch.py b/custom_components/adaptive_lighting/switch.py
index 0cf670a5..c86504d8 100644
--- a/custom_components/adaptive_lighting/switch.py
+++ b/custom_components/adaptive_lighting/switch.py
@@ -1475,11 +1475,12 @@ class AdaptiveSwitch(SwitchEntity, RestoreEntity):
         if current_lux is not None and self._target_lux > 0:
             ambient_lux: float | None = current_lux
             if current_lux > self._target_lux:
-                lux_reduction = round(
-                    min(self._target_lux / current_lux, 1.0) * 100,
-                )
+                factor = min(self._target_lux / current_lux, 1.0)
+                # Report the *reduction*, not the retained factor: 0 % means
+                # no dimming (curve passes through), 100 % means fully cut.
+                lux_reduction = 100 - round(factor * 100)
             else:
-                lux_reduction = 100
+                lux_reduction = 0
         else:
             ambient_lux = current_lux
             lux_reduction = None
diff --git a/openspec/specs/output-sensors/spec.md b/openspec/specs/output-sensors/spec.md
index f43717d6..ff5a5f61 100644
--- a/openspec/specs/output-sensors/spec.md
+++ b/openspec/specs/output-sensors/spec.md
@@ -175,31 +175,29 @@ The `ambient_lux` output sensor SHALL read the configured `lux_sensor` entity's
 - **THEN** `outputs["ambient_lux"]` SHALL be `None`
 - **AND** the `ambient_lux` sensor entity state SHALL be `unknown`
 
-### Requirement: Lux reduction sensor exposes the applied factor as a percentage
+### Requirement: Lux reduction sensor exposes the applied reduction as a percentage
 
-The `lux_reduction` output sensor SHALL publish `factor × 100` to `hass.data[DOMAIN][entry.entry_id]["outputs"]["lux_reduction"]`, where `factor` is the value computed by the lux gate. A value of `100` means no reduction (curve passes through); `50` means brightness was halved. When the lux gate is inactive (sensor below target or unavailable), the value SHALL be `100`.
+The `lux_reduction` output sensor SHALL publish the brightness reduction the lux gate is applying, as a percentage, to `hass.data[DOMAIN][entry.entry_id]["outputs"]["lux_reduction"]`. The value SHALL be `100 − round(factor × 100)`, where `factor` is the retained brightness factor computed by the lux gate (`target_lux / current_lux`, capped at `1.0`). A value of `0` means no reduction (curve passes through); `50` means brightness was halved; values approaching `100` mean the lights are almost fully cut. When ambient lux is at or below target, the value SHALL be `0`. When the lux gate is inactive (no sensor configured, sensor unavailable, or `target_lux` is 0), the value SHALL be `None` (the sensor renders `unknown`).
 
-When the lights were turned off due to lux reduction (factor drove brightness below `min_brightness`), the value SHALL be `0`.
-
-#### Scenario: Lux reduction shows the applied factor
+#### Scenario: Lux reduction shows the applied reduction
 
 - **GIVEN** `target_lux` is 500 and `lux_sensor` reads 700
 - **WHEN** a curve evaluation tick completes
-- **THEN** `outputs["lux_reduction"]` SHALL be `71` (rounded from 71.4)
-- **AND** the `lux_reduction` sensor entity state SHALL be `"71"`
+- **THEN** `outputs["lux_reduction"]` SHALL be `29` (100 − 71, the retained 71.4% rounded)
+- **AND** the `lux_reduction` sensor entity state SHALL be `"29"`
 
-#### Scenario: No reduction shows 100%
+#### Scenario: No reduction shows 0%
 
 - **GIVEN** `target_lux` is 500 and `lux_sensor` reads 300
 - **WHEN** a curve evaluation tick completes
-- **THEN** `outputs["lux_reduction"]` SHALL be `100`
-
-#### Scenario: Lights-off due to lux shows 0%
-
-- **GIVEN** the lux gate drove brightness below `min_brightness`
-- **WHEN** a curve evaluation tick completes
 - **THEN** `outputs["lux_reduction"]` SHALL be `0`
 
+#### Scenario: Gate inactive shows unknown
+
+- **GIVEN** no `lux_sensor` is configured (or it is unavailable, or `target_lux` is 0)
+- **WHEN** a curve evaluation tick completes
+- **THEN** `outputs["lux_reduction"]` SHALL be `None`
+
 ### Requirement: Lux output sensors follow the same dispatcher pattern as existing sensors
 
 The `ambient_lux` and `lux_reduction` sensors SHALL use the same `SIGNAL_OUTPUTS_UPDATED` dispatcher subscription as the three existing sensors. They SHALL NOT poll. `_attr_should_poll` SHALL be `False`. The dispatcher unsubscribe handle SHALL be tracked via `async_on_remove`.
diff --git a/tests/test_sensor_platform.py b/tests/test_sensor_platform.py
index c8aeb495..e967cfa7 100644
--- a/tests/test_sensor_platform.py
+++ b/tests/test_sensor_platform.py
@@ -20,6 +20,8 @@ from pytest_homeassistant_custom_component.common import MockConfigEntry
 
 from custom_components.adaptive_lighting import sensor as sensor_module
 from custom_components.adaptive_lighting.const import (
+    CONF_LUX_SENSOR,
+    CONF_TARGET_LUX,
     CONFIG_ENTRY_VERSION,
     DOMAIN,
     OUTPUT_SENSORS,
@@ -194,6 +196,32 @@ async def test_curve_tick_publishes_outputs(hass) -> None:
     assert outputs["updated_at"] >= before
 
 
+async def test_lux_reduction_reports_reduction_not_retained_factor(hass) -> None:
+    """lux_reduction is the percentage *reduced* (0 = none), not the kept factor."""
+    entry = await _setup_entry(
+        hass,
+        options={CONF_LUX_SENSOR: "sensor.test_lux", CONF_TARGET_LUX: 500},
+    )
+    al_switch = hass.data[DOMAIN][entry.entry_id]["switch"]
+    al_switch._settings.update({"brightness_pct": 80, "color_temp_kelvin": 3000})
+    hass.states.async_set("sun.sun", "above_horizon", {"elevation": 10.0})
+
+    # Above target -> dimming. factor = 500/700 = 0.714 -> reduction = 29 %.
+    hass.states.async_set("sensor.test_lux", "700")
+    al_switch._publish_outputs_and_wake_sensors()
+    assert hass.data[DOMAIN][entry.entry_id]["outputs"]["lux_reduction"] == 29
+
+    # At/below target -> no reduction -> 0 % (the at-rest value).
+    hass.states.async_set("sensor.test_lux", "300")
+    al_switch._publish_outputs_and_wake_sensors()
+    assert hass.data[DOMAIN][entry.entry_id]["outputs"]["lux_reduction"] == 0
+
+    # No usable reading -> gate inactive -> None (sensor renders unknown).
+    hass.states.async_set("sensor.test_lux", "unavailable")
+    al_switch._publish_outputs_and_wake_sensors()
+    assert hass.data[DOMAIN][entry.entry_id]["outputs"]["lux_reduction"] is None
+
+
 # ---------------------------------------------------------------------------
 # 4.7 — Firing the dispatcher signal updates the three sensors
 # ---------------------------------------------------------------------------