Occupancy Sensing & Daylight Harvesting

Sensors are where lighting control projects most often fail, and almost never because the sensor was faulty. They fail because the wrong technology went into the space, the placement was decided from a reflected ceiling plan rather than the room, or nobody calibrated anything after installation.

Choosing the sensing technology

Passive infrared detects heat movement across its field of view. It needs line of sight, and it is least sensitive to motion coming straight toward it — which is exactly how people enter a room through a doorway. It is reliable, immune to false triggering from noise, and wrong for spaces with partitions or obstructions.

Ultrasonic detects frequency shift from movement and does not require line of sight, so it works around partitions and in restrooms with stalls. It is more prone to false triggering from air movement, which makes it a poor choice near HVAC diffusers.

Dual-technology sensors require both to agree before turning lights on, and either to hold them on. This is the correct default for spaces where a false-off is genuinely disruptive — conference rooms, and any space where someone might sit still for twenty minutes.

Occupancy versus vacancy

An occupancy sensor turns lights on automatically when someone enters. A vacancy sensor requires a manual switch-on and only handles the switch-off. Vacancy saves more energy, because it never turns lights on for someone walking past a doorway or in a room with adequate daylight.

Energy code increasingly requires manual-on in specific space types, so this is often not a preference. Where it is a choice, vacancy is usually the better one in private offices and the wrong one in corridors and restrooms.

Daylight harvesting done properly

Daylight harvesting dims electric lighting in response to available daylight. Where there is meaningful daylight it is one of the most effective measures available; where there is not, it adds cost and delivers nothing.

Getting it right requires the daylit zone to be defined honestly, the photosensor placed where it reads the work plane rather than a light patch on the floor, and calibration performed after the space is furnished. A sensor commissioned in an empty shell will be wrong once the furniture, partitions and finishes are in.

• Primary and secondary daylight zones defined from actual glazing and orientation
• Photosensor placement verified in the room, not from the ceiling plan
• Gradual dimming rates — visible steps make occupants disable the system
• Calibration after furniture and finishes are installed
• Seasonal verification, because sun angle changes what the sensor sees

What the 2021 IECC requires

Colorado has adopted the 2021 IECC as its statewide commercial baseline. Beyond reducing lighting power allowances, it expanded mandatory controls: daylight-responsive control extended to secondary daylight zones, plus specific requirements for plug load control, parking garage lighting, and corridor lighting.

These are mandatory provisions, which means they apply regardless of whether you take the prescriptive or performance compliance path. Controls are also the most common reason a lighting installation fails final inspection — usually because the compliance path was chosen after installation instead of before design.