Laser Safety
Classification, hazards, protection standards etc.
During the last few decades, Lasers have emerged as the most innovative tool, having wide ranging applications starting from the very common supermarket bar code readers to the highly advanced systems such as nuclear fusion systems for power generation, directed energy weapon in an antimissile role. There is hardly any sphere of our life, which is untouched by Lasers. A common man encounters Lasers not only in light shows, but also with the beauticians, eye specialists, orthopedists, to sight a few. In industry lasers are being widely used for material processing, alignment etc. Doctors use it for variety of applications like Surgery, dermatology, dentistry, ophthalmology etc. The use of lasers by medical community, industry and by the academic community continues to increase. Many educational institutions are using a wide variety of lasers on regular basis for demonstration of various experiments. Departments such as biology, chemistry, physic and even civil engineering, earth and planetary sciences, and biomedical research now recognize the laser as an essential element in their teaching program. In this section on laser safety, laser fundamentals and various technological aspects will not be addressed as the same have been discussed in great detail in the earlier sections, though they are very much relevant here. The intention here is to educate and create awareness among laser developers, users and the general public regarding the details related to temporary as well as permanent damage that can be caused by lasers and laser based systems. The injuries can be caused not only by laser radiation, but also by the associated hazards due to electrical, chemical, mechanical, fire, explosion etc. Most of the laser accidents reported occur because of inadequate knowledge about the Laser safety regarding handling laser beams, associated power supplies etc. As per one of the reports, the hazards can be listed into following categories: According to the most recent figures This section will provide necessary safety measures that have to be taken while working with lasers and laser systems to protect the personnel from radiation as well as from associated hazards as listed above. For the reader, who is interested in learning more about laser safety, a number of references are provided at the end of this section.

Laser Hazards can be classified into following categories
Laser Beam Hazards
Eye Hazards
A primary danger to the human eye posed by lasers results from the fact that the eye itself works as a focusing optical device for light within a certain wavelength range. Utilizing lasers in conjunction with microscope optics further adds to the probability for damage to the eye. It is common for educational and research institutions to house large number of lasers on optical benches. Reflections from external reflective surfaces, such as walls, any other reflective surfaces in the room or even belt buckles, watches, jewelry can equally be a cause of hazard. Even a split-second exposure to a fraction of reflected portion of a laser beam may be sufficient to cause permanent injury and loss of vision. The extent of damage to the eye depends upon which part of the eye absorbs the beam energy. Injury to various parts of the eye (see adjoining figure), such as cornea, lens and retina depends on the wavelength, intensity and absorption characteristics of the eye tissues.
Skin Hazards
The risk of skin injury by lasers may look to be greater than the risk of eye damage because larger area is exposed to lasers beams. However, the potential risks to the skin are in fact much less than as compared to the risks to the eyes. This is mainly because skin injuries are never that serious and are not of permanent nature. Skin injuries may affect only the external dead layer of the skin cells; and even more penetrating damage usually will heal after some time. Laser radiation can affect the skin thermally or photochemically. Skin consists of two main layers: the surface layer (epidermis) and the underneath layer (dermis). Surface layer is about 8 - 20 micron thick and consists mainly of dead cells and protects us against water loss, abrasion, dust, air, and radiant energy. Underneath layer on the other hand houses many specialized cells and glands. The skin reflects most visible and near-infrared radiation. The surface layer, however absorbs wavelengths in the ultraviolet range of 200 - 300 nm, thus protecting the underneath layer. However, when the higher power and longer duration, the incident radiation of any wavelength in the optical spectrum can penetrate the surface layer and may cause deep internal injury. Laser emission in the ultra-violet region from 200nm to 300nm produce skin cancer and accelerated skin aging. Exposure to radiation from 300nm to 400nm results in increased pigmentation. Lasers in the visible region (400nm to 700nm) produce photosensitisation. Direct exposure to very high power radiation above 800nm may produce irreparable damage like, ulceration, depigmentation, blisters, skin burn, scarring etc. If the power of laser is very high, underlying connecting tissues and organs like sweat glands, blood vessels, nerve cells, hair follicles could also be damaged. Photochemical reaction is the principal reason for tissue level damage. Laser-induced thermal change to the skin is most pronounced at far-infrared wavelengths such as are produced by CO2 lasers. Thermal damage also can be caused by visible and near-infrared wavelengths, but at much higher irradiance values as compared to far-infrared laser beams. Following table summarizes the possible biological effects of various lasers.
Biological Effects of lasers

Wavelength Lasers Damage Remarks
(200-280 nm) Argon Fluoride, Krypton chloride, Krypton Fluoride Eye Photokeratitis
Skin Erythema (Sunburn), Skin Cancer
(280-315 nm) Xenon chloride Eye Photokeratitis
Skin Erythema (Sunburn), Accelerated Skin Aging, Increased Pigmentation
(315-400 nm) Xenon Fluoride, Nitrogen, Helium Chloride Eye Cataract
Skin Skin Burn, Pigment Darkening
(400-780 nm) Helium Chloride, Helium Neon, Argon, Krypton, Copper Vapour, Frequency doubled Nd:YAG, Gold Vapour, Dye lasers (Visible), Ruby, Ti:Saphire(Visible), Diode laser (Visible) Eye Photochemical and Thermal Retinal Injury, Color and Night Vision Degradation
Skin Skin Burn, Photosensitive Reactions
(780-1400 nm) Ga As, Nd:YAG/Glass, chemical oxygen iodine laser (COIL) Eye Retinal Burns, Cataract
Skin Skin Burn
(>1400 nm) HF, Diode Lasers ( IR), He-Ne (IR), Erbium doped YAG/Glass, DF, Carbon Dioxide Eye Corneal Burn
Skin Skin Burn

Non-Beam Hazards
In addition to the direct hazards to the eye and skin from the laser beam itself, it is also important to consider other hazards associated with the use of lasers. Generation of laser requires a number of support systems like high voltage, high current as well as radio frequency power supplies, high pressure arc and flash lamps, heavy duty capacitor banks, gases at high pressure in heavy containers, toxic gases and fumes, carcinogenic and inflammable materials, cryogenic systems etc. These sub-systems also cause injuries to the personnel working with lasers. The non-beam hazards can be summarized as follows:
Maximum Permissible Exposure (MPE)
MPE is defined in ANSI Z-136.1 as "the level of radiation to which a person may be exposed without hazardous effect or adverse biological changes in the eye or skin". The biological effects of laser radiation depend on the wavelength, exposure duration, repetition rate and power / energy levels. The MPE is usually expressed either in terms of radiant exposure in J/cm2 for pulsed lasers or as irradiance in W/cm2 for continuous lasers for a given wavelength and exposure duration. In general, the longer the wavelength, the higher the MPE and for longer exposure times, the MPE is lower. It may be mentioned that MPE is only a guideline. In certain cases it is necessary to define an area in which potentially dangerous laser hazard whether direct, reflected or scattered laser radiations exist. This is referred to as Nominal Hazard Zone (NHZ), where the level of laser radiation is more than MPE and it is necessary to enforce various laser safety control measures to protect the users.

The following tables give the typical MPE for various types of lasers. (Collected from various references)

It may be mentioned that these values are representative only for a single pulse. For multiple pulses, the empirical relation MPE is reduced by a factor of n0.25, where n is the number of number of pulses in maximum 10 seconds.
Pulsed Lasers (MPE for 10 seconds duration - J/cm2)

Laser Wavelength Pulse Width Eye Skin
Argon Fluoride, Krypton chloride, Krypton Fluoride (200-280 nm) Nanosecond to tens of secs 3 x 10-3 3 x 10-3
Xenon chloride (280-315 nm) Nanosecond to tens of secs 10-2 - 0.1 0.1
Xenon Fluoride, Nitrogen, Helium Chloride (315-400 nm) Nanosecond to tens of secs 0.6 1.0
Helium Chloride, Frequency doubled Nd:YAG, Gold Vapour, Dye lasers (Visible), Ruby, Ti:Saphire(Visible), Diode laser (Visible) (400-780 nm) Nanosecond to microsecond 5 x 10-7 2 x 10-2
Ga As 905 nm Nanosecond to microsecond 1 x 10-6 1.5 x 10-2
Nd:YAG 1064nm millisecond 5 x 10-5 1.0
Nd:YAG 1064nm Nanosecond to microsecond 5 x 10-6 0.1
Erbium doped YAG/Glass 1500nm nanosecond to millisecond 0.1 0.1

Continuous Lasers. (MPE for 8-hour duration - W/cm2)

Laser Wavelength Eye Skin
Argon ion 488 / 514 nm 1 x 10-6 0.2
Frequency doubled Nd:YAG 532nm 1 x 10-6 0.2
He-Ne 632.8nm 1.7 x 10-5 0.2
Nd:YAG 1064nm 1.6 x 10-3 0.2
COIL 1.354 micron 4 x 10-2 0.2
HF / DF 2.8 - 4.0 micron 0.1 0.1
CO2 10.6 micron 0.1 0.1

In certain laser applications, it is essential to have laser beams in an open area. As mentioned earlier, the area, which is potentially hazardous to personnel, is defined as the Nominal Hazard Zone (NHZ). In this space laser radiation level is more than that is permitted (MPE), whether it is direct, scattered or reflected. Inside this area, one should use laser safety equipments where strict control measures are required to protect the user from exposure to radiation above MPE. For calculation of NHZ, one must consider beam divergence, power of the lasers etc. In general, NHZ is given as
Where W is the power of laser, Φ is the divergence of the laser beam and MPE is the maximum permissible exposure. For example, if CO2 laser has a power of 2 kW and beam divergence of 4mrad, NHZ is approximately equal to 400 meter.
Laser Classifications
Of the many laser safety standards developed by both governmental and other agencies, the one most often relied upon in the United States is the American National Standards Institute's Z136 series. The ANSI Z136 laser safety standards are the basis for the Occupational Safety and Health Administration (OSHA) technical rules used to evaluate laser hazard issues, and are also the reference for many states' occupational safety rules pertaining to laser use. All laser products sold in the USA since 1976 are required to be certified by the manufacturer as meeting specified product safety standards for their designated classification, and they must be labeled as to their class. Research results combined with an accumulated understanding of the hazards of sunlight and other light sources have led to the establishment of estimated nominal safe exposure limits for most types of laser radiation. A system of laser hazard categories, based on the known Maximum Permissible Exposures (MPE) and experience gained from years of laser use, has been developed to simplify the application of safety procedures to minimize or prevent accidents. The laser manufacturer is required to certify that a laser product falls into one of the categories, or risk classes, and to label it accordingly. Laser classification is based on wavelength, exposure duration, repetition rate and power / energy level. For pulsed lasers, total energy per pulse, pulse width, pulse repetition rate and total radiant exposure are considered, whereas for continuous wave lasers, average power output and limiting exposure time are taken into account. The general classifications of laser categories are summarized in the following list. It must be emphasized that this is an abbreviated summary, and is not intended to be a complete statement of any agency's laser classification regulations.
General Laser Safety and Control Measures
Power levels given above are only guidelines and the following general protection measures have to be adhered to.
Safety Rules for Various Types of Laser
Since power levels of lasers are of prime importance, the same dictates basic safety rules.
Class I, IM Lasers
Since these type of lasers do not create any damage to the eye or the skin, even after long and direct exposures or are designed in such a way that laser output is embedded inside and is never exposed to the environment outside, they are exempt from any control measures (~50µw). But laser-warning sign has to be located at the access panel.
Class II, IIM Lasers
Since the visible lasers (400nm-700) with power levels less than 1 mlliwatt does not produce any damage even with long exposures (maximum 1000 seconds), a laser caution label and indicator lamp to indicate laser operation is are the basic requirements. However the person should not stare in to the laser beam from within the beam.
Class III R, III B lasers
Since the intra-beam viewing of these lasers can create potential hazard to the eye, elimination of the same is sufficient as laser safety measure and hardly any diffuse reflection hazard exists. The general safety measures should be adopted as given in the earlier section. However, for class III B, following additional safety measures may be taken.
Class IV lasers
These lasers exist in research laboratories, industrial locations and military areas. Education and training of personnel working with these lasers are of paramount importance, since exposure to high power laser radiation may result in serious eye and skin injury. Non-beam laser hazards such as electrical and chemical are also of serious concern. In industrial and military systems remote operation is normally adopted. Laser system operation has to be carried out by trained and authorized personnel only. The "high-power" lasers present the most serious of all laser hazards. Besides presenting serious eye and skin hazardous, these lasers may ignite flammable targets, create hazardous airborne contaminants, and may also have a potentially lethal, high current/ high voltage power supply. In addition to safety measures as mentioned for class III B lasers, following additional instructions should also be followed.
Laser safety guidelines
The American National Standard Institute (ANSI) has set up standards and guidelines for manufacturers, users as well as general public. The reader may specifically refer to the ANSI Z 136.1 (1993) standards revised in 2000 and 2007 for details regarding the safety limits and computations of laser hazards.
References
References given below are not exhaustive, but are very informative for further understanding of the importance of safe use of lasers.

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Updated: 6 April, 2015
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