Nominal Ocular Hazard Distance (NOHD)
Distance beyond which a direct beam is within every eye exposure limit for an exposure of up to t, from the beam's power, diameter and divergence.
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.
The standards use 1/e values; 1/e² values are divided by √2. If unsure, pick 1/e²: it errs on the safe side.
The linear form is the standards'. A beam with its waist at the aperture widens more slowly, so the Gaussian NOHD is a little longer.
NOHD
97.8 m
For 100 mW, exposures up to 0.250 s
Governing limit
Retina, thermal
1/e² diameter at the NOHD
9.98 cm
Smallest beam that is within the governing limit
| Limit | Aperture at t (mm) | H at t (J/m²) | Safe 1/e² diameter | Hazard distance |
|---|---|---|---|---|
| Retina, thermal | 7.00 | 6.36 | 9.98 cm | 97.8 m |
| Retina, photochemical (blue light) | 7.00 | — | any size | 0 m |
The safety factor multiplies the power, which is the same as dividing every limit by it. The photochemical limit applies from 10 s; before that only the thermal limit does.
Nominal ocular hazard distance of a direct beam on the ICNIRP 2013 eye limits (Health Phys. 105:271), at any wavelength from 180 nm to 1 mm, for one exposure of up to t. For each limit the beam is safe once the irradiance averaged over that limit's aperture (7 mm for the retina; 1–3.5 mm on the cornea) stays within it for every exposure up to t. 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 NOHD is where the beam's diameter reaches that size. For beams much wider than the aperture this is the standards' NOHD = (√(4P/(πE)) − a)/φ with 1/e values; at the defaults it gives 97.8 m where the peak-irradiance formula gives 98.0 m. Not modelled: atmospheric attenuation, optical aids (binoculars and telescopes lengthen it), pulse trains, and elliptical beams (see the diode page).
Exposure over limit against distance for each eye limit, with the beam's diameter growing as chosen. The beam is within the limits where every line is below the dashed limit line; the NOHD is where the highest line crosses it.
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 eye limits (180 nm – 1 mm, one exposure up to t) for a Gaussian beam averaged over each limit's aperture (its peak if under 1 mm (1/e) on the cornea below 0.35 s, ICNIRP Table 5 note c), spreading as a + rφ or √(a² + (rφ)²). Not modelled: atmosphere, optical aids, pulse trains.
- 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, 5 and 8. doi:10.1097/HP.0b013e3182983fd4
- ANSI Z136.1-2014, American National Standard for Safe Use of Lasers. Appendix B: nominal hazard zone, NOHD and optical density.
- IEC TR 60825-14:2004, Safety of laser products – Part 14: A user's guide. Hazard distances and protective eyewear.