Two rangefinders can quote the same maximum range and still behave completely differently on the same vehicle. In my experience the difference usually comes down to two numbers that are buried in the datasheet, or missing from it entirely: spot size at the target and measurement latency.
This note is a practical worksheet for comparing laser rangefinder modules before you bolt one onto a drone, a robot or a gimbal. It is written from the vendor side (disclosure at the end), but the arithmetic is the arithmetic.
1. Spot size: the number that connects the datasheet to the installation
Beam divergence is the angle at which the beam spreads after it leaves the module. For small angles, the spot diameter at distance R is approximately:
spot_diameter ≈ R × θ
with θ in radians. Millimetres of milliradians are easier: 1 mrad ≈ 1 mm per metre.
- 4.5 mrad at 100 m → about 0.45 m across
- 10 mrad at 100 m → about 1.0 m across
That is the difference between "illuminating a drone-sized target" and "illuminating the hillside behind it". A wide beam is forgiving to aim and friendly on a moving gimbal; a narrow beam concentrates the return on small targets but demands better pointing stability and a stiffer mount.
Two practical consequences:
- Small targets at long range: if the spot is wider than the target, most of the emitted energy lands on the background. The receiver still gets a return, but it is the background's return. The reading can look plausible and still be wrong.
- Alignment tolerances: a 1 mrad pointing error moves the spot by 10 cm at 100 m. On a flexing carbon arm, that is not a rounding error.
Before trusting a range figure, ask for the divergence, the receiver aperture and the target reflectivity used in the test. If a datasheet gives only "max range", it has told you almost nothing about your installation.
2. Latency and update rate: the number your control loop actually feels
A 10 Hz update rate is one sample every 100 ms. At 5 m/s, the vehicle has moved 0.5 m between two readings. A 1 kHz rate is one sample every millisecond — roughly 1 mm of travel at the same speed.
That gap is not cosmetic. If the loop closes on stale data, it will overshoot, oscillate, or brake late. The fix is not always "buy the fastest module": it is to match the rate to the motion.
A rough way to size it:
required_rate ≈ (allowed position error) / (max closing speed)
If you can tolerate 20 cm of error while closing at 4 m/s, you need at least 20 Hz and you need the latency of the whole chain — module, UART, parser, control loop — to be inside that budget. The module datasheet is only the first term.
Some modules auto-switch: they run fast at short range and drop to a lower rate at long range, where more averaging is needed to get a usable return. That is a reasonable engineering trade, but it means the "update rate" line on the datasheet may be a best case. Ask what the rate is at the range you care about.
3. A comparison checklist that survives procurement
When you compare modules, write these down for each candidate:
- Divergence (mrad) and the resulting spot at your operating range.
- Update rate at your operating range, not just the headline maximum.
- Latency from measurement to UART frame out, plus the frame size at your baud rate.
- Interface and protocol: UART TTL levels, frame format, checksum, unit of the distance field.
- Behaviour when the return is weak: last-value hold, error flag, or a plausible-looking wrong number.
- What the test target was — reflectivity, size, ambient light.
Point 5 is the one that bites in the field. A module that holds the last good value silently is harder to debug than one that returns an explicit error. Whichever behaviour your module has, make the parser handle it explicitly rather than treating every frame as truth.
4. Why I wrote this down
We build 905 nm and 1535 nm laser rangefinder modules, and most integration questions we get are not about the laser at all — they are about spot size, timing and protocol framing. Two of our engineering notes go deeper on the geometry and on update-rate selection:
- Beam divergence and spot size: https://erdilrf.com/knowledge/rangefinder-beam-divergence-spot-size/
- Update rate for moving platforms: https://erdilrf.com/knowledge/rangefinder-update-rate-moving-platforms/
- Module list and datasheets: https://erdilrf.com/models/
If you are comparing modules, the checklist above is the part worth copying into your own spreadsheet. The range number is the least interesting column in it.
Disclosure: I work on integration support at ERDI TECH LTD (erdilrf.com), which makes laser rangefinder modules. The arithmetic and the checklist above apply to any module; the links point to our own engineering notes.
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