Aversion Response Time

Retinal limit at the 0.25 s blink response and the power it allows into a 7 mm pupil, about 1 mW: the basis of Class 2. Visible beams only.

Model: Exact

Educational only — not compliance-grade

These pages implement the published exposure limits for the cases each one states, but they are educational tools: no laser safety officer has signed them off.

Do not use these results alone for PPE selection, NHZ / NOHD approval, controlled-area sign-off, enclosure/interlock approval, or formal classification.
Always verify against ANSI Z136.1, IEC 60825-1, your local procedures, and the responsible laser safety officer / CLSO.

Wavelength (nm, visible 400–700)

Radiant Exposure Limit at t = 0.25 s

0.636 mJ/cm²

λ = 632 nm, retinal thermal limit

Irradiance Limit (mW/cm²)

2.546

Averaged over a 7 mm aperture

Maximum Power into a 7 mm Pupil

0.980 mW

98.0 % of the Class 2 value

Class 2 Power Limit (IEC 60825-1)

1.0 mW

Visible only (400–700 nm)

Formulas

t_aversion = 0.25 s (blink reflex, visible light only)

H = 18 · t^0.75 J/m² (400–700 nm, 5 µs – 10 s; ICNIRP 2013 Table 5, C_A = 1)

E = H / t

P_max = E · π (D/2)², D = 7 mm

Class 2 limit: P ≤ 1 mW (IEC 60825-1, relies on the aversion response)

Only visible light (400–700 nm) gets an aversion response. An invisible beam does not make you blink, so 0.25 s does not apply to it: use the actual exposure duration instead.

For the retinal limit the 7 mm aperture applies whatever the beam size: ICNIRP allows no reduction for a beam narrower than the pupil, so the beam diameter and divergence do not change the limit. In the visible the thermal limit has no wavelength dependence (C_A = 1), so the curve is the same from 400 to 700 nm up to 10 s; the photochemical (blue-light) limit starts at 10 s.

Model and references

Exact. The equations are exact within the stated assumptions, or are a standard's tabulated values. The results are as good as the inputs. About the tiers

ICNIRP retinal thermal limit 18 t^0.75 J/m² at t = 0.25 s, 400–700 nm only, and the power it allows through a 7 mm pupil: 0.98 mW, rounded to the 1 mW Class 2 limit. Not modelled: extended sources.

ICNIRP (2013). Guidelines on limits of exposure to laser radiation of wavelengths between 180 nm and 1,000 µm. Health Phys. 105(3), 271–295. Table 5 (eye) and Table 8 (apertures). doi:10.1097/HP.0b013e3182983fd4
IEC 60825-1:2014, Safety of laser products: the Class 2 AEL of 1 mW for 400–700 nm, with a 0.25 s time base.

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