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Blind zone and minimum range: the first metre is the hard part

Datasheets lead with maximum range. The number that causes most installation problems is the minimum one.

A laser rangefinder needs the outgoing pulse to have left the optics before the receiver can accept the return. That timing, plus the receiver's recovery behaviour, sets a blind zone: a distance below which the module either reports nothing, reports a fixed value, or reports a reflection from its own window.

Where the blind zone comes from​

  • Pulse width and gating. The receiver is effectively deaf while the transmitter fires. A return that arrives inside that window cannot be separated from the transmit pulse.
  • Optical crosstalk. A protective window, a filter or a dirty lens can reflect a small part of the pulse straight back into the receiver. At short distances that reflection can be stronger than the target return.
  • Recovery time. After a strong return (a wall at 30 cm), the receiver may need time to return to normal sensitivity.

The practical consequence: a module specified for 0.3 m may still be unreliable at 0.35 m against a dark, angled target, and perfectly fine at 0.3 m against a white plate. The blind zone is not a single number - it is a curve that depends on target reflectivity, angle and ambient light.

Mounting around it​

  1. Offset the module so the closest surface you must measure is outside the blind zone, or accept that the closest reading will be invalid and handle it in software.
  2. Never let the beam hit the vehicle. On a small drone, the skid or the camera housing can sit inside the blind zone. Tilt the module a few degrees or move it forward.
  3. Treat the first samples after power-up as suspect. Many modules need a few frames to settle; discard them explicitly rather than smoothing them into the output.
  4. Define the invalid value. Decide in advance whether "no return" is -1, 0, a NaN, or a held previous value, and make the parser express that rather than folding it into a plausible distance.

A quick bench check​

Put a target at the shortest distance you care about. Move it 5 cm closer in steps until the reported distance stops tracking. Record where it breaks and what the module reports there (no return, zero, or the last good value).

Repeat with a dark target and with a tilted target. The break point will move. That range - not the datasheet's minimum - is what your control loop must tolerate.

Further reading:

Disclosure: maintained by integration support at ERDI TECH LTD (erdilrf.com), a manufacturer of laser rangefinder modules.