Occupancy Sensors: PIR vs Microwave vs Dual Tech

After the fixture swap itself, controls are usually the largest energy saving available in a building, and in a lot of spaces energy code now requires them. The three sensing technologies fail in completely different ways, and picking the wrong one is what generates the callback where lights drop out on somebody sitting still at a desk.

How each one actually detects you

Type	What it senses	Fails when
PIR (passive infrared)	A heat source moving across its field of view, in segments	Somebody sits still, or moves straight toward the sensor rather than across it. Cannot see through anything.
Microwave (active, doppler)	Its own emitted signal bouncing back changed by movement	It is too sensitive. It sees through thin walls, doors and racking, and will hold lights on for the corridor outside the room.
Ultrasonic	Reflected sound waves shifted by movement	Air movement and vibration set it off. Less common on commercial LED fixtures.
Dual technology	Usually PIR and microwave together	Rarely. It is the fix for the other two, at a higher price.

Why PIR misses a seated person

A PIR sensor does not see a continuous picture. Its lens divides the space into segments, and it triggers when a heat source crosses from one segment into the next. Someone typing at a desk barely crosses a segment boundary, which is exactly the situation where lights drop out and everybody in the room waves their arms at the ceiling.

It also means PIR is most sensitive to movement across its view and least sensitive to movement directly toward it. A sensor at the end of a corridor is in the worst possible orientation. One mounted to the side of the walking path works far better.

Why microwave sees too much

Microwave is active. It emits a signal and watches for the reflection to change. That signal passes through drywall, glass, doors and thin partitions, so a sensor in a private office can quite happily hold the lights on because somebody is walking down the corridor outside. It also picks up on ceiling fans, HVAC dampers and hanging signs moving in airflow.

The upside is real though: it covers a much larger area than PIR from the same mounting point, and it does not need line of sight. In a racked warehouse aisle, where a PIR would be blind past the first pallet, microwave is usually the only thing that works.

Dual technology, and how it is wired

Dual tech normally requires both technologies to detect motion before it turns lights on, then allows either one to hold them on. That combination kills nuisance switch-on from the corridor while still keeping the lights up for the person sitting still at the desk. It is the right default for conference rooms, private offices and classrooms, and it is worth the extra money in any space where a false trigger is embarrassing.

Picking one by space

Space	Use	Why
Warehouse aisles	Microwave	Racking blocks line of sight. PIR will see the first pallet and nothing else.
Open warehouse floor	PIR or microwave	Either works. PIR is cheaper and less prone to holding on for a forklift two aisles over.
Private office	Dual tech	Seated occupants plus a corridor on the other side of a thin wall.
Conference room	Dual tech	People sit still for an hour. This is the classic PIR failure.
Open plan office	PIR	Plenty of movement, clear sight lines, and you want tight zone control.
Restroom	Dual tech or ultrasonic	Stall partitions block PIR.
Corridor	PIR, mounted side on	Cheap and reliable, as long as it is not staring down the length of the corridor.
Stairwell	Microwave	Landings and half flights break line of sight, and these usually run bi-level rather than off.
Parking garage	Microwave	Columns everywhere, and the fixtures run around the clock so the saving is large.

Coverage and mounting height

Coverage area grows with mounting height, but sensitivity falls. A sensor rated for a 40 foot ceiling is not the same part as one rated for 10 feet, and the lens is the difference.

Mounting height	Lens	Rough coverage
8 to 12 ft	Low bay lens, wide	Broad and shallow, good for offices and corridors
12 to 25 ft	Standard high bay lens	Moderate cone, the general warehouse answer
25 to 40 ft	High bay lens, narrow	Tighter cone so it still reaches the floor with usable sensitivity
Aisle applications	Aisle lens	Long and narrow rather than circular, matched to the aisle shape

Settings that decide whether people complain

• Time delay. How long the lights hold after the last detection. Too short and people get switched off on them, too long and you lose the saving. Fifteen to twenty minutes for offices and conference rooms, five for storage and corridors, one to five for warehouse aisles.
• Step dimming instead of off. In warehouses, stairwells and garages, dropping to 10 to 20 percent rather than switching off is nearly always the better answer. You keep the saving, nobody walks into a dark aisle, and the driver is not being hard cycled all day.
• Sensitivity. Turn it down on microwave in any room with a thin wall. This is the single most common field adjustment.
• Daylight threshold. If the sensor also does daylight harvesting, set the level with real daylight in the space, not from the spec sheet.
• Walk test mode. Almost every commercial sensor has one. Use it before you leave, with the actual furniture and racking in place.

Three mounting mistakes

• Near an HVAC diffuser. Moving warm or cold air across a PIR field reads as a moving heat source. Keep sensors clear of supply registers and unit heaters.
• Pointed down a corridor. PIR is least sensitive to motion coming straight at it. Mount to the side of the path, not at the end of it.
• Microwave against a shared wall. It does not stop at the drywall. In a row of private offices, either turn the sensitivity down or go dual tech.

Integral, sensor base, or ceiling mounted

Option	Trade-off
Integral to the fixture	Cheapest and fastest. Each fixture controls itself, so zoning is fixed by where the fixtures are.
Sensor base or twist lock	The fixture ships ready and the sensor plugs in, so you can change or replace the sensor without touching the fixture. Most of what we carry offers this.
Separate ceiling sensor	Most flexible zoning, controls a group of fixtures, but it is another device, another location and more wire.
Networked control	Individual addressing, scheduling and reporting. Worth it on large buildings and on projects where a utility wants measured savings.

The code and rebate angle

Most current commercial energy codes require automatic shutoff in the majority of enclosed spaces, and many require it in open areas too. If you are pulling a permit, controls are usually not optional.

On the incentive side, controls are frequently what moves a lighting project from the flat prescriptive rebate onto the custom path, where the incentive is calculated from measured savings and is often considerably larger. If you are already speccing sensors, it is worth checking whether the job qualifies. See how utility rebates work.