The safety of laser equipment ultimately comes down to a specific value: at what distance is the human eye safe? This value is NOHD (Nominal Ocular Hazard Distance).
It is not a fixed empirical value, but a safety boundary derived from rigorous calculations and actual measurement verification based on laser power, wavelength, divergence angle and other parameters. For Class 3B and Class 4 laser products, this value directly determines the safety planning of operating areas and the compliant labeling of user manuals.
01 What is NOHD?
NOHD (Nominal Ocular Hazard Distance) refers to the distance at which the laser beam energy attenuates to the maximum permissible exposure (MPE) threshold for human eyes.
Simply put: within the NOHD, direct viewing of the laser with naked eyes carries risks; beyond the NOHD, it is relatively safe.
Examples:
A varicose vein laser therapy device has an NOHD of approximately 5 meters. Strict protection is required within 5 meters.
A 10W, 1064 nm laser can have an NOHD up to 160 meters, which explains why high-power lasers require large-space control.
02 Which products require NOHD testing?
Class 3B and Class 4 laser products are mandatory test subjects, mainly including:
Industrial: cutting machines, welding machines, marking machines
Medical: therapeutic devices, surgical lasers
Scientific research: high-power lasers, LiDAR
Measurement: laser projectors, rangefinders
In addition, for Class 1M / 2M products, ENOHD (Extended Nominal Ocular Hazard Distance) needs to be assessed if observation via optical instruments is possible.
03 Test Procedures: 5 Key Steps
Standards: IEC 60825, ANSI Z136, ISO 11554
The workflow can be condensed into five steps:
Parameter collection — optical power, wavelength, beam divergence, pulse duration
MPE calculation — look up tables and calculate values based on wavelength and exposure duration
Theoretical NOHD calculation — derive theoretical results by formula substitution
Physical verification — measure with beam analyzers and revise the results
Report issuance — record all data and generate formal assessment documents
Notes: All operating modes (continuous wave / pulsed) shall be covered. Separate calculations are required if the divergence angle is adjustable.
04 NOHD vs Laser Safety Classifications — Do Not Confuse Them
Laser safety classes (Class 1~4) are static ratings, determined by power and wavelength, which are qualitative.
NOHD is a dynamic distance metric, quantifying the safe separation distance.
Relationship: Class 3B and Class 4 products carry high risks and must undergo NOHD testing. The two complement each other.
05 What are the purposes of NOHD testing?
Personnel protection — laser-induced eye damage is irreversible; NOHD defines safety zones.
Market compliance — mandatory under international standards; test reports are essential for export.
Design optimization — support adjustment of power and beam characteristics.
Risk management — used to develop safety procedures and training materials.
06 Professional Testing Services
As a third-party laser testing laboratory, we provide:
Laser safety classification testing (IEC 60825-1)
NOHD spatial radiation hazard testing
Compliance consultation and rectification suggestions
Technical support for global market access (CE / FDA / CB)
