A perimeter is a line, and a laser measures along one
Most perimeter intrusion systems try to cover an area. A laser distance module does something much narrower and much cheaper: it watches one line — the gap between two posts, the approach to a gate, the space in front of a fence — and reports when that line is interrupted.
That makes it a good fit for the places where an area system is overkill and a passive infrared sensor is too easy to fool: gate approaches, tunnel mouths, yard edges, road crossings into a site, transformer compounds, and the gap between two buildings.
Modules used along perimeter lines
| Model | Published range | Price | Where it fits |
|---|---|---|---|
| LRF200H | 0.05–200 m at 90% reflectivity; 0.05–70 m at 10% | $250 | Gate and doorway lines, short fence runs |
| LRF300H | 0.05–300 m at 90%; 0.05–100 m at 10% | $280 | Standard fence span between posts |
| LRF600H | 0.05–600 m at 90%; 0.05–200 m at 10% | $350 | Long boundary runs with few mounting points |
| LRF1500H | 0.05–1,500 m at 90%; 0.05–300 m at 10% | $400 | Very long sight lines, port and utility sites |
| LR1000D23 | 4–1,000 m on a white building; vegetation ≥ 750 m | $110 | Lowest-cost long line, with a stated vegetation figure |
| SPD1200S2 | Catalog range 0.2–1,200 m, sunlight-dependent | $150 | Single-photon line with a very low minimum range |
The design question is sampling, not range
An intruder crossing a line does not stop there. A person walking at 1.4 m/s crosses a 0.5 m deep detection zone in about 350 ms. If the module is sampled at 10 Hz you get three or four samples of the crossing — enough. If the integration polls it once a second, you get one sample, sometimes, and sometimes none. Most “the sensor missed the intruder” reports are a sampling design problem, not a sensor problem.
The second design question is what counts as an alarm. A gap that changes by 40 cm is a person; a gap that changes by 2 cm is a leaf, a bird or thermal expansion of the mast. A dead band between those two, with a confirmation count, removes most nuisance alarms. Threshold hysteresis describes the same technique at a smaller scale.
Weather is the specification
Visibility is a term inside every published range figure on this site. Fog, heavy rain and blowing dust reduce it, and a perimeter product that is specified on a clear day will underperform in the weather that actually produces incidents. Two practical consequences:
- Mount a cooperative target at the far end if the geometry allows it. A retroreflective plate turns a variable (whatever the far post looks like) into a constant.
- Budget for a contamination and alignment check in the maintenance schedule. A window with a season of dust on it is a range reduction that nobody notices until an alarm fails. See protective window acceptance.
What this is not
A component module is not a certified intruder detection system and it is not a fence-mounted sensor. It reports a distance; whether that distance constitutes an alarm, and what happens next, is your controller’s decision. If the site requires a certified product to a standard, the certified product is the system that carries that rating — the module is one input to it.
Next step
Send the line length, the mounting geometry, the target at the far end and the weather exposure to tommy@erdimail.com. You will get a model and the condition behind its range figure within one business day (Chengdu, UTC+8).
Where two source documents for a model conflict, the model page states the conflict rather than choosing one value, and the current datasheet governs the order.