Laser safety

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 laser radiation may be unaware that damage is occurring to the eye because pain does not always eye damage. Some lasers are so powerful that even the diffuse reflection from a surface can be dangerous.

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, localised burning and permanent damage can occur within a fraction of a second, perhaps faster then the blink of an eye. Certain short UV wavelengths can cause cataracts and high energy laser can cause boiling of the vitreous humour, the fluid in the eyeball.

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, will 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 100C resulting in localised explosive boiling accompanied by the immediate creation of a permanent blind spot. Additionally, the brain can edit out a limited number of distracting blind spots from vision, particularly from peripheral vision. As a result eye a number injures may go unnoticed until a certain damage threshold is met.

Since 1990 there have been 400 incidence of lasers directed at aircraft within the United States. Concerns led to an enquiry at the US congress [1]. Exposure to laser light under such circumstances may seem a 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; for example some 18 to 35% of the population posses an autosomal dominant genetic trait Photic Sneeze [2], that causes the affected individual to experience an involuntary sneezing fit when exposed to a sudden flash of light. Though others claim that the danger is greatly exaggerated, at least for small hand held lasers [3].

Classification

In the United States lasers are classified by wavelength and maximum output power into the following safety classes, (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.

class IIIa: Lasers in this class are mostly dangerous in combination with optical instruments which change the beam diameter. Output powers 1–5 mW. This class is for visible light lasers only. Most 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 powers from 5–500 mW. Diffuse reflection is not considered hazardous and the laser doesn't present a fire hazard.

class IV: Highly dangerous; even indirect scattering of light from the beam can lead to eye or skin damage. This generally applies to laser powers of more than 500 mW, or lasers that produce intense pulses of light. Although the intensity of the beam may be only a few times that of bright sunlight, when it enters the eye the beam can be focused on a very small spot near its diffraction limit.

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. Also, even a high power laser may be assigned to a low safety class if it is enclosed so that no laser radiation can leave the case and injure a person.

Revised system

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)[4]

Protective eyewear

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.

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.

Electrical safety

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

A discussion of laser safety would not be complete without general electrical safety. Lasers are often high voltage, typically 400V upward for a small 5mJ pulsed laser, and exceeding many kilovolts for higher powered lasers. This coupled with high pressure water for cooling the lasers and often many other associated electrical pieces of equipment can in combination often provide more of a hazard than the laser beam itself. It is absolutely essential that every piece of electric equipment is at least 10 inches off of the floor to reduce electric risk in the case of flooding. The optical table, laser and any other equipment should be fully grounded.

External links

References

Breitenbach RA, "The photic sneeze reflex as a risk factor to combat pilots." Mil Med. Dec 1993, 158:806-9, PMID 8108024.