Eye-Safe Wavelength Region

Why 1.4–2.6 µm is called "eye-safe": up to 1400 nm the eye focuses the beam on the retina, so the single-pulse limit is a few mJ/m² over the 7 mm pupil (2 mJ/m² × C_A below 5 µs; 20 mJ/m² × C_C from 1050 nm below 13 µs). From 1400 nm water in the cornea absorbs the beam, nothing reaches the retina, and the limit is the corneal one: 10³ J/m² below 1 ms (10⁴ J/m² at 1.5–1.8 µm), averaged over 1 mm. That is 10⁴–10⁶ times more energy for the same beam. "Eye-safe" is relative: a pulse above the corneal limit still burns the cornea. A train lowers the limit per pulse: ICNIRP's repetitive-pulse rules hold every group of pulses to the limit for the time it spans (the average power) and, on the retina, reduce the single-pulse limit by C_P; the "Limited by" column names the rule that binds. Above 1400 nm only the single-pulse and average rules apply, so at higher repetition rates the corneal average limit takes over. Limits from ICNIRP 2013 (Health Phys. 105:271, Tables 3, 5 and 8, repetitive pulses p. 287, with the sub-ns rows) and a round Gaussian beam centred on the aperture. A beam under 1 mm (1/e diameter, d/√2 for the 1/e² d) is held to its peak on the cornea below 0.35 s (ICNIRP 2013 Table 5 note c). The pulsed MPE page shows the rules in detail.