What is NOHD (Nominal Ocular Hazard Distance)?

Glossary Definition

NOHD is short for Nominal Ocular Hazard Distance. Laser safety experts recommend not having direct naked eye exposure to a laser’s beam closer than the rated NOHD for that laser. Lasers are extremely dense forms of light (irradiance), and the permissible exposure limit is 2.54 milliwatts per square centimeter. If a person is outside the NOHD, the beam is considered completely eye safe since the irradiance falls below the hazard distance.

However, this does not mean that exposure within the NOHD is immediately dangerous and will cause inevitable eye injury, or even that eye injury is probable. The NOHD is a nominal hazard distance, not an actual hazard distance. It turns out that even at only 31.6% of the NOHD, there is a 50/50 chance that an exposure will cause a minimally detectable change to the retina. We call this the ED50 (Exposure Dose 50%) distance.

So while the NOHD is a good guideline to follow for safety, not all eye exposure within the NOHD is immediate cause for panic.