Daylight Harvesting with KNX and DALI-2 Sensors
Constant-illuminance lux setpoint control using DALI-2 combined sensors — with occupancy integration, EN 15193 energy factors, and calibration guidance.
What is daylight harvesting?
Daylight harvesting — also called daylight-responsive control — automatically adjusts artificial lighting output based on the available natural daylight in the space. A photometric sensor measures the total illuminance (lux) at the work surface. When daylight contributes sufficient lux, the DALI-2 drivers dim down accordingly, maintaining a constant target lux level while reducing electricity consumption.
The principle is simple: if the target is 500 lux and daylight provides 300 lux, the artificial lights need only deliver 200 lux — roughly 40% of their output. On a sunny day near south-facing windows, lights may dim to near-zero for hours at a time.
EN 15193 compliance requirement
EN 15193 (Energy performance of buildings — energy requirements for lighting) requires daylight-linked control for buildings seeking energy compliance in the EU. Projects without daylight harvesting carry a higher LENI value and may fail building energy certification thresholds.
DALI-2 combined sensors
The sensing component is a combined presence (PIR) and daylight (photocell) sensor mounted flush to the ceiling, aimed toward the work plane. A single sensor handles both occupancy detection and lux measurement — one device, one cable run, one DALI-2 address.
Helvar DALI-2 Sensor
Ceiling mount combined PIR + photocell. DALI-2 Part 303 (occupancy) + Part 304 (light sensor). Configurable detection zone. Adjustable lux setpoint via DALI-2 commands from gateway.
Tridonic MSP (Multi Sensor)
High-sensitivity PIR + lux sensor. DALI-2 bus powered (no separate 24V supply). Detection range 10 m diameter at 2.5 m ceiling. Popular in commercial LED retrofit projects.
Lunatone DALI-2 Sensor
Compact 70 mm ceiling unit. Lux setpoint and hold time configurable via Lunatone DALI Cockpit software. Outputs presence state and measured lux to DALI-2 controller.
Theben HTS (KNX)
KNX native multi-sensor (not DALI). Combined PIR + lux + temperature. Sends lux value on KNX DPT 9.004 (lux). Used where KNX logic handles dimming via DALI gateway, rather than DALI-native setpoint.
Lux setpoint control algorithm
The core algorithm is constant illuminance control. The controller maintains a fixed target lux level at the work plane regardless of how much daylight is available. The calculation runs in the DALI-2 gateway or KNX logic module:
Constant illuminance control logic
Target lux (E_target): 500 lux (office work plane, EN 12464-1)
Measured lux (E_sensor): reported by DALI-2 photocell sensor
Artificial lux required: E_target − E_sensor (clamped to 0–max)
DALI-2 arc level = f(E_required / E_max_artificial)
→ sent to DALI group as DPT 5.001 (0–100%)
Dead band (hysteresis): ±50 lux
→ prevents hunting (constant micro-adjustments every second)
Typical setpoints:
Office workplane: 500 lux (EN 12464-1 Class A)
Meeting room: 300 lux
Corridor / reception: 200 lux
Storage: 100 luxCalibration first step
Before enabling the algorithm, calibrate with the room fully lit by artificial light and no daylight (night or blackout). Measure actual lux at the work plane with a calibrated lux meter while stepping arc level from 0–100%. This establishes the arc-level-to-lux transfer curve for that sensor position and room geometry.
Occupancy integration
Daylight harvesting and occupancy detection are typically provided by the same ceiling sensor. The two functions interact to produce four operating states:
Occupied + sufficient daylight
Lights at minimum dim level (e.g. 5–10%) or off. Daylight alone meets the target.
Occupied + insufficient daylight
Lights at calculated arc level to reach target lux. Algorithm runs continuously.
Unoccupied (hold time expired)
Lights off. Hold time: 10–15 min office, 30 min corridor, 5 min WC.
First entry detected (PIR leading edge)
Lights immediately to comfort scene — no delay. Then daylight algorithm takes over after 30 s stabilisation.
The 30-second stabilisation delay on entry prevents the daylight algorithm from immediately dimming lights as the occupant enters — the PIR leading edge triggers a fixed comfort scene first, giving the sensor time to measure a stable lux value before handing control to the algorithm.
Energy savings and EN 15193
EN 15193 defines the LENI (Lighting Energy Numeric Indicator, kWh/m²/year) using correction factors for control quality. Daylight-linked control earns a lower FD factor; occupancy-based control earns a lower FO factor. Lower factors = lower LENI = better energy rating.
| Control type | FD factor | FO factor | Combined | Energy reduction vs manual |
|---|---|---|---|---|
| Manual switch only | 1.00 | 1.00 | 1.00 | Baseline |
| Manual + occupancy (PIR) | 1.00 | 0.85 | 0.85 | ~15% |
| Daylight-linked only | 0.90 | 1.00 | 0.90 | ~10% |
| Daylight + occupancy (combined) | 0.90 | 0.85 | 0.77 | ~23% |
| Daylight + occ. + personal control | 0.90 | 0.80 | 0.72 | ~28% |
Typical real-world energy savings from presence + daylight harvesting vs fixed manual lighting: office spaces 30–50%, corridors 40–60%, stairwells 60–80%. Savings are highest in south-facing spaces with large glazed facades.
Common calibration mistakes
Do not: Calibrating in winter or on a clear day
Use a uniformly overcast day (CIE overcast sky) for sensor calibration — this gives a reproducible, diffuse daylight condition. Clear-sky calibration varies with time of day and season. Winter minimum daylight gives a false baseline that causes over-dimming in spring.
Do not: Sensor aimed toward the window
The sensor must measure the lux level at the work surface — not the incoming sunlight. Aim the photocell lens perpendicular to the ceiling, pointed at the floor plane. A sensor aimed at a window will read high lux even when the work plane is poorly lit.
Do not: No dead band configured
Without a dead band (hysteresis of ±40–80 lux), the control loop hunts — lights adjust every few seconds as cloud cover fluctuates. This is visible, irritating for occupants, and causes unnecessary ECG switching cycles. Set dead band in the DALI gateway commissioning tool.
Watch out: High-reflectance surfaces inflating lux readings
White gloss walls, polished concrete floors, and light-coloured furniture all reflect light back toward the ceiling sensor, inflating the measured lux. Either apply a calibration offset in the gateway (subtract 80–150 lux) or use matte finishes in the space.
DALI-2 sensor integration and lux commissioning
We include DALI-2 sensor wiring, lux setpoint calibration, and occupancy hold-time configuration in our panel delivery — documented and verified on-site.
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