Laser Safety Training Guide

The Laser Safety Training Guide provides basic information for working safely with low, medium and high-powered lasers. It is intended to supplement, but not replace the Laser Safety Training that all Class 3 and 4 laser users are required to attend.

Section 1: Laser Fundamentals

Notice: The materials found on these pages are provided for the use of Princeton University faculty, staff and students to meet training needs specific to Princeton University.

Introduction (top)

The word laser is an acronym for Light Amplification by Stimulated Emission of Radiation.  Lasers are used as research aides in many departments at Princeton University.

In this document, the word laser will be limited to electromagnetic radiation-emitting devices using light amplification by stimulated emission of radiation at wavelengths from 180 nanometers to 1 millimeter.  The electromagnetic spectrum includes energy ranging from gamma rays to electricity.  Figure 1 illustrates the total electromagnetic spectrum and wavelengths of the various regions.

Electromagnetic Spectrum

Firgure 1. Electromagnetic Spectrum

The primary wavelengths for lasers used at Princeton University include the ultraviolet, visible and infrared regions of the spectrum.  Ultraviolet radiation for lasers consists of wavelengths between 180 and 400 nanometers (nm).  The visible region consists of radiation with wavelengths between 400 and 700 nm. This is the portion we call visible light.  The infrared region of the spectrum consists of radiation with wavelengths between 700 nm and 1 mm.

The color or wavelength of light being emitted depends on the type of lasing material being used.  For example, if a Neodymium:Yttrium Aluminum Garnet (Nd:YAG) crystal is used as the lasing material, light with a wavelength of 1064 nm will be emitted.  Table 1 illustrates various types of material currently used for lasing and the wavelengths that are emitted by that type of laser.  Note that certain materials and gases are capable of emitting more than one wavelength. The wavelength of the light emitted in this case is dependent on the optical configuration of the laser.

 Common Lasers and Their Wavelengths

Table 1. Common Lasers and Their Wavelengths


Laser Theory And Operation (top)

A laser generates a beam of very intense light.  The major difference between laser light and light generated by white light sources (such as a light bulb) is that laser light is monochromatic, directional and coherent.  Monochromatic means that all of the light produced by the laser is of a single wavelength.  White light is a combination of all visible wavelengths (400 - 700 nm).  Directional means that the beam of light has very low divergence.  Light from a conventional sources, such as a light bulb diverges, spreading in all directions, as illustrated in Figure 2.  The intensity may be large at the source, but it decreases rapidly as an observer moves away from the source.

Divergence of Conventional Light Source

Figure 2. Divergence of Conventional Light Source

In contrast, the output of a laser, as shown in Figure 3, has a very small divergence and can maintain high beam intensities over long ranges.  Thus, relatively low power lasers are able to project more energy at a single wavelength within a narrow beam than can be obtained from much more powerful conventional light sources.

Divergence of Laser Source

Figure 3. Divergence of Laser Source

Coherent means that the waves of light are in phase with each other.  A light bulb produces many wavelengths, making it incoherent.

Coherence of conventional vs. laser light

Figure 4. Incoherent light bulb vs. coherent laser


Components of a Laser (top)

Figure 5 illustrates the basic components of the laser including the lasing material, pump source or excitation medium, optical cavity and output coupler.

Solid State Laser Diagram

Figure 5. Solid State Laser Diagram

The lasing material can be a solid, liquid, gas or semiconductor, and can emit light in all directions.  The pump source is typically electricity from a power supply, lamp or flashtube, but may also be another laser.  It is very common in Princeton University laboratories to use one laser to pump another.

The excitation medium is used to excite the lasing material, causing it to emit light.  The optical cavity contains mirrors at each end that reflect this light and cause it to bounce between the mirrors.  As a result, the energy from the excitation medium is amplified in the form of light.  Some of the light passes through the output coupler, usually a semi-transparent mirror at one end of the cavity.  The resulting beam is then ready to use for any of hundreds of applications.

The laser output may be steady, as in continuous wave (CW) lasers, or pulsed.  A Q-switch in the optical path is a method of providing laser pulses of an extremely short time duration.  The Q-switch may use a rotating prism, a pockels cell or a shutter device to create the pulse.  Q-switched lasers may produce a high-peak-power laser pulse of a few nanoseconds duration.

A continuous wave laser has a steady power output, measured in watts (W).  For pulsed lasers, the output generally refers to energy, rather than power.  The radiant energy is a function of time and is measured in joules (J).  Two terms are often used to when measuring or calculating exposure to laser radiation.  Radiant Exposure is the radiant energy divided by the area of the surface the beam strikes.  It is expressed in J/cm2.  Irradiance is the radiant power striking a surface divided by the area of the surface over which the radiant power is distributed.  It is expressed in W/cm2.  For repetitively pulsed lasers, the pulse repetition factor (prf) and pulse width are important in evaluating biological effects.

Types of Lasers (top)

The laser diode is a light emitting diode that uses an optical cavity to amplify the light emitted from the energy band gap that exists in semiconductors.  (See Figure 6.)  They can be tuned to different wavelengths by varying the applied current, temperature or magnetic field.

Semiconductor laser diagram

Figure 6. Semiconductor laser diagram

Gas lasers consist of a gas filled tube placed in the laser cavity as shown in Figure 7.  A voltage (the external pump source) is applied to the tube to excite the atoms in the gas to a population inversion.  The light emitted from this type of laser is normally continuous wave (CW). One should note that if Brewster angle windows are attached to the gas discharge tube, some laser radiation may be reflected out the side of the laser cavity. Large gas lasers known as gas dynamic lasers use a combustion chamber and supersonic nozzle for population inversion.

Gas laser diagram

Figure 7. Gas laser diagram

Dye lasers employ an active material in a liquid suspension.  The dye cell contains the lasing medium.   These lasers are popular because they may be tuned to several wavelengths by changing the chemical composition of the dye.  Many of the commonly used dyes or liquid suspensions are toxic.

Free electron lasers such as in Figure 8 have the ability to generate wavelengths from the microwave to the X-ray region. They operate by having an electron beam in an optical cavity pass through a wiggler magnetic field. The change in direction exerted by the magnetic field on the electrons causes them to emit photons.

Free electron laser

Figure 8. Free electron laser


 

Section 2: Laser Hazards

The hazards of lasers may be separated into two general categories – beam related hazards to eyes and skin and non-beam hazards, such as electrical and chemical hazards.


Beam Related Hazards

Improperly used laser devices are potentially dangerous. Effects can range from mild skin burns to irreversible injury to the skin and eye. The biological damage caused by lasers is produced through thermal, acoustical and photochemical processes.

Thermal effects are caused by a rise in temperature following absorption of laser energy. The severity of the damage is dependent upon several factors, including exposure duration, wavelength of the beam, energy of the beam, and the area and type of tissue exposed to the beam.

Acoustical effects result from a mechanical shockwave, propogated through tissue, ultimately damaging the tissue. This happens when the laser beam causes localized vaporization of tissue, causing the shockwave analogous to ripples in water from throwing a rock into a pond.

Beam exposure may also cause photochemical effects when photons interact with tissue cells. A change in cell chemistry may result in damage or change to tissue. Photochemical effects depend greatly on wavelength. Table 2 summarizes the probable biological effects of exposure of eyes and skin to different wavelengths.

Photobiological
Spectral domain
Eye
 
skin
 

Ultraviolet C

(200 nm - 280 nm)

Photokeratitis

 

Erythema (sunburn)
Skin Cancer
Accelerated skin aging

Ultraviolet B

(280 nm - 315 nm)

Photokeratitis

 

Increased pigmentation

Ultraviolet A

(315 nm - 400 nm)

Photochemical cataract

Pigment darkening
Skin burn

Visible

(400 nm - 780 nm)

Photochemical and thermal retinal injury

Pigment darkening
Photosenstive reactions
Skin burn

Infrared A

(780 nm - 1400 nm)

Cataract and retinal burn

Skin burn

Infrared B

(1.4mm - 3.0 mm)

Corneal burn, aqueous flare, cataract

Skin burn

Infrared C

(3.0 mm - 1000 mm)

Corneal burn only

Skin burn


Types of Beam Exposures (top)

Exposure to the laser beam is not limited to direct beam exposure. Particularly for high powered lasers, exposure to beam reflections may be just as damaging as exposure to the primary beam.

Intrabeam exposure means that the eye or skin is exposed directly to all or part of the laser beam. The eye or skin is exposed to the full irradiance or radiant exposure possible.

Specular reflections from mirror surfaces can be nearly as harmful as exposure to the direct beam, particularly if the surface is flat. Curved mirror-like surfaces will widen the beam such that while the exposed eye or skin does not absorb the full impact of the beam, there is a larger area for possible exposure.

A diffuse surface is a surface that will reflect the laser beam in many directions. Mirror-like surfaces that are not completely flat, such as jewelry or metal tools, may cause diffuse reflections of the beam. These reflections do not carry the full power or energy of the primary beam, but may still be harmful, particularly for high powered lasers. Diffuse reflections from Class 4 lasers are capable of initiating fires.

Whether a surface is a diffuse reflector or a specular reflector will depend upon the wavelength of the beam. A surface that would be a diffuse reflector for a visible laser may be a specular reflector for an infrared laser beam.

Eye (top)

The major danger of laser light is hazards from beams entering the eye. The eye is the organ most sensitive to light. Just as a magnifying glass can be used to focus the sun and burn wood, the lens in the human eye focuses the laser beam into a tiny spot than can burn the retina. A laser beam with low divergence entering the eye can be focused down to an area 10 to 20 microns in diameter.

The laws of thermodynamics do not limit the power of lasers. The second law states that the temperature of a surface heated by a beam from a thermal source of radiation cannot exceed the temperature of the source beam. The laser is a non-thermal source and is able to generate temperatures far greater than it's own. A 30 mW laser operating at room temperature is capable of producing enough energy (when focused) to instantly burn through paper.

Per the law of the conservation of energy, the energy density (measure of energy per unit of area) of the laser beam increases as the spot size decreases. This means that the energy of a laser beam can be intensified up to 100,000 times by the focusing action of the eye. If the irradiance entering the eye is 1 mW/cm2, the irradiance at the retina will be 100 W/cm2. Thus, even a low power laser in the milliwatt range can cause a burn if focused directly onto the retina.

NEVER point a laser at someone's eyes no matter how low the power of the laser.

Structure Of The Eye (top)

Damage to the eye is dependent upon the wavelength of the beam. In order to understand the possible health effects, it is important to understand the functions of the major parts of the human eye.

The cornea is the transparent layer of tissue covering the eye. Damage to the outer cornea may be uncomfortable (like a gritty feeling) or painful, but will usually heal quickly. Damage to deeper layers of the cornea may cause permanent injury.

Cross section of the human eye

Cross section of the human eye

The lens focuses light to form images onto the retina. Over time, the lens becomes less pliable, making it more difficult to focus on near objects. With age, the lens also becomes cloudy and eventually opacifies. This is known as a cataract. Every lens develops cataract eventually.

The part of the eye that provides the most acute vision is the fovea centralis (also called the macula lutea). This is a relatively small area of the retina (3 to 4%) that provides the most detailed and acute vision as well as color perception. This is why eyes move when you read or when you look as something; the image has to be focused on the fovea for detailed perception. The balance of the retina can perceive light and movement, but not detailed images (peripheral vision).

If a laser burn occurs on the fovea, most fine (reading and working) vision may be lost in an instant. If a laser burn occurs in the peripheral vision it may produce little or no effect on fine vision. Repeated retinal burns can lead to blindness.

Fortunately the eye has a self-defense mechanism -- the blink or aversion response. When a bright light hits the eye, the eye tends to blink or turn away from the light source (aversion) within a quarter of a second. This may defend the eye from damage where lower power lasers are involved, but cannot help where higher power lasers are concerned. With high power lasers, the damage can occur in less time than a quarter of a second.

Symptoms of a laser burn in the eye include a headache shortly after exposure, excessive watering of the eyes, and sudden appearance of floaters in your vision. Floaters are those swirling distortions that occur randomly in normal vision most often after a blink or when eyes have been closed for a couple of seconds. Floaters are caused by dead cell tissues that detach from the retina and choroid and float in the vitreous humor. Ophthalmologists often dismiss minor laser injuries as floaters due to the very difficult task of detecting minor retinal injuries. Minor corneal burns cause a gritty feeling, like sand in the eye.

Several factors determine the degree of injury to the eye from laser light:

  • pupil size - The shrinking of pupil diameter reduces the amount of total energy delivered to the retinal surface. Pupil size ranges from a 2 mm diameter in bright sun to an 8 mm diameter in darkness (night vision).

  • degree of pigmentation - More pigment (melanin) results in more heat absorption.

  • size of retinal image - The larger the size, the greater the damage because temperature equilibrium must be achieved to do damage. The rate of equilibrium formation is determined by the size of the image.

  • pulse duration - The shorter the time (ns versus ms), the greater the chance of injury.

  • pulse repetition rate - The faster the rate, the less chance for heat dissipation and recovery.

  • wavelength - determines where the energy deposits and how much gets through the ocular media.


Eye Absorption Site vs. Wavelength (top)

The wavelength determines where the laser energy is absorbed in the eye.

Ocular absorption site vs. wavelength

Source: Sliney & Wolbarsht, Safety with Lasers and Other Optical Sources, Plenum Press, 1980

Lasers in the visible and near infrared range of the spectrum have the greatest potential for retinal injury, as the cornea and the lens are transparent to those wavelengths and the lens can focus the laser energy onto the retina. The maximum absorption of laser energy onto the retina occurs in the range from 400 - 550 nm. Argon and YAG lasers operate in this range, making them the most hazardous lasers with respect to eye injuries. Wavelengths of less than 550 nm can cause a photochemical injury similar to sunburn. Photochemical effects are cumulative and result from long exposures (over 10 seconds) to diffuse or scattered light. Table 3 summarizes the most likely effects of overexposure to various commonly used lasers.

Summary of Bioeffects of Commonly Used Lasers


Skin (top)

Lasers can harm the skin via photochemical or thermal burns. Depending on the wavelength, the beam may penetrate both the epidermis and the dermis. The epidermis is the outermost living layer of skin. Far and Mid-ultraviolet (the actinic UV) are absorbed by the epidermis. A sunburn (reddening and blistering) may result from short-term exposure to the beam. UV exposure is also associated with an increased risk of developing skin cancer and premature aging (wrinkles, etc) of the skin.

Thermal burns to the skin are rare. They usually require exposure to high energy beams for an extended period of time. Carbon dioxide and other infrared lasers are most commonly associated with thermal burns, since this wavelength range may penetrate deeply into skin tissue. The resulting burn may be first degree (reddening), second degree (blistering) or third degree (charring).

Some individuals are photosensitive or may be taking prescription drugs that induce photo-sensitivity. Particular attention must be given to the effect of these (prescribed) drugs, including some antibiotics and fungicides, on the individual taking the medication and working with or around lasers.

 


Non-Beam Hazards (top)

In addition to the hazards directly associated with exposure to the beam, ancillary hazards can be produced by compressed gas cylinders, cryogenic and toxic materials, ionizing radiation and electrical shock.

Electrical Hazards (top)

The use of lasers or laser systems can present an electric shock hazard. This may occur from contact with exposed utility power utilization, device control, and power supply conductors operating at potentials of 50 volts or more. These exposures can occur during laser set-up or installation, maintenance and service, where equipment protective covers are often removed to allow access to active components as required for those activities. The effect can range from a minor tingle to serious personal injury or death. Protection against accidental contact with energized conductors by means of a barrier system is the primary methodology to prevent electrical shock.

Additional electrical safety requirements are imposed upon laser devices, systems and those who work with them by the federal Occupational Safety and Health Administration OSHA, the National Electric Code and related state and local regulations. Individuals who repair or maintain lasers may require specialized electric safety-related work practices training. Contact the University Safety Engineer at 258-5294 for an electrical safety inspection and/or required training.

Another particular hazard is that high voltage electrical supplies and capacitors for lasers are often located close to cooling water pumps, lines, filters, etc. In the event of a spill or hose rupture, an extremely dangerous situation may result. During times of high humidity, over-cooling can lead to condensation which can have similar effects. A potentially lethal accident occurred at Princeton University when a graduate student opened a laser to wipe condensation from a tube.

The following are recommendations for preventing electrical shocks for lasers for all classifications:

  • All equipment should be installed in accordance with OSHA and the National Electrical Code.
  • All electrical equipment should be treated as if it were “live”.
  • Working with or near live circuits should be avoided. Whenever possible, unplug the equipment before working on it.
  • A “buddy system” should be used when work on live electrical equipment is necessary, particularly after normal work hours or in isolated areas. Ideally, the person should be knowledgeable of first aid and CPR.
  • Rings and metallic watchbands should not be worn, nor should metallic pens, pencils, or rulers be used while one is working with electrical equipment.
  • Live circuits should be worked on using one hand, when it is possible to do so.
  • When one is working with electrical equipment, only tools with insulated handles should be used.
  • Electrical equipment that upon touch gives the slightest perception of current should be removed from service, tagged and repaired prior to further use.
  • When working with high voltages, consider the floor conductive and grounded unless standing on a suitably insulated dry matting normally used for electrical work.
  • Live electrical equipment should not be worked on when one is standing on a wet floor, or when the hands, feet or body is wet or perspiring.
  • Do not undertake hazardous activities when truly fatigued, emotionally stressed, or under the influence of medication that dulls or slows the mental and reflex processes.
  • Follow lockout/tagout procedures when working with hard-wired equipment.

Section 3: Laser Hazard Classification

Lasers are classified according to their potential to cause biological damage.  The pertinent parameters are:

  • Laser output energy or power
  • Radiation wavelengths
  • Exposure duration
  • Cross-sectional area of the laser beam a the point of interest

In addition to these general parameters, lasers are classified in accordance with the accessible emission limit (AEL), which is the maximum accessible level of laser radiation permitted within a particular laser class.

The ANSI standard laser hazard classifications are used to signify the level of hazard inherent in a laser system and the extent of safety controls required.  These range from Class 1 lasers (which are inherently safe for direct beam viewing under most conditions) to Class 4 lasers (which require the most strict controls).  The laser classifications are described below:


Class 1-Exempt Lasers

Class 1 laser cannot, under normal operating conditions, produce damaging radiation levels.  These lasers must be labeled, but are exempt from the requirements of the Laser Safety Program. A laser printer is an example of a Class 1 laser.

Class 1M lasers cannot, under normal operating conditions, produce damaging radiation levels unless the beam is viewed with an optical instrument such as an eye-loupe (diverging beam) or a telescope (collimated beam).  This may be due to a large beam diameter or divergence of the beam. Such lasers must be labeled, but are exempt from the requirements of the Laser Safety Program other than to prevent potentially hazardous optically aided viewing.


Class 2-Low Power Visible Lasers (top)

Class 2  lasers are low power lasers or laser system in the visible range (400 - 700 nm wavelength) that may be viewed directly under carefully controlled exposure conditions.  Because of the normal human aversion responses, these lasers do not normally present a hazard, but may present some potential for hazard if viewed directly for long periods of time.  A continuous wave (cw) HeNe laser above Class 1, but not exceeding 1 mW radiant power is an example of a Class 2 laser.

Class 2M  lasers are low power lasers or laser system in the visible range (400 - 700 nm wavelength) that may be viewed directly under carefully controlled exposure conditions.  Because of the normal human aversion responses, these lasers do not normally present a hazard, but may present some potential for hazard if viewed with certain optical aids.


Class 3-Medium Power Lasers and Laser Systems (top)

Class 3 lasers are medium power lasers or laser systems that require control measures to prevent viewing of the direct beam.  Control measures emphasize preventing exposure of the eye to the primary or specularly reflected beam.

Class 3R denotes lasers or laser systems potentially hazardous under some direct and specular reflection viewing condition if the eye is appropriately focused and stable, but the probability of an actual injury is small.  This laser will not pose either a fire hazard or diffuse-reflection hazard.  They may present a hazard if viewed using collecting optics.  Visible CW HeNe lasers above 1 mW, but not exceeding 5 mW radiant power, are examples of this class.

Class 3B denotes lasers or laser systems that can produce a hazard if viewed directly.  This includes intrabeam viewing or specular reflections.  Except for the higher power Class 3b lasers, this class laser will not produce diffuse reflections.  Visible cw HeNe lasers above 5 mW, but not exceeding 500 mW radiant power, are examples of this class.


Class 4-High Power Lasers and Laser Systems (top)

A high power laser or laser system that can produce a hazard not only from direct or specular reflections, but also from a diffuse reflection.  In addition, such lasers may produce fire and skin hazards.  Class 4 lasers include all lasers in excess of Class 3 limitations.


Classification (top)

Commercial lasers are classified and certified by the manufacturer.  When a commercial laser is modified or when a new laser is constructed in the laboratory, it is the responsibility of the principal investigator to classify and label the laser per the ANSI Standard.  EHS can assist in determining the appropriate classification.  See Table A for a summary of typical laser classifications.

Section 4: Laser Control Measures

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) (top)

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

Optical density equation

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.

Table 4 provides a summary of optical density needed for particular lasers, based on the worst case exposure duration.

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

 Normal Hazard Zone (NHZ) (top)

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:

Nominal Hazard Zone Formula

Where f is the emergent beam divergence measured in radians; F 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.


Control Measures by Laser Classification (top)

Potential hazards exist to all individuals working near a laser system.  Such individuals should be warned of the existence and location of lasers, and of the meaning of the warning labels for all classes of lasers.

Particular attention should be given to the environment where the laser is used.  This factor should be considered together with the class and application of the laser for determining the control measures to be applied.  Basic elements to be considered are:

  • number and class of lasers
  • laser location
  • presence (access) of uninformed, unprotected personnel
  • permanence of beam paths
  • presence of objects that may have specular surfaces or reflecting objects near the beam path
  • use of optical devices such as lenses, microscopes, etc.

Control measures may be broken down to two types: administrative controls, such as signage, procedures, etc., and engineering controls, such as beam housings, shutters, etc.  The following are general considerations for work with lasers, per laser hazard class.  Table 5 provides a summary of these control measures.

Class 1 (top)

Many Class 1 lasers have higher class lasers enclosed within a protective housing.  If the Class 1 laser has an enclosed Class 3b or 4 laser, interlocks should be provided on any removable parts of the housing, or the laser should have a service access panel that is either interlocked or requires a tool for removal.  If the protective housing is removed, control measures appropriate for the enclosed laser class should be followed.

All Class 1 lasers must be labeled.

Class 2 (top)

Class 2 lasers must be labeled.

The laser beam should not be purposefully directed toward the eye of any person.  Alignment of the laser optical systems (mirrors, lenses, beam deflectors, etc.) should be performed in such a manner that the primary beam, or specular reflection of the primary beam, does not expose the eye to a level above the MPE for direct irradiation of the eye.

The work area should be posted with a warning label or sign cautioning users to avoid staring into the beam or directing the beam toward the eye of individuals.

If the MPE is exceeded, design viewing portals and/or display screens to reduce exposure to acceptable levels.

If the Class 2 laser has an enclosed Class 3b or 4 laser, interlocks should be provided on any removable parts of the housing, or the laser should have a service access panel that is either interlocked or requires a tool for removal.  If the protective housing is removed, control measures appropriate for the enclosed laser class should be followed.

Class 3a (top)

Class 3a lasers must be labeled accordingly.  The work area should be posted with a warning label or sign cautioning users to avoid staring into the beam or directing the beam toward the eye of individuals.

Removable parts of the housing and service access panels should have interlocks to prevent accidental exposure.  A permanent beam stop or attenuator may also be used.

If the MPE is exceeded, design viewing portals and/or display screens to reduce exposure to acceptable levels.  Alignment procedures should be designed to ensure the MPE is not exceeded.

Class 3b (top)

Class 3b lasers and laser systems must be labeled accordingly.  These lasers are used in areas where entry by unauthorized individuals can be controlled.  If an individual who has not been trained in laser safety must enter the area, the laser operator or supervisor should first instruct the individual as to safety requirements and must provide protective eyewear, if required.

If the entire beam is not enclosed or if a limited open beam exists, the laser operator, supervisor or laser safety officer should determine a Nominal Hazard Zone (NHZ).  An alarm, warning light or verbal countdown should be used during use or start up of the laser.

The controlled area should

  • have limited access to spectators,
  • have beam stops to terminate potentially dangerous laser beams,
  • be designed to reduce diffuse and specular reflections,
  • have eye protection for all personnel,
  • not have a laser beam at eye level,
  • have restrictions on windows and doorways to reduce exposure to levels below the MPE, and
  • require storage or disabling of the laser when it is not being used.

If the MPE is exceeded, design viewing portals and/or display screens to reduce exposure to acceptable levels.  Alignment procedures and collecting optics should be designed to ensure the MPE is not exceeded.

Only authorized, trained individuals should service the laser.  Approved, written standard operating, maintenance and service procedures should be developed and followed.

Class 4 (top)

In addition to the control measures described for Class 3b, Class 4 lasers should be operated by trained individuals in areas dedicated to their use.  Failsafe interlocks should be used to prevent unexpected entry into the controlled area, and access should be limited by the laser operator to persons who have been instructed as to the safety procedures and who are wearing proper laser protection eyewear when the laser is capable of emission.

Laser operators are responsible for providing information and safety protection to untrained personnel who may enter the laser controlled areas as visitors.

The laser area should be

  • restricted to authorized personnel only
  • designed to allow for rapid emergency egress
  • equipped with a device that allows for deactivation of the laser or reduction of the output to below the MPE
  • designed to fulfill Class 3b controlled area requirements
  • designed with entry safe controls
  • designed such that the laser may be monitored and fired from a remote location
  • (for pulsed systems) have interlocks designed to prevent firing of the laser by dumping the stored energy into a dummy load
  • (for continuous wave systems) have interlocks designed to turn off the power supply or interrupt the beam by means of shutters.

The beam path must be free of specularly reflective surfaces and combustible objects and the beam terminated in a non-combustible, non-reflective barrier or beam stop.



 Warning Signs and Labels (top)

All Class 2, 3 and 4 laser equipment must be labeled indicating hazard classification, output power/energy, and lasing material or wavelength with words and symbols as indicated below:

  • Class 2 laser equipment: CAUTION, Laser Radiation (or laser symbol), Do Not Stare Into Beam

Class 2 laser Warning Sign

  • Class 3R laser equipment, below MPE: Danger, Laser Radiation (or laser symbol), Do Not Stare into Beam or View Directly with Optical Instruments

Class 3R  Laser Warning Sign

  • Class 3R laser equipment, above MPE: DANGER, Laser Radiation (or laser symbol), Avoid Direct Eye Exposure

Class 3R (>MPE) Laser Warning Sign

  • Class 3B laser equipment: DANGER, Laser Radiation (or laser symbol), Avoid Direct Exposure to Beam

Class 4 Laser Warning Sign

  • Class 4 laser equipment: DANGER, Laser Radiation (or laser symbol), Avoid Eye or Skin Exposure to Direct or Scattered Radiation

Class 4 Laser Warning Sign

Labels and warning signs should be displayed conspicuously in areas where they would best serve to warn individuals of potential safety hazards.  Normally, signs are posted at entryways to laser controlled areas and labels are affixed to the laser in a conspicuous location.


Table 5. Control Measures for the Four Laser Classes

Control Measures

Classification

Engineering Controls

1

1M

2

2M

3R

3B

4

Protective Housing

X

X

X

X

X

X

X

Without protective housing

Laser Safety Officer establishes alternative controls

Interlocks on protective housing

à

à

à

à

à

X

X

Service Access Panel

à

à

à

à

à

X

X

Key Control

--

--

--

--

--

·

X

Viewing Portals

Assure viewing limited < MPE

Collecting Optics

             

Totally Open Beam Path

--

--

--

--

--

X
NHZ

X
NHZ

Limited Open Beam Path

--

--

--

--

--

X
NHZ

X
NHZ

Enclosed Beam Path

None required if protective housing and interlocks in place

Remote Interlock Connector

--

--

--

--

--

·

X

Beam Stop or Attenuator

--

--

--

--

--

·

X

Activation Warning Systems

--

--

--

--

--

·

X

Indoor Laser Controlled Area

--

--

--

--

--

--

X

Class 3B Indoor Laser Controlled Area
--
--
--
--
--
X
--
Class 4 Laser Controlled Area
--
--
--
--
--
--
--
Outdoor Control Measures
--
--
--
--
--
--
--
Laser in Navigable Airspace

X

·
NHZ

X
NHZ

·
NHZ

X
NHZ

X
MPE

X
MPE

Temporary Laser Controlled Area

à
MPE

à
MPE

à
MPE

à
MPE

à
MPE

--

--

Controled Operation

--

--

--

--

--

--

·

Equipment Labels

X

X

X

X

X

X

X

Laser Area Warning Signs

--

--

--

X

·

X

X

 

Administrative and Procedural Controls

Standard Operating Procedure

--

--

--

--

--

·

X

Output Emission Limitations

--

--

--

--

--

LSO Determines

Education and Training

--

·

·

·

·

X

X

Authorized Personnel

--

*

--

*

--

X

X

Alignment Procedures

à

à

à

à

à

X

X

Protective Equipment

--

*

--

*

--

·

X

Spectator

--

*

--

*

--

·

X

Service Personnel

à

à

à

à

à

X

X

Demonstration with Public

--

*

X

*

X

X

X

Laser Fiber Optic Systems

MPE

MPE

MPE

MPE

MPE

X

X

Laser Robotic Installation

--

--

--

--

--

X
NHZ

X
NHZ

Protective Eyewear

--

--

--

--

--

·
MPE

X
MPE

Window Protection

--

--

--

--

--

X

X
NHZ

Protective Barriers and Curtains

--

--

--

--

--

·

·

Skin Protection

--

--

--

--

--

X

X
MPE

Warning Signs and Labels

--

--

·

·

·

X
NHZ

X
NHZ

Skin Protection

--

 

--

--

--

X
MPE

X
MPE

LEGEND

X = shall       · = should      -- = no requirement    NHZ = NHZ analysis required

à = shall if enclosed Class 3b or 4            MPE = shall if MPE is exceeded


Protective Equipment (top)

Enclosure of the laser equipment or beam path is the preferred method of control, since the enclosure will isolate or minimize the hazard.  When engineering controls do not provide adequate means to prevent access to direct or reflected beams at levels above the MPE, it may be necessary to use personal protective equipment.  Note that use of personal protective equipment may have serious limitations when used as the only control measure with higher power Class 4 lasers or laser systems.  The protective equipment may not adequately reduce or eliminate the hazard and may be damaged by the incident laser radiation.

Protective Eyewear (top)

Protective eyewear is necessary for Class 3 and 4 laser use where irradiation of the eye is possible.  Such eye protection should be used only at the wavelength and energy/power for which it is intended.  Eye protection may include goggles, face shields, spectacles or prescription eyewear using special filter materials or reflective coatings (or a combination of both) to reduce exposure below the MPE.  Eye protection may also be necessary to protect against physical or chemical hazards.

Laser Protective Eyewear  Infrared Laser Protective Eyewear

The following factors should be considered in selecting the appropriate laser protective eyewear:

  • wavelength(s) of the laser output
  • potential for multi-wavelength operation
  • radiant exposure or irradiance levels for which protection (worst case) is required
  • exposure time criteria
  • MPE
  • optical density (OD) requirement of the eyewear filter at laser output wavelength
  • angular dependence of protection afforded
  • visible light transmission requirement and assessment of the effect of the eyewear on the ability to perform tasks while wearing the eyewear
  • need for side shield protection and peripheral vision
  • radiant exposure or irradiance and the corresponding time factors at which laser safety eyewear damage (penetration) occurs, including transient bleaching
  • need for prescription glasses
  • comfort and fit
  • degradation of absorbing media, such as photobleaching
  • strength of materials (resistance to mechanical shock or trauma)
  • capability of the front surface to produce a hazardous specular reflection
  • requirement for anti-fogging design or coatings

Laser Eye Protection Selection Process (top)

  1. Determine the wavelength of the laser.  Eye protection is wavelength-specific.  Eyewear that provides protection for CO2 lasers will not necessarily protect against Nd:YAG lasers.
  2. Determine the maximum anticipated viewing duration.  Viewing duration usually fall into one of three categories:
    1. Unintentional, accidental exposure to visible lasers (400-700 nm), use 0.25 seconds
    2. Unintentional, accidental viewing of near infrared (700-1000 nm) beams, use 10 seconds
    3. For all other lasers, use 600 seconds or laser on time, up to 8 hours.
  3. Determine the maximum irradiance or radiant exposure to which the eye may be exposed.  Consider the following:
    1. If the emergent beam is not focused down to a smaller spot and is greater than 7 mm in diameter, the emergent beam radiant exposure/irradiance may be considered the maximum intensity that could enter the eye. 
    2. If the beam is focused after emerging from the laser or if the beam diameter is less than 7 mm, assume that all of the laser energy/power could enter the eye.  In this case, use the columns titled Maximum Output Power/Energy in Table 6.
  4. Determine the optical density needed.
  5. Select the type of eye protection needed.  Laser eye protection is available in the form of glasses and goggles.  The lens may be made out of glass or crystalline filter material or plastic.  Generally, glass or crystalline lenses are recommended for harsh environments, such as areas where solvents and corrosives are used.
  6. Test the eye protection.  Always check the integrity of the lens before use.  At very high beam intensities, filter materials become bleached out or otherwise damaged.  A continuous wave power exceeding 10 W can fracture glass and burn through plastics.

Table 6. Selecting Laser Eye Protection for Intrabeam Viewing for 400 - 1400 nm Wavelengths

Q-Switched
(1 ns - 0.1 ms)
Non-Q-Switched
(0.4 ms - 10 ms)

CW Momentary View (0.25 s to 10 s)

CW Starting (more than 3 hours)

Attenuation Factor

Max Output Energy (J)

Max Beam Radiant Exposure (j/cm^2)
Max Laser Output Energy (J)
Max Beam Radiant Exposure (J/cm^2)
Max Power Output (W)
Max Beam Irradiance (W/cm^2)
Max Power Output (W)
Max Beam Irradiance (W/cm^2)
10
20
100
200
na
na
na
na
100,000,000
1
2
10
20
na
na
na
na
10,000,000
10^-1
2x10^-1
1
2
na
na
na
na
1,000,000

10^-2

2x10^-2
10^-1
2x10^-1
na
na
10^-1
2x10^-1
100,000
10^-3
2x10^-3
10^-2
2x10^-2
10
20
10^-2
2x10^-2
10,000
10^-4
2x10^-4
10^-3
2x10^-3
1
2
10^-3
2x10^-3
1,000
10^-5
2x10^-5
10^-4
2x10^-4
10^-1
2x10^-1
10^-4
2x10^-4
100
10^-6
2x10^-6
10^-5
2x10^-5
10^-2
2x10^-2
10^-5
2x10^-5
10

Other Protective Equipment (top)

It is important that protective equipment such as beam stops, shields, safety interlocks, and warning lights and horns be maintained in proper operating condition and be utilized whenever indicated to prevent harmful exposure to laser radiation. 

Special Controls for UltraViolet and Infrared Lasers (top)

Since infrared (IR) and ultraviolet (UV) wavelengths are normally invisible, particular care must be taken when using these types of lasers.  In addition to the recommended control measures that apply for each laser classification, the following should also be employed:

Infrared 
  1. The collimated beam from a Class 3 laser should be terminated by a highly absorbent backstop wherever practicable.Many surfaces which appear dull visually can act as reflectors of IR.
  2. The beam from a Class 4 laser should be terminated in a fire resistant material wherever practicable.Periodic inspection of the absorbent material is required since many materials degrade with use.
  3. Areas that are exposed to reflections from Class 3 or 4 lasers, at levels above the MPE, should be protected by appropriately screening the beam or target area with IR absorbent material.  This material should be fire-resistant for use with Class 4 lasers.
UV
  1. Exposure to UV should be minimized by using shield material which attenuates the radiation to levels below the appropriate MPE for the specific wavelength.
  2. Special attention should be given to the possibility of producing undesirable reactions in the presence of UV, for example, ozone formation.