Extended Source Correction (C_E)
ICNIRP 2013 extended-source correction C_E (C₆ in IEC 60825-1), α_max and T₂, and the retinal thermal limit of a source larger than 1.5 mrad against a point source.
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.
C_E at t
33.3
α_max(t) = 100 mrad
T₂
31.1 s
from T₂ the thermal limit is a constant irradiance
Extended / point limit
33.7×
Point-source thermal limit
500 J/m²
50.0 W/m², 1.92 mW through 7 mm
Extended-source thermal limit
16870 J/m²
1687 W/m², 64.9 mW through 7 mm
An apparent source subtending α larger than α_min = 1.5 mrad forms a larger retinal image, so the retinal thermal limit rises by C_E (C₆ in IEC 60825-1), from ICNIRP 2013 (Health Phys. 105:271, Tables 2, 4 and 5): C_E = α/α_min up to α_max, then α_max/α_min, measured with a field of view γ = α_max. α_max grows with exposure time: 5 mrad below 625 µs, 200 t^0.5 mrad, then 100 mrad from 0.25 s. Eye movements spread a long exposure, so from T₂ = 10 × 10^((α − 1.5)/98.5) s (100 s above 100 mrad) the limit is the constant irradiance 18 C_A C_C C_E T₂^−0.25 W/m²; a point source keeps 10 C_A C_C W/m² from 10 s. Only the retinal thermal limit (400–1400 nm) has C_E: the photochemical limit (400–600 nm, from 10 s) is not raised, though its exposure is measured over an 11 mrad field of view, and the corneal limits outside 400–1400 nm don't depend on α. The power is for a beam inside the 7 mm aperture. A collimated laser beam is a point source; α matters for diffuse reflections, diffusers and arrays.
Retinal thermal limit against exposure time for a point source and for the chosen α.
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 2013 C_E with γ = α_max, the time-dependent α_max and T₂, on the retinal thermal limit (400–1400 nm). Not modelled: the photochemical field of view and the radiance limits.
- 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. Tables 2, 3, 4 and 5. doi:10.1097/HP.0b013e3182983fd4