Lux

Lux (lx) measures illuminance: the amount of light falling on an area. One lux is one lumen per square metre. NIST defines the unit

BetterDisplay can report ambient-light sensor readings in lux. Think of this as light arriving at the sensor, rather than brightness coming out of the display. Display luminance uses a different unit, nits.

The reading depends on the sensors available and where they are positioned. The System Colors menu shows the combined ambient reading when sensors are available.