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Beam divergence, receiver aperture, and why both decide range
Range is a photon budget: how many you send, how tightly you send them, and how much of the return you collect. Divergence and aperture are the two knobs on that budget.
Divergence: spot size on target
Spot diameter ≈ divergence × distance, plus the initial beam diameter. A 0.3 mrad beam is 1.5 m at 5 km; a 1.3 mrad beam is 6.5 m. If your target is smaller than the spot, most of the energy misses it and range falls off faster than 1/D². This is why the long-range parts in the catalogue carry 0.3–0.6 mrad and the short-range parts are happy at 4–10 mrad.
Aperture: how much return you collect
Received power scales with receiver area, so doubling aperture diameter quadruples collected energy — the single largest lever after pulse energy. The catalogue spans 6.5 mm (905 nm LiDAR sensors) to 60 mm (long-range 1535 nm modules). That 60 mm aperture is a large part of why a 410 g module reaches 23 km and a 5 g sensor reaches 1.5 km.
The integration traps
- Putting a 60 mm aperture behind a 30 mm window throws away most of the range you paid for.
- A window that is flat in the visible but not AR-coated at 1535 nm loses several percent per surface — twice per pass.
- Transmit and receive axes must stay parallel within the divergence budget across temperature; a common mount with different thermal paths will drift.
- Dust and condensation on the window is the usual field failure; a heated window is part of the optical design, not an accessory.
Reading the numbers together
Pulse energy × aperture / divergence² is a crude but useful figure of merit. Compute it for two candidates and compare with the published test conditions — if a module wins on the figure of merit but quotes a much easier target, the numbers are not comparable.