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Laser safety

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A typical laser warning symbol.
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A typical laser warning symbol.

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 because the cornea, lens and retina of the eye do not possess pain sensory nerves. Some lasers are so powerful that even the diffuse reflection from a surface can be dangerous. Laser radiation predominately causes eye injury via thermal effects on the retina of the eye. A transient increase of only 10 °C can cause death of retinal photoreceptors.

Diagram of a human eye.
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Diagram of a human eye.

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 then 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, a temperature rise of just 10 °C can kill retinal photoreceptors. Whereas, exposure to laser radiation with wavelengths less then 400 nm and greater then 1400 nm are largely absorbed by the cornea and lens, leading to the development of eye cataracts or burn injuries [link].

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 1063 nm radiation, may not feel pain or notice immediate damage eye 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 [link].

Since 1990 there have been 400 incidences of lasers directed at aircraft within the United States, concerns have led to an enquiry at the US congress [link]. 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 posses the autosomal dominant genetic trait Photic Sneeze [link], 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 [link].

Contents

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 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):

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 is for visible light lasers 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. Many laser pointers are in this category.
class IIIb
Can cause damage if the beam enters the eye directly or if the beam is directly reflected into the eye. This generally applies to lasers powered from 5–500 mW. Diffuse reflection is not considered hazardous and the laser doesn't usually present a fire hazard.
class IV
Highly dangerous; even indirect scattering of light from the beam can lead to eye or skin damage. 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.

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)[link]

Protective eyewear

Laser goggles
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Laser goggles

Inside an environment with exposed laser beams, it is recommended that everyone wears suitable eye protection.

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 the European norm EN 207. In addition to the EN 207 norm, there is the European norm EN 208 norm that 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, the European norm EN 60825 specifies the required optical densities in extreme situations.

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.

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

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 opthamologists 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-infomred, 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. A police officer seeing a red dot painted on his chest may conclude that a sniper is targeting him and take unnecessarily aggressive action. 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.

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 400V upward for a small 5mJ 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 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 eximer 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.

External links

References

 

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