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Laser Control Measures--General Concepts

Individuals who operate lasers should follow the guidelines in this section to protect both themselves and others in the area.  Supervisors and operators should be properly trained before working with or around Class 2, 3, and 4 lasers

Features of a laser device, such as power output, beam diameter, pulse length, wavelength, beam path, beam divergence, and exposure duration determine the capability for injuring personnel.  The potential for injury from use of a laser is determined by its classification, therefore, the control measures are also determined by laser class. 

Concepts such are the maximum permissible exposure (MPE), accessible emission level (AEL) and  nominal hazard zone (NHZ) are important for the laser operator to use and understand.

Maximum Permissible Exposure (MPE)

MPE is the maximum level of laser radiation to which a person may be exposed without hazardous effects or biological changes in the eye or skin.  The MPE is determined by the wavelength of of laser, the energy involved, and the duration of the exposure.  The ANSI 136.1 standard tables 5, 6, and 7 (See Appendix A) summarize the MPE for particular wavelengths and exposure durations.  

MPE is a necessary parameter in determining the appropriate optical density and the nominal hazard zone.

Optical Density (OD)

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The OD (absorbance) is used in the determination of the appropriate eye protection.  OD is a logarithmic function defined by:

Where H0 is the anticipated worst case exposure conditions (in joules/cm2 or watts/cm2) and the MPE is expressed in the same units as H0.  The OD values for various lasers, computed for various appropriate exposure times, are listed below.  Keep in mind that these values are for intrabeam viewing (worst case) only.  Viewing Class 4 diffuse reflections (such as alignment tasks) requires, in general, less OD.  These should be determined for each situation and would be dependent upon the laser parameters and viewing distance.

The table belowprovides a summary of optical density needed for particular lasers, based on the worst case exposure duration:

 

 Optical Densities for Protective Eyewear for Various Laser Types

Laser Type/ Power

Wavelength

(mm)

OD

0.25 seconds

OD

10 seconds

OD for

600 seconds

OD for

30,000 seconds

XeCl

50 watts

0.308a

---

6.2

8.0

9.7

XeFl

50 watts

0.351a

---

4.8

6.6

8.3

Argon

1.0 watt

0.514

3.0

3.4

5.2

6.4

Krypton

1.0 watt

0.530

3.0

3.4

5.2

6.4

Krypton

1.0 watt

0.568

3.0

3.4

4.9

6.1

HeNe

0.005 watt

0.633

0.7

1.1

1.7

2.9

Krypton

1.0 watt

0.647

3.0

3.4

3.9

5.0

GaAs

50 mW

0.840c

---

1.8

2.3

3.7

Nd:YAG

100 watt

1.064a

---

4.7

5.2

5.2

Nd:YAG

(Q-switch)b

1.064a

---

4.5

5.0

5.4

Nd:YAGc

50 watts

1.33a

---

4.4

4.9

4.9

CO2

1000 watts

10.6a

---

6.2

8.0

9.7

a Repetitively pulsed at 11 Hertz, 12 ns pulses, 20mJ/pulse

b OD for UV and FIR beams computed using 1 mm limiting aperture which presents a “worst case scenario.  All visible/NIR computation assume 7 mm limiting aperture.

c Nd:YAG operating at a less common 1.33 mm wavelength.

NOTE:  All OD values determined using MPE criteria of ANSI Z-136.1

Nominal Hazard Zone (NHZ)

The NHZ relates to the space within which the level of direct, reflected, or scattered radiation during normal operation exceeds the appropriate MPE.  Exposure levels beyond the NHZ are below the appropriate MPE level, thus no control measures are needed outside the NHZ.  The NHZ may be calculated using the following formula:

Where ø is the emergent beam divergence measured in radians;  Φ is the radiant power (total radiant power for continuous wave lasers or average radiant power of a pulsed laser) measured in watts; and a is the diameter of the emergent laser beam, in centimeters.

nominal hazard zone diagram

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