Home › Guide › 1535 vs 1550
1535 nm vs 1550 nm: same safety argument, different sources
Both sit in the "eye-safe" band where the cornea absorbs before the retina. The difference is what makes the light: erbium-glass microchip sources at 1535 nm, fibre or diode-pumped sources at 1550 nm.
The physics is shared
Above roughly 1400 nm, water absorption in the cornea and lens removes the beam before it can be focused on the retina, so the permissible exposure is roughly two orders of magnitude higher than at 905 nm. Any product in this band can carry far more pulse energy than a silicon-detector rangefinder — that is the whole reason the 3–23 km category exists.
The sources are not
- 1535 nm (erbium-glass): a small passively Q-switched rod, hundreds of microjoules to millijoules, compact and robust, 0.5–10 Hz. This is what the ERDI long-range catalogue uses.
- 1550 nm (fibre / diode-pumped): higher average power and higher rates are available, at more volume, more thermal design and a different cost structure. Common in laser radar and designator work.
- 1550 nm eyesafe lasers are also the standard choice where a beam must propagate over kilometres in a collimated form factor.
What changes in the datasheet
Look at pulse energy, pulse width, divergence and the source's own safety statement. A 1535 nm module rated at 350 µJ with 1.3 mrad and a 1550 nm source at 1 mJ with 0.5 mrad are not interchangeable even though both are "eye-safe" — the first is a rangefinder transmitter, the second is a designator-class source.
Does the 15 nm matter to the detector?
Both land on InGaAs. Filter design changes slightly, and atmospheric transmission is essentially identical. The choice is set by the source and the system, not by the 15 nm.