Understanding NOHD (Nominal Ocular Hazard Distance)
NOHD is the distance beyond which the laser exposure is below the Maximum Permissible Exposure (MPE) for eye safety. It's calculated according to GB 7247-2012, ANSI Z136.1, and ISO 11553 standards.
Safety Zone Marking
- Danger Zone (0 to NOHD): Extreme eye hazard, protective eyewear mandatory
- Caution Zone (NOHD to 1.5× NOHD): Reduced hazard, protective measures recommended
- Safe Zone (> 1.5× NOHD): Minimal hazard, normal operation
Laser Classification System
- Class 1: Safe under all conditions (<0.5 mW)
- Class 2: Low-power visible (<1 mW) - blink reflex provides protection
- Class 3R: Low risk (<5 mW) - viewing hazardous, but low injury probability
- Class 3B: Medium risk (5-500 mW) - direct viewing hazardous, diffuse reflections usually safe
- Class 4: High risk (>500 mW) - hazardous to eyes and skin, fire hazard, diffuse reflections dangerous
Wavelength-Specific Hazards
UV Lasers (200-400nm):
- Absorbed by cornea and lens - cataract risk
- Skin damage (sunburn-like) possible
- Lower MPE due to photochemical damage
Visible Lasers (400-700nm):
- Focused on retina - burn risk
- Aversion response (blink) provides some protection for Class 2
- Green (532nm) most visible, appears brighter than same power red
Near-IR Lasers (700-1400nm):
- Invisible but focused on retina (1064nm fiber lasers)
- No aversion response - more dangerous than visible
- Most common industrial lasers
Far-IR Lasers (1400nm+):
- Absorbed by cornea and water in eye (CO₂ at 10.6μm)
- Lower retinal risk but corneal burn possible
- Higher MPE than visible/near-IR
Factors Affecting NOHD
- Laser Power: NOHD ∝ √Power - doubling power increases NOHD by 1.4×
- Beam Divergence: NOHD ∝ 1/divergence - larger divergence reduces NOHD
- Wavelength: Different wavelengths have different MPE (UV lowest, far-IR highest)
- Exposure Duration: Longer exposure requires stricter limits
- Beam Quality (M²): Better beam quality (lower M²) = smaller spot = longer NOHD
Eye Protection Selection
Optical Density (OD) indicates the attenuation factor: OD 5 = 10⁵ = 100,000× reduction.
Always verify eyewear is certified for:
- Specific wavelength (±50nm tolerance)
- Correct OD rating for maximum laser power
- Continuous wave or pulsed operation
- Sufficient Visible Light Transmission (VLT) for task visibility
⚠️ Critical Safety Note:
Never rely solely on calculations. Always implement multiple safety barriers: beam enclosure, interlocks, warning labels, training, and supervision. NOHD calculations assume direct beam exposure - scatter and reflections can extend hazard zones.