Unit 4: Maximum Permissible Exposure (MPE), Nominal Ocular Hazard Distance (NOHD) and Nominal Hazard Zone (NHZ)

C(E)

Correction factor use for calculating the extended source MPE for the eye from the small-source MPE, when the laser source subtends a visual angle exceeding a(min)

C(C)

Correction factor which increases the MPE values for ocular exposure because of pre-retinal absorption of radiant energy in the spectral region between 1150-1400 nm

C(B)

Correction factor which increases the MPE values in part of the visible spectrum because of greatly reduced photochemical epithelium

C(A)

Correction factor which increases the MPE values in the near IR spectral band based upon reduced absorption properties of melanin pigment granules found in the skin and in the retinoal pigment epithelium

C(P)

Correction factor which reduces the MPE for repetitive-pulse exposure of the eye

Maximum Permissible Exposure (MPE)

Define the level of laser radiation to which a person may be exposed without hazardous effects or adverse biological changes in the eye or skin Depend on wavelength, exposure duration, output, and exposure conditions Ocular, skin

Factors in NHZ

Laser power or energy output Beam diameter Beam divergence Pulse repetition frequency Wavelength Beam optics and beam path Reflection values for surfaces impacted by laser beam Maximum anticipated exposure duration

Nominal Ocular Hazard Distance (NOHD)

The distance from the laser to where the energy of the laser beam will eventually fall to levels that are below the ocular MPE

Nominal Hazard Zone (NHZ)

The space within which the level of the direct, reflected, or scattered radiation during normal operation exceeds the applicable MPE, which defines the space where control measures are required Indoors, is often limited by walls and ceilings that confine the laser beam within the room that the laser is used Outdoors, may be limited somewhat by terrain, but can extend miles in any direction that the laser can be expected to aim its beam

Exposure Duration

UV (180-400 nm): 30,000 s (full work day) Visible (400-700): 0.25 s (aversion response) IR (700-10000 nm): 10 s All wavelengths, single pulse: pulse width

Extended vs Point Source

a(min) = 1.5 mrad if a<a(min), point source if a>a(min), extended source Extended source is less hazardous than a point source