Laser safety

A laser is a light source that can be dangerous to people exposed to it. Even low power lasers can be hazardous to a person's eyesight. The coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localised burning and permanent damage in seconds. Certain wavelengths of laser light can cause cataracts or even boiling of the vitreous humor, the fluid in the eyeball. Infrared and ultraviolet lasers are particularly dangerous, since the body's "blink reflex", which can protect an eye from excessively bright light, works only if the light is visible.

Classification

Lasers are classified by wavelength and maximum output power into the following safety classes:

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 will prevent eye damage. Most laser pointers are in this category, with output powers of around 1 to 5 milliwatts.

class IIIa: similar to IIIb, but with large beam diameters, such that the pupil will only allow a 'class-II'-amount of light to enter the eye. Lasers in this class are mostly dangerous in combination with optical instruments which change the beam diameter.

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 milliwatts up to several hundred milliwatts. Diffuse reflection is not considered hazardous and 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 a few hundred milliwatts, 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 rough indications; 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.

Guidelines

The use of eye protection when operating lasers of class IIIb and IV is strongly recommended and required in the workplace by U.S. Occupational Safety and Hazard Administration (OSHA). However, it is common practice in scientific research that operators do not use eye protection even while working with class-IV lasers. The problem is that the use of safety glasses over longer times 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 e.g. 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.

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 (e.g. 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 (e.g. 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.
  • 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.

• Adaquate 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.

• Particular care is to be taken when optical elements such as mirrors are inserted or removed. Alignment can also be dangereous because it can e.g. make a laser beam hit some metallic post, from where it can be reflected.

Dangerous styles of working are advanced (but not justified) by various factors, e.g.

• the difficulty to get 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.

External link

• Laser safety article in the Encyclopedia of laser physics and technology (http://www.rp-photonics.com/encyclopedia_l.html#k_laser_safety)