How a passive infrared sensor actually decides something moved
A motion light does not see. It has a pyroelectric element behind a segmented lens that divides the field into a fan of narrow zones, and it fires when a heat source moves from one zone into the next, producing a rapid change in what the element receives. Every property that follows from that is a property of the segmented lens.
Which is why direction of travel matters so much. Somebody walking across the pattern crosses zone boundaries constantly and produces a strong, immediate signal. Somebody walking directly at the sensor stays within roughly one zone for a long time, and the signal is a slow warm drift rather than a sharp change. The published range figure is measured on the first case. The second is the one that produces the complaint that a light does not come on until you are almost at the door.
Nothing about this is a defect and no amount of sensitivity adjustment removes it — turning sensitivity up to catch head-on approach mostly buys you more false triggers. Aiming the head so that the route people take crosses the pattern rather than points at it fixes it properly.
The pattern is a band, not a circle
Detection coverage is usually drawn as a fan on the ground and thought of as a circle out to the rated range. In reality it is a band with a hole in the middle of it. The sensor sits at a height and looks down at an angle, with a vertical spread of perhaps forty degrees. The bottom edge of that fan hits the ground some distance out, and inside that distance nothing is covered at all. The top edge either hits the ground further out or, if the head is barely tilted, misses the ground entirely and looks off at the horizon and whatever is on it.
That near blind zone is the source of the most familiar failure: the light comes on as you approach and switches off while you are standing at the door searching for keys, because you have walked out of the bottom of the pattern. Tilting the head down shrinks it and pulls in the far edge; tilting up does the opposite and starts including the street. There is no setting that does both, which is why one head rarely covers a driveway and a doorway properly.
Detection coverage and lit coverage are different shapes
The sensor and the lamp on a combined fixture point roughly the same way and cover different areas, because they are governed by different angles. A lamp with a 120 degree flood beam at nine feet lights a circle about thirty-one feet across, roughly 750 square feet, with the brightness falling off toward the edge. The sensor on the same fixture may watch out to forty feet, over a much larger area than the lamp reaches.
The consequence is that the light will trip for people it does not illuminate. Whether that matters depends on the job. If the point is to signal that something moved, it is fine. If the point is to see who arrived, or to give a camera enough light to record a usable image, the fix is more fixtures covering the approach rather than one brighter one, because brightness does not change the shape of a beam. The camera side of that is the pixel density calculator, which assumes you have light to work with, and the interior half of the detection plan is the alarm sensor coverage planner, which uses the same PIR geometry indoors.
| Adjustment | Detection | Lit area | False triggers |
|---|---|---|---|
| Tilt the head further down | Blind zone shrinks, reach drops | Pool moves closer in | Fall, especially street traffic |
| Raise the mounting height | Wider footprint, less sensitive | Larger and dimmer | Fall a little |
| Turn sensitivity up | Longer effective range | Unchanged | Rise sharply |
| Mask part of the lens | Removes a direction entirely | Unchanged | Fall in that direction |
| Fit a brighter lamp | Unchanged | Brighter, not larger | Unchanged |
False triggers, and why they end with the light switched off
The realistic end state of a badly placed motion light is not a nuisance. It is a fixture somebody disabled, which is worse than never having installed it. The causes are consistent and mostly avoidable at the planning stage. Warm air from a dryer vent, boiler flue or extract fan moving through the pattern reads exactly like a moving heat source. A car parked on a driveway cools for an hour after it arrives and triggers repeatedly while it does. Cats, foxes, raccoons and possums cross at sensor height all night. Branches and hedges move in wind, hold the day heat and release it unevenly. A masonry wall or a dark driveway inside the pattern heats in sun and cools after dusk, and the change moves across zones. And a pattern that reaches a public street will fire for every passing vehicle and pedestrian, forever.
The fixes are physical rather than electronic: point the head away from vents and parking spaces, tilt down so the pattern stops short of the boundary, mask the segment of the lens facing a street, and cut back planting that grows into the pattern. Sensitivity adjustment is the last resort because turning it down to stop false triggers also stops real ones.
Aiming, neighbours and the rules that are not yours to set
Outdoor lighting aimed outward rather than downward puts glare onto neighbouring property and into bedroom windows, and it is one of the most common ways a security installation turns into a dispute. Shielded fixtures aimed at the ground light what you want lit and spill far less. Some jurisdictions have outdoor lighting ordinances, many HOAs have rules, and rental agreements often restrict fixtures entirely. Those are local questions and none of them are decided here — find out before installing rather than after a complaint.
It is also worth being honest about what bright buys. A very intense fixture creates hard shadows at the edge of its pool and wrecks the dark adaptation of anyone looking toward it, which can leave you seeing less of an approach than moderate even lighting would. Even light across the whole route beats one blinding pool, and it is also the condition under which a camera produces footage worth reviewing rather than a white blob against black.
For the low-voltage garden fixtures that often share the same project, the landscape lighting calculator handles transformer sizing and voltage drop, and indoor fixture spacing is in the lighting layout calculator. The home security guide puts all of it in proportion: lighting helps, and the door hardware and the sightlines do more of the work than any fixture on the wall.
Questions people ask
How high should a motion sensor light be mounted?
Within the range the manufacturer states, which for most domestic heads is somewhere between about eight and twelve feet, and there is a reason for both ends. Too low and the pattern is short, the fixture is within reach, and pets and low animals fill the beams. Too high and the sensor is looking down steeply at everyone in the pattern, so the apparent movement is small and sensitivity falls off exactly as the footprint widens. The near blind zone also grows with height. If the head has to go high for reach or for security, expect to tilt it down and accept a shorter working range.
Why does my motion light not come on until I am right at the door?
Almost always because you are approaching head-on. A passive infrared sensor detects heat crossing between the zones of its segmented lens, so movement across the pattern produces a sharp signal and movement straight along it produces a slow drift that takes much longer to cross a threshold. The rated range was measured on a crossing target. The second cause is the near blind zone: the bottom edge of the pattern meets the ground some feet from the mount, and inside that nothing registers, which is also why the light goes off while you are standing at the door. Turning the head so the path crosses the pattern fixes both far better than raising the sensitivity.
How do I stop a motion light triggering all night?
Find the heat source rather than reaching for the sensitivity dial. The usual culprits, in rough order: a pattern that reaches a street or pavement, a dryer or boiler vent or extract fan moving warm air through the beams, a car cooling on a driveway for an hour after it parks, animals crossing at sensor height, and vegetation that moves in wind and radiates stored heat after dusk. The fixes are physical. Tilt the head down so the pattern falls short of the boundary, mask the lens segment facing the street, aim away from vents and parking spaces, and cut back planting. Reducing sensitivity works and is a last resort, because it reduces detection of the things you wanted detected too.
How many motion lights do I need for the front of a house?
Work it from coverage width rather than from brightness. A 180 degree head at a working range of twenty or thirty feet covers a band roughly twice that wide, so one head handles a modest frontage and a long driveway or a wide lot needs two or three with deliberate overlap, since patterns are weakest at their edges and butting them exactly leaves a soft seam. The other consideration is that separate fixtures let you aim each one at a specific approach and mask each one differently, which a single wide head cannot do. Two heads aimed thoughtfully outperform one bright one aimed at everything.
Is brighter better for a security light?
Up to a point, and past it brightness works against you. A very intense fixture produces hard shadow at the edge of its pool that a person can stand in, and it destroys the dark adaptation of anyone looking toward it, so you can end up seeing less of the approach than moderate light would give you. It also spills onto neighbouring property, which is the most common way one of these becomes a dispute. Even, shielded, downward light across the whole route is more useful than one intense pool, and it is also what a camera needs — cameras handle a moderate, uniformly lit scene far better than a bright patch against black.