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1535 nm vs 905 nm laser rangefinders
The two wavelengths are not two grades of the same product. They are two different engineering compromises, and the choice is usually made by the eye-safety limit rather than by any optical property.
Why 1535 nm carries more energy
A 905 nm beam passes through the cornea and is focused onto the retina, so the permissible exposure is low and the pulse energy a product can emit is small. At 1535 nm the light is absorbed in the cornea and lens before it reaches the retina, so the permissible exposure is roughly two orders of magnitude higher. That is the whole reason the long-range catalogue exists at 1535 nm: more pulse energy out means more photons back from a distant, poorly reflective target.
Cost, size and rate go the other way
| 905 nm | 1535 nm | |
|---|---|---|
| Detector | Silicon APD or SiPM — cheap, fast | InGaAs APD — costlier, more supply chain risk |
| Source | Laser diode | Erbium-glass rod, pumped, passively Q-switched |
| Typical mass | 1–20 g | 30–410 g |
| Typical update rate | 10 Hz – 10 kHz | 0.5–10 Hz |
| Typical usable range (published) | 9 m – 3 km | 3–23 km |
| Haze and dust | Strong attenuation | Much less attenuation |
| System price driver | Optics and volume | Source and InGaAs detector |
Reading a 1535 nm datasheet
Four figures do the work: pulse energy (µJ), beam divergence (mrad), receiver aperture (mm) and the test condition for the range. A 350 µJ source with 1.3 mrad divergence in a 410 g housing is a different instrument from a 200 µJ, 0.6 mrad, 32 g module — and both may print "4 km" if the targets differ.
Reading a 905 nm datasheet
Look for the two reflectance columns (90 % and 10 %), because the 10 % figure is your real world: 1,500 m at 90 % often collapses to 300 m at 10 %. Also check the ambient-light immunity in klx and the reported resolution, which is not the accuracy.
When the answer is "both"
Dual-band systems exist for a reason: 905 nm for the fast, short-range channel and 1535 nm for the long-range designation channel, with a common interface. If your product needs both a kilohertz proximity channel and kilometre reach, say so — that is a two-module architecture, not one compromise module.