Constant Light Control: How Daylight-Regulated Dimming Actually Works

The principle

A constant light control sensor measures the actual illuminance at the working plane (usually via a light sensor built into the presence/light sensor housing) and continuously adjusts the dimming level of the luminaire to maintain a set lux target — regardless of how much daylight is contributing. As daylight increases, the sensor dims the artificial light down; as daylight fades, it brings the artificial light back up to hold the target.

This is different from simple daylight harvesting on a timer or step-dim schedule. Constant light control is a closed feedback loop: sensor reads lux → compares to setpoint → adjusts output → re-measures.

Why commissioning is the part that matters

The sensor doesn't measure the illuminance on the desk — it measures illuminance at the sensor, which is typically ceiling-mounted. The relationship between "lux at the sensor" and "lux at the working plane" has to be calibrated during commissioning, usually by:

1. Setting artificial light to 100%, with no daylight contribution (blinds closed or after dark)
2. Measuring actual lux at the working plane with a handheld meter
3. Recording the corresponding sensor reading
4. Setting that sensor reading as the calibration reference point, so the control loop's internal target maps correctly to a real-world lux level

Skip this step and the system will hold whatever lux level happens to correspond to the sensor's factory default — which may be nowhere near the design illuminance for the space.

Common failure points

• Sensor mounted too close to a luminaire it isn't controlling, picking up contribution from neighbouring fittings and never allowing full output
• No cut-off or deadband set, causing hunting — the loop continuously overshoots and corrects in small oscillations, visible as light "breathing"
• Ignoring reflectance changes — a room repainted a darker colour or re-carpeted changes the sensor's baseline reading without any change to actual working-plane lux

Constant light control is one of the more genuinely energy-effective features in a lighting control system — perimeter zones near windows can run at a fraction of nominal output for most of the day — but only when it's calibrated, not just switched on.