Laser safety

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A laser is a light source that can be dangerous to the people exposed to it. Even low power lasers can be hazardous to eyesight. A person exposed to, in particular invisible laser radiation, may be unaware that damage is occurring. Some lasers are so powerful that even the diffuse reflection from a surface can be hazardous to the eye. Laser radiation predominantly causes eye injury via thermal effects on the retina. A transient increase of only 10 °C can destroy retinal photoreceptors.

The coherence, the low divergence angle of laser light and the focusing mechanism of the eye, means that laser light can be concentrated into an extremely small spot on the retina. If the laser is sufficiently powerful, permanent damage can occur within a fraction of a second, even faster than the blink of an eye. Sufficiently powerful visible to near infrared laser radiation (400-1400 nm) will penetrate the eye ball and may cause heating of the retina, whereas exposure to laser radiation with wavelengths less than 400 nm and greater than 1400 nm are largely absorbed by the cornea and lens, leading to the development of cataracts or burn injuries [1].

Infrared and ultraviolet lasers are particularly hazardous, since the body's protective "blink reflex" response only operates if the light is visible. For example, some people exposed to high power Nd:YAG laser emitting invisible 1064 nm radiation, may not feel pain or notice immediate damage to their eye sight. A pop or click noise emanating from the eyeball may be the only indication that retinal damaged has occurred i.e. the retina was heated to over 100 °C resulting in localised explosive boiling accompanied by the immediate creation of a permanent blind spot [2].

Since 1990 there have been 400 incidences of lasers directed at aircraft within the United States, laser/aviation safety concerns have led to an enquiry at the US congress [3]. Exposure to hand held laser light under such circumstances may seem trivial given the brevity of exposure, the large distances involved and beam spread of up to several metres. However, laser exposure may pose a dangerous distraction such as flash blindness. If this occurs during a critical moment in aircraft operation the aircraft may be endangered. In addition, some 18 to 35% of the population possess the autosomal dominant genetic trait Photic Sneeze [4], that causes the affected individual to experience an involuntary sneezing fit when exposed to a sudden flash of light. Some observers believe that the danger is greatly exaggerated, at least for small hand held lasers [5].

Contents 1 Classification 1.1 Old system 1.2 Revised system 2 Protective eyewear 3 Guidelines 3.1 Laser goggles 4 Laser pointers 5 Non-beam hazards – electrical and other 6 See also 7 External links 8 References

[edit] Classification

Lasers have been classified by wavelength and maximum output power into four classes and a few subclasses since the early 1970s. The classifications were intended to categorize lasers according to their ability to produce damage in exposed people. In the United States the following safety classes have been established in consensus standards (IEC 825, later IEC 60825, and ANSI Z136.1) and in Federal and state regulations (the international classification is slightly different):

[edit] Old system

class I

Inherently safe; no possibility of eye damage. This can be either because of a low output power (in which cases eye damage is impossible even after hours of exposure), or due to an enclosure that cannot be opened in normal operation without the laser being switched off automatically, such as in CD players.

class II

The blinking reflex of the human eye (aversion response) will prevent eye damage, unless the person deliberately stares into the beam. Output powers up to 1 mW. This class contains lasers that emit visible light only. Some laser pointers are in this category.

class IIa

A region in the low-power end of Class II where the laser requires in excess of 1000 seconds of continuous viewing to produce a burn to the retina. Supermarket laser scanners are in this subclass.

class IIIa

Lasers in this class are mostly dangerous in combination with optical instruments which change the beam diameter. Output powers 1–5 mW. Maximum power density in the beam is 2.5 mW/square cm. Many laser pointers are in this category.

class IIIb

Will cause damage if the beam enters the eye directly. This generally applies to lasers powered from 5–500 mW. Lasers in this category can easily cause permanent eye damage from exposures of 1/100th of a second or less depending on the strength of the laser. A diffuse reflection is generally not hazardous but specular reflections can be just as dangerous as direct exposures. Protective eyewear is always recommended when experimenting with IIIb lasers. Lasers at the high power end of this class may also present a fire hazard and can lightly burn skin.

class IV

Highly dangerous. Lasers in this class have output powers of more than 500 mW in the beam, or produce intense pulses of light. These are cutting, etching and surgical lasers and can cause damage without being magnified by the optics of the eye. Diffuse reflections of the laser beam can be hazardous to skin or eye within the Nominal Hazard Zone.

The laser powers mentioned above are typical values; the classification is also dependent on the wavelength and on whether the laser is pulsed or continuous. In addition a laser of any power may be classified as a Class 1 Laser Product if it is enclosed so that there can be no access to laser radiation during normal use.

[edit] Revised system

In 2002 the system of Laser Classes was revised as part of a revision of the international laser safety standard, IEC 60825. The revision was based on the greater knowledge of lasers that had accumulated since the original classification system was devised, and was intended to permit certain types of lasers to be recognized as having a lower hazard than was implied by their placement in the original classification system. The revised system is expected to be adopted for use in the US in the next revision of the ANSI Laser Safety Standard (ANSI Z136). The FDA, which regulates lasers offered in commerce in the United States, does not object to its use on imported laser products' labels and markings.

class I

A class 1 laser is safe for use under all reasonably-anticipated conditions of use; in other words, it is not expected that the maximum permissible exposure (MPE) can be exceeded. This class may include lasers of a higher class whose beams are confined within a suitable enclosure so that access to laser radiation is physically prevented.

class IM

Class 1M lasers produce large-diameter beams, or beams that are divergent. The MPE for a Class 1M laser cannot normally be exceeded unless focusing or imaging optics are used to narrow down the beam. If the beam is refocused, the hazard of Class 1M lasers may be increased and the product class may be changed.

class II

A class 2 laser emits in the visible region. It is presumed that the human blink reflex will be sufficient to prevent damaging exposure, although prolonged viewing may be dangerous.

class IIM

A class IIM laser emits in the visible region in the form of a large diameter or divergent beam. It is presumed that the human blink reflex will be sufficient to prevent damaging exposure, but if the beam is focused down, damaging levels of radiation may be reached and may lead to a reclassification of the laser.

class IIIR

A class 3R laser is a continuous wave laser which may produce up to five times the emission limit for Class 1 or class 2 lasers. Although the MPE can be exceeded, the risk of injury is low. The laser can produce no more than 5 mW in the visible region.

class IIIB

A class 3B laser produces light of an intensity such that the MPE for eye exposure may be exceeded and direct viewing of the beam is potentially serious. Diffuse radiation (i.e., that which is scattered from a diffusing surface) should not be hazardous. CW emission from such lasers at wavelengths above 315 nm must not exceed 0.5 watts.

class IV

Class 4 lasers are of high power (typically more than 500 mW if cw, or 10 J/cm² if pulsed). These are hazardous to view at all times, may cause devastating and permanent eye damage, may have sufficient energy to ignite materials, and may cause significant skin damage. Exposure of the eye or skin to both the direct laser beam and to scattered beams, even those produced by reflection from diffusing surfaces, must be avoided at all times. In addition, they may pose a fire risk and may generate hazardous fumes.

(From The Physical and Theoretical Chemistry Laboratory Oxford University)[6]

[edit] Protective eyewear

In an environment with potential exposure to laser beams, suitable eye protection is recommended for beams of Class 3B and Class 4.

In the U.S., guidance for the use of protective eyewear, and other elements of safe laser use, is given in the ANSI Z136 series of standards. They are:

ANSI Z136.1 - Safe Use of Lasers

ANSI Z136.2 - Safe Use of Lasers in Optical Fiber Communication Systems Utilizing Laser Diode and LED Sources

ANSI Z136.3 - Safe Use of Lasers in Health Care Facilities

ANSI Z136.5 - Safe Use of Lasers in Educational Institutions

ANSI Z136.6 - Safe Use of Lasers Outdoors

In the European Community, eye protection requirements are specified in European norm EN 207. In addition to EN 207 norm, European norm EN 208 norm specifies requirements for goggles for use during alignment. These transmit a small fraction of the laser light in order to allow the operator to see where the beam is. The latter does not provide sufficient protection against a direct hit of the laser beam. Finally, European norm EN 60825 specifies the required optical densities in extreme situations.

[edit] Guidelines

The use of eye protection when operating lasers of classes IIIb/IIIB and IV is strongly recommended and required in the workplace by the U.S. Occupational Safety and Hazard Administration (OSHA). It is common in scientific research, however, for operators to remove their eye protection during certain procedures, or even to avoid wearing it altogether. The problem is that the use of safety glasses over a long time is often uncomfortable, and in many types of optical experiments it is also somewhat inconvenient. For example in spectroscopy, the experimental arrangement is constantly being modified and fine-tuned, during which it often is necessary to see where the beam is going. This is often most simply achieved with the naked eye, rather than (for example) with a camera. In this situation, many scientists assign a higher priority to convenience and comfort than to safety, and routinely breach the laser safety regulations. Sometimes it is unavoidable when working with, for example, an RGB laser, which would technically require completely black goggles.

Although not everybody agrees on these practices, most scientists involved with lasers agree about the following guidelines.

• Everyone who touches a laser should be aware of the risks. This awareness is not just a matter of time spent with lasers; to the contrary, long-term dealing with invisible risks (such as from infrared laser beams) tends to reduce risk awareness, rather than to sharpen it.

• Many experimentalists feel quite secure when dealing with an experiment carried out on an optical table, where all laser beams travel in the horizontal plane only, and all beams are stopped at the edges of the table. Experimentalists just make sure never to put their eyes at the level of the horizontal plane where the beams are travelling, in case that a reflected beam accidentally leaves the table. This guideline significantly reduces the risk, but a lot of hazards still remain when no protecting glasses are used:
  • In a non-trivial optical setup, it is very hard to ensure that all mirrors, filters, and lenses are strictly kept in a vertical position at all times, particularly when the setup is constantly modified.
  • Accidental upward reflections can be caused by watches and jewelry. Even if those are banned, operators often use metallic tools (like screwdrivers), which can get into a beam path. Note that reflections normally stay unnoticed until an accident occurs.
  • When picking up something from the floor, closing the eye may not give sufficient protection against multi-watt laser beams, as the eye's lid is partially transparent, particularly for infrared light. Closing both eyes when kneeling becomes second nature and automatic for workers in such fields.
  • Nobody can guarantee that all these hazards can be safely avoided without wearing protecting glasses, when infrared laser beams with non-negligible powers are used in the experiment. Working without glasses under these circumstances means trading safety for convenience. This is commonplace, but not safe, and for this reason not allowed by any official safety regulations.

• Adequate eye protection is required by anyone in the room, not just the one who tweaks an experiment.
• High-intensity beam paths (say, above 200 mW) that are not frequently modified should be guided through black tubes. For ultraviolet beams, this is necessary even for much lower power levels due to the risk of skin cancer. When modifying and aligning the beam it is often sufficient to drastically reduce the energy to a safer level, increasing back to full strength when in use.

• Particular care is to be taken when optical elements such as mirrors are inserted or removed. Alignment can also be dangerous because it can (for example) make a laser beam hit some metallic post, from where it can be reflected. Spray painting such metallic posts in matte black is preferred.

Dangerous styles of working are encouraged (but not justified) by various factors, including:

• the difficulty of obtaining adequate eye protection (particularly when working with multiple wavelengths);
• highly inconvenient or uncomfortable safety devices;
• irrational assessment of risks;
• nonsensical safety regulations, which encourage their breach as a usual procedure; or
• a lack of general knowledge on safety issues.

[edit] Laser goggles

Laser goggles may be recommended for protection from the reflected or scattered beams of lasers with sufficient beam power, as well as from exposure to a sufficiently powerful direct beam. To provide appropriate eye protection, goggles must be selected for the specific type of laser. For example, blue goggles are needed for red (650 nm) lasers, and red goggles are needed for green (532 nm) lasers. Goggles are rated for their optical density, or OD. Optical density is the logarithm of the amount by which the optical filter reduces beam power. For example, goggles with with OD=2 will reduce beam power by a factor of 100, and goggles with OD=6 will offer 10,000 times the protection of goggles with OD=2.

[edit] Laser pointers

In recent years, increasing attention has been paid to the risks posed by so called laser pointers and laser pens. Laser pointers in the US are class IIIa or less, <5 mW. While in the UK, laser pointers are nominally limited to <1mW. However, most laser pointers in the UK are class IIIa, in disregard of local laws.

Van Norren et al. (1998) could not find a single example in the medical literature of a <1mW class II laser causing eyesight damage. Mainster et al. (2003) provide one case, an 11 year old child who temporarily damaged her eyesight by holding an approximately 5mW red laser pointer close the eye and staring into the beam for 10 seconds, she experienced scotoma but fully recovered after 3 months. Luttrulla & Hallisey (1999) describe a similar case, a 34 year old male who stared into the beam of a class IIIa red laser for 30 to 60 seconds, causing temporary central scotoma and visual field loss. His eyesight fully recovered within 2 days, at the time of his eye exam. An intravenous fundus flourescein angiogram, a technique used by ophthalmologists to visualise the retina of the eye in fine detail, identified subtle discoloration of the fovea.

Thus, it appears that brief 0.25 second exposure to a <5 mW laser does not pose a threat to eye health. Apart from an aggressive act, briefly (0.25 second) shining a <5mW laser at another persons eye from a distance of several metres, will not affect their vision. On the other hand there is a potential for injury if a person deliberately stares into a beam of a class IIIa laser for few seconds or more at close range. Even if injury occurs, most people will fully recover their vision. With regard to green lasers, the safe exposure time may be less. These conclusions must be qualified with recent theoretical observations that certain prescription medications may interact with some wavelengths of laser light, causing increased sensitivity (phototoxicity).

The best course of action is to inform the victim of a laser pointer "attack" that medical science presently expresses the belief that brief exposure to a <5mW laser, although annoying, cannot harm eyesight. Claims of Injury from laser pointers, in particular if the claim is embellished with descriptions of eye pain, headaches and nausea, are likely to be false, mis-informed, or based more on concern than physical effects.

Beyond the question of physical injury to the eye from a laser pointer, several other undesirable effects are possible. These include short-lived flash blindness if the beam is encountered in darkened surroundings, as when driving at night. This may result in momentary loss of vehicular control. Lasers pointed at aircraft are a hazard to aviation. A police officer seeing a red dot painted on his chest may conclude that a sniper is targeting him and take unnecessarily aggressive action. (On February 4, 2005, a Florida man was killed by police after shining a laser at three officers, who did not know "if someone was aiming a gun at them or not", according to a sheriff's spokeswoman.^[1]) In addition, the startle reflex exhibited by some exposed unexpectedly to laser light of this sort as been reported to have resulted in cases of self-injury or loss of control. For these and similar reasons, the US Food and Drug Administration has advised that laser pointers are not toys and should not be used by minors except under the direct supervision of an adult.

[edit] Non-beam hazards – electrical and other

For the main article on general electrical safety, see High-voltage hazards.

A discussion of laser safety would not be complete without mention of non-beam hazards that are often associated with use of laser systems. Many lasers are high voltage devices, typically 400 V upward for a small 5 mJ pulsed laser, and exceeding many kilovolts in higher powered lasers. This, coupled with high pressure water for cooling the laser and other associated electrical equipment can create a greater hazard than the laser beam itself.

Electric equipment should generally be installed at least 250 mm / 10 inches above the floor to reduce electric risk in the case of flooding. Optical tables, lasers, and other equipment should be well grounded. Enclosure interlocks should be respected and special precautions taken during troubleshooting.

In addition to the electrical hazards, lasers may create chemical, mechanical, and other hazards specific to particular installations. Chemical hazards may include materials intrinsic to the laser, such as beryllium oxide in argon ion laser tubes, halogens in excimer lasers, organic dyes dissolved in toxic or flammable solvents in dye lasers, and heavy metal vapors and asbestos insulation in helium cadmium lasers. They may also include materials released during laser processing, such as metal fumes from cutting or surface treatments of metals or the complex mix of decomposition products produced in the high energy plasma of a laser cutting plastics.

Mechanical hazards may include moving parts in vacuum and presssure pumps; implosion or explosion of flashlamps, plasma tubes, water jackets, and gas handling equipment.

High temperatures and fire hazards may also result from the operation of high-powered Class IIIB or any Class IV Laser.

In commercial laser systems, hazard mitigations such as the presence of fusible plugs, thermal interrupters, and pressure relief valves reduce the hazard of, for example, a steam explosion arising from an obstructed water cooling jacket. Interlocks, shutters, and warning lights are often critical elements of modern commercial installations. In older lasers, experimental and hobby systems, and those removed from other equipment (OEM units) special care must be taken to anticipate and reduce the consequences of misuse as well as various failure modes.

[edit] See also

• Lasers and aviation safety
• Audience scanning -- use of lasers in light shows, where they are deliberately directed into the audience to create special effects

[edit] External links

• Laser safety article in the Encyclopedia of laser physics and technology
• Laser safety fact sheet at University of Kentucky
• Laser safety classes at TU Vienna
• U.S. Navy Laser Safety Website
• Laser safety from eyesafety.4ursafety.com
• Laser Safety products and advice from Lasermet
• Laser Safety Glasses for Common Lasers used for Holography
• OSHA safety guidelines

[edit] References