Laser Safety

Safety with Lasers

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:

• Beam related hazards: 64%
• Non-beam related hazards: 28%
• Others like malfunctioning etc: 8%

According to the most recent figures

• 73% of all reported laser injuries are eye-related
• 35% of eye-related injuries are associated with the alignment process
• 68% of eye-related injuries result in permanent injury

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, which include the damage to eye and damage to skin. The factors responsible for causing biological damage depend on the laser wavelengths, nature of output i.e. pulsed or continuous wave (CW), power/energy levels, and duration and repetition rate of the pulses. Eye and skin are the biological systems, which are manly affected by the laser radiation.
• Non-Beam Hazards: 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 include electrical hazards, explosion and fire hazards, chemical hazards.

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 figure below), such as cornea, lens and retina depends on the wavelength, intensity and absorption characteristics of the eye tissues.

• Wavelengths between 100nm to 300nm are absorbed by the cornea, resulting in photokeratitis by the photochemical process, which produces denaturation of protein in the cornea. Though the corneal tissues regenerate rather quickly, this is still a cause of concern. Lens absorbs laser wavelength between 300nm to 400nm. The photochemical process denatures proteins in the lens and cataract is formed.
• Cornea, lens and the vitreous humor transmit laser wavelengths between 400nm to 1400nm. Retinal tissue absorbs the light and heating takes place producing damage. The focusing effect of lens increases the irradiation density of the laser by about 100,000 times on the retina, resulting in the creation of highly increased power density region in the retina. Therefore, even the diffused reflection could be potentially dangerous. For visible light between 400nm to 700nm, if the person moves the eye away from the light source in less than 0.25sec due to reflex action, the damage may not occur unless the irradiation density is too high due to the focusing property of lens. Therefore the exposure time of the laser radiation is of great importance.
• Since cornea absorbs radiation above 1400nm, corneal damage occurs due to the absorption of the laser radiation by the tissue water, causing temperature rise and subsequent denaturation of proteins. Lasers in the wavelength range of 2000nm to 3000nm penetrate deeper, leading to the development of cataract in the lens due to heating of proteins in the lens.

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

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:

• Electrical: High voltage power supply, with output in the kilovolt range, is energized continuously for a long time. Large amount of energy is stored in the capacitor bank, which may still be alive even after the power is switched off. The capacitor banks release large amount of energy in a very short time generating current levels in the range of hundreds of kiloamperes. These high currents can produce magnetically induced currents in adjacent conductors, leading to its malfunctioning or destruction. Many of the batteries like lead acid batteries generate hydrogen fumes, which can be a fire hazard. Control, diagnostic and data acquisition systems are also potentially dangerous areas. Injuries may happen due to the explosion of flash lamps and high-pressure arc lamps. Discharge tubes, arc lamps and welding equipments may generate ultraviolet outputs, which can harm the eyes of the personnel irreversibly. Electrical shocks caused by high voltages associated with power supplies employed in laser systems are a major hazard area. Electrocution occurs not only due to carelessness of the users, but also due to the manufactures not adhering to standard procedures for electrical safety. These mishaps occur in research laboratories, where breadboard developments take place and at shop floors where material processing takes place.
• Chemical: Some of the chemicals employed in lasers are carcinogenic, toxic, potentially inflammable and explosive in nature. Highly dangerous and hazardous contaminants are released in air during the development of lasers and also during laser material processing. High intensity laser radiation vaporizes target materials during welding, cutting and heat treatment activities. These pollutants are extremely dangerous to general public as well. Some laser systems use cryogenic liquids for cooling. It is most likely that these cryogenic liquids may evaporate under intense heat and replace oxygen. These, as well as other coolants like liquid oxygen, hydrogen etc also have a tendency to explode, if not kept in suitably designed containers. Laser interaction with certain material can generate X-rays, which can be a serious health hazard. High power lasers employ compressed gases and the same are potentially dangerous hazards. Serious health problems may arise, if the cylinders containing toxic, corrosive and flammable gases are not stored in well-ventilated enclosures.
• Fire hazards: Class III Class IV lasers are capable of setting fire to the plastic enclosure materials and the inflammable gases produced during laser operation. Fire resistant materials should be used as laser enclosures.
• Mechanical: Many of the equipments, high-pressure gas containers, high voltage power supplies etc, are heavy and they can cause injuries to personnel, if the same happen to fall on them. Noise pollution is another major hazard for the people working with lasers.
• Associated radiations: In a laser environment, apart from laser radiation, radiations like, X-rays, ultraviolet, radio- frequency emissions, and plasma may also be present. Electric discharge lasers, rectifiers and thyratrons can generate X-rays. Electrical discharge laser tubes and pump lamps are sources of UV radiation. Skin cancer and eye damage can occur, if the UV out put level goes above permitted limits. When high power laser radiation impinges on targets, plasma is generated and same could be a source of hazardous UV radiation. Unshielded RF excited components like plasma tubes and certain q-switches generate high power radio frequency fields, which may be detrimental to control systems.

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 n^0.25, where n is the number of number of pulses in maximum 10 seconds.

Pulsed Lasers (MPE for 10 seconds duration - J/cm²)

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

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.

• Class I lasers are considered safe, based upon current knowledge, under any exposure condition inherent in the design of the product. These Laser systems cannot emit laser radiation levels greater than the Maximum Permissible Exposure and are considered to be incapable of causing eye damage under normal operating or viewing conditions. Maximum power output is of the order of a few microwatts. These low powered devices that use lasers of this category include laser printers, CD players, and survey equipment, and they are not permitted to emit levels of optical radiation above the exposure limits for the eye. Lasers of this class are found in compact disc players. No safety requirements are specified for the use of this class of laser. It may be pointed out that the lasers which are totally enclosed system where access to higher levels of laser radiation is not possible during normal operation also falls in this category. However, whenever the instrument is opened for servicing or repairs, then these lasers are no longer fall in Class 1 and all the necessary precautions applicable to the embedded laser must be followed until the service is complete and the system is again enclosed.
• Class 1M Lasers are considered incapable of producing hazardous exposure conditions during normal operations unless the beam is viewed through collecting optics like magnifying optical instruments such as telescope. These lasers produce either a large diameter beam or a highly divergent beam. Some of the lasers used for fibre-optic communication systems are Class 1M laser products. These lasers are exempted from any safety measures other than to prevent potentially hazardous optically aided viewing.
• Class II is a low-power laser that usually emits in the visible portion of the spectrum (0.4 - 0.7 μm). The brightness of the beam normally causes the eyes to blink, well before any permanent damage can occur. These lasers are limited to a radiant power of less than 1 milliwatt, which is below the maximum permissible exposure for momentary exposure of 0.25 second or less. The natural aversion reaction to visible light of this brightness is expected to protect the eyes from damage, but any intentional viewing for extended periods greater than 1000 sec can result in damage. So deliberate staring into the beam should be avoided. Some examples of this class of laser are demonstration lasers for classroom use, laser pointers, laser printers and supermarket scanners.
• Class II M class is similar to Class II in terms of power and wavelength. Normally these lasers produce either a large diameter beam or a highly divergent beam and as such the total output may be more as compared the output observed in Class II lasers. However, the power densities are safe for accidental viewing because of the diverging nature of laser beams. When viewed with the naked eye, the hazards are the same as for a Class II laser. But these lasers are potentially hazardous if viewed with collecting optics or certain optical aids like magnifying optical instruments e.g. binoculars or a telescope. Lasers used for surveying come under this Class.
• Class III R lasers are continuous wave visible and infrared lasers with intermediate power levels of 1-5 milliwatts. These lasers have similar applications as encountered for Class II lasers, including laser scanners and pointers. They are considered safe for momentary viewing (less than 0.25 second i.e. blink response), but should not be viewed directly (intrabeam), or with any kind of magnifying optics. These medium-power laser systems though do not pose a fire hazard threat or even eye damage hazard through viewing of diffuse reflections, but nevertheless may be hazardous under direct and reflected beam viewing conditions. Lasers in this class may be used in alignment products.
• Class III B lasers can be both in the visible and as well as infrared band and are of medium power: continuous wave (5-500 milliwatt), or pulsed (10 joules per square centimeter) These lasers are hazardous to the eye for direct intrabeam viewing and from specular reflections. In general these lasers are safe so far as diffuse reflections are concerned, except towards the high power end. However, longer wavelength and high powers can cause some skin damage.. Specific safety measures are recommended in the standards for control of hazards with this laser class. Examples of applications of this laser type are spectroscopy, confocal microscopy, and entertainment light shows. These lasers are also used in medical applications and research.
• Class IV lasers emit high power, in excess of 500 mW, the limit for Class IIIB devices, and require stringent controls to eliminate hazards in their use. These lasers can be both continuous as well as pulsed in the visible and infrared ranges. These high-power laser systems are hazardous both to the eye as well as skin. Direct intrabeam viewing, specular and even diffuse reflection can cause severe eye and skin damage. These lasers can also have sufficient energy to ignite materials and thus are fire hazards. Also these lasers can produce hazardous plasma radiation, laser-generated air contaminants, hazardous fumes and byproduct emissions as a result of laser matter interaction. Since most laser eye injuries involve reflections of Class IV laser light, their use requires extreme caution .All reflective surfaces must be kept away from the beam, and appropriate eye protection worn at all times when working with these lasers. In case of pulsed lasers of this class, the power supplies can be fatal and thus all electrical safety precautions must be taken. Class IV lasers can be found in the metal industry, research laboratories, and laser light shows. These lasers are employed for surgery, cutting, drilling, micromachining, and welding.

Power levels given above are only guidelines and the following general protection measures have to be adhered to.

General Laser Safety and Control Measures

• Employing Laser Safety Officers
• Display of laser warning/caution signs/labels on the equipments at the work as well as in the experimental area, Warning lights, emission Indicators can be put at suitable places. Some of caution signs / labels are shown here.
• Standard operating Procedures should be strictly followed.
• Laser eye protection goggles depending on the wavelength of the lasers, clothing, hand gloves etc. should be provided. The reader may refer to the booklet "Guide for selection of Laser Eye Protection" by the Laser Institute of America.
• Designate the laser hazard area and post-warning signs at the entrance and warning light should be used during activation of laser.
• Protective housing and service panel should be provided on various lasers systems. Further Interlocks on the protective housing should also be provided.
• Safety interlocks for entrance area. Door interlocks and remote control connector should be provided.
• Minimize specular and diffuse reflections, especially in the case of Class 3 and Class 4 lasers.
• If possible, all class IV lasers should have Remote firing and/or Monitoring mechanisms.
• Beam traverse should be restricted during the operation, adjustments etc and laser energy terminated at the end of their path. There should be preferably Lasers Beam enclosure.
• First Aid boxes must be provided for rendering immediate relief.
• All laser equipments should be appropriately labeled as per the laser classification, including circuit breakers in an emergency to shut down the system.
• All hazardous materials should be properly stored and controlled.
• Entry in the Laser room should be limited for visitors.

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.

• Assuring that individuals who operate Class 3b lasers are trained in laser safety and authorized to operate a laser.
• A key switch should be used if untrained persons are likely to gain access to the laser. A warning light or buzzer may be used to indicate when the laser is operating.
• Enclosing as much of the beam's path as practical.
• Terminating the primary and secondary beams if possible at the end of their useful paths.
• Using low power settings, beam shutters and laser output filters to reduce the beam power to less hazardous levels when the full output power is not required.
• Attempting to operate the laser only in a well-controlled area. For example, it can be housed in a closed room with covered or filtered windows and controlled access.
• Labeling lasers and room with appropriate Class Warning sign.

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.

• Enclose the entire laser beam path if possible.
• Confine open beam indoor laser operations to a light-tight room.
• Interlock entrances to assure that the laser cannot emit when the door is open.
• Insure that all personnel wear adequate eye protection, or ensure that a suitable shield is present between the laser beam and personnel.
• Use remote firing and video monitoring or remote viewing through a laser safety shield where feasible.
• Use lower power settings; a beam shutter or laser output filters to reduce the laser beam irradiance to less hazardous levels whenever the full beam power is not required.
• Assure that the laser device has a key- switch master control to allow only authorized personnel to operate the laser.
• Labeling lasers and room with appropriate Class IV warning sign.
• Use dark, absorbing, diffuse, fire resistant target and backstops where feasible
• The laser rooms should be declared as restricted areas and must be locked at all times when unattended.
• All visitors must be accompanied by the authorized users.
• Safety goggles labeled with the appropriate wavelength and optical density shall be available at the entry where each door sign is posted.
• Glass windows should be covered with shades or filters of appropriate optical density whenever a laser system is operational.
• Laser keys should be kept in a secured area and be taken out only by authorized persons only.
• Proper emergency instructions should be displayed.

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.

• American National Standard Institute, American National Standard for the Safe Use of Lasers, ANSI Z136.1 (1993), 2000, 2007 Laser Institute of America, New York, NY .
• OSHA Technical Manual - Section III- Chapter 6, US Department of Labour, USA.
• Laser Safety Handbook, Northwestern University, 2011
• Laser Safety Manual, University of western Ontario, 2006
• Laser Safety Manual, University of Pennsylvania, USA.
• Laser Safety Manual, University of Iowa, USA.
• Laser Safety Manual, University of Florida, USA.