FEATURE
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Safety first

Stephen Mounsey looks into the requirements that laser safety equipment must meet, and takes a look at the various precautions that are available and advisable to laser users

Human eyes are remarkable organs, capable of a dynamic range of around 1,000,000:1, allowing us to tread carefully on star-lit nights, or see our way to an oasis in the brightest desert. Our eyes are well adapted to seeing visible light at wavelengths from 380-750nm, as these colours are abundant in nature. Nature, however, presents no light sources as intense as modern lasers. Even at relatively low powers, laser radiation can cause permanent damage to the retina of the eye, and it can do so whether the wavelength emitted is visible or not. Higher powers can burn through clothing and damage skin, potentially causing deep burns that cause a lot of pain, take a long time to heal, and leave scars behind them.

The safety of lasers has been a consideration since the early days of their commercialisation. Standards of laser safety were put in place in the beginning of the 1970s, starting with a classification system that has remained largely unchanged to this day (see box). The type of protection required depends on the classification of the laser. The laser classification standards are maintained by the International Electrotechnical Commission (IEC), a not-for-profit, non-governmental international standards organisation that prepares and publishes standards for all electrical and electronic technologies.

Eyewear

High-powered lasers, such as those used in industry and academia, will always have several layers of safety preventing a user from being exposed to dangerously intense radiation; where an industrial laser operates at powers that would make it Class 4 laser product, for example, it may be enclosed in a cabinet in such a way as to make the whole system a Class 1 product, i.e. entirely safe. Nonetheless, accidents happen, and laser safety eyewear is worn as a precaution. Bernard Russel, of Laser Components, a company producing laser safety eyewear, states that ‘our customers always have other safety measures in place, but safety spectacles are used as a last line of defence.’

Laser safety eyewear is recommended for anybody working with lasers of Class 3 or above (see box). John O’Connor, marketing manager of distributor Photonic Solutions, explains how the design criteria are decided upon: ‘Safety eyewear is designed on a worst case scenario basis, in terms of power, energy levels, and the level of attenuation at a given coverage wavelength.’

Laser safety products sold within the European Union must conform to EU safety standards covering personal protective equipment, or PPE. The standard for laser safety eyewear is EN207, and any eyewear certified to this standard is awarded the CE mark. Standard EN207 dictates minimum values for the optical densities of laser safety eyewear, which varies dependant on the wavelength of the laser used, whether the laser is CW or pulsed, and on the duration of those pulses. Logarithmic equations are used, a different one for each wavelength and modulation type, in order to come up with a laser safety factor, or ‘L-number’, which gives designers an idea of how much attenuation they should plan into their eyewear. In addition to optical density requirements, European-certified eyewear must be able to retain this minimum protection while withstanding a direct hit from the laser source they are to be rated for, either for 10 seconds or for 100 pulses, depending on the modulation scheme. In the US, the applicable safety standard is ANSI Z 136, but this only dictates the minimum optical density, and does not include the durability testing required for the EN207 standard.

Signage is a simple, but important, part of creating an environment in which it is safe to use a powerful laser. Image courtesy of Lasermet.

Laser safety lenses are made of either polycarbonate or glass. ‘Most of the eyewear NoIR provides is based around polycarbonate lenses,’ explains O’Connor. ‘This polycarbonate is impregnated with laser-reactive dyes.’ Polycarbonate has an advantage over coated glass in that the protective effect is relatively unaffected by surface damage to the lenses, meaning that scratches on the surfaces of the lenses will not compromise their safety. In addition, the polymer-based eyewear is cheaper, less cumbersome, and more comfortable to wear than bulky goggles incorporating glass lenses. Polycarbonate lenses can be made to have good visible light transmission in order for the wearer to be able to work effectively while wearing them. At higher intensities, however, such as those found at Class 4 and above, glass lenses become necessary in order to ensure protection. In particular, glass lenses are required when laser safety standards dictate that a high optical density is needed at a short wavelength.

As mentioned, laser safety eyewear is sold on a wavelength and power specific basis, and so the user must have a good understanding of the laser parameters before they will be able to choose the correct protection for the job. O’Connor believes that continuing to accommodate the ever-broadening capabilities of available lasers can be a challenge: ‘The challenge for us over the past few years has been more and more wavelengths of laser becoming available, and so we have had to keep developing new safety ranges in order to cover the new ranges of lasers available,’ he says. In addition, wavelength tuneable lasers, such as those based on quantum cascade effects, have become popular, meaning that a single piece of eyewear can be used to cover a broader bandwidth in order to remain within the safety standards for a given application. ‘The glasses need to be transmitting 400-700nm light in order for the user to be able to see through the glasses,’ explains O’Connor. ‘The more of that range you’re actually blocking out and filtering, the less useful the glasses are going to be. Peak eye-sensitivity is around 530-550nm, and as coverage gets closer to these wavelengths, the darker the glasses become.’ There does not currently seem to be a way around this problem, and so laser users working with a variety of types tend to buy a range of different eyewear for use at different wavelengths. ‘It’s a balance,’ says O’Connor, ‘between having good protection, and also having good visible light transmission.’

Class 3 and 4 lasers must be used with an interlock mechanism, meaning that the laser is shut off when a door is opened. Image courtesy of Lasermet.

While increasing availability of high-powered lasers presents an additional challenge to safety eyewear developers, in practice many of the highest power lasers are made safe by way of Class 1 enclosures; the workpiece may be behind plates of protective glass, which falls under the same stringent standards as eyewear. In addition, many industrial lasers make use of fibre beam delivery, meaning that laser light stays well out of view during normal operation. The only people likely to require protection around this type of laser are the technicians who build and service the equipment.

Another challenge highlighted by O’Connor is that of ultrafast lasers; these have become more popular in recent years as innovative applications for these classes of device continue to be introduced. Systems incorporating ultrafast lasers have their own rating, called an M-rating, for laser filters. The main consideration for protection against ultrafast pulsed radiation, be they pulses of picosecond or femtosecond duration, is the peak power of the pulses. ‘A whole lot of work has gone into that in the last few years, and now there’s a whole range of these M-rated safety glasses for the ultrafast community,’ says O’Connor.

Gloves

The certification process for European laser safety products must be carried out at a specified test centre, and there are only a couple of these in Europe. Safety products imported from the US must be re-certified at one of these test houses before they can be given the CE mark for sale in the European Union. The tests are well established for eyewear, but when developing new safety products, new tests must also be developed.

This is a difficulty that German laser safety company LaserVision recently faced after having developed a novel safety product in the form of protective gloves for the prevention of burns. While serious accidents involving laser radiation on the skin are rare, Thomas Froehlich, product manager at LaserVision, explains that there are a few serious accidents reported each year, and that a burn with a laser can take many months to heal. The wavelength of the laser used determines the depth of the burn, with CO² lasers only affecting the surface tissues, and solid-state lasers causing more severe burns. The laser gloves are sold on a wavelength-specific basis, just as eyewear is.

Until recently, there was no standard in place for laser-safe clothing. European PPE guidelines require a product to be externally validated in order for it to be CE rated, and so LaserVision has had to work closely with the test centres in order to establish one. A standardised test was established in June of this year, which makes use of a specialised laser energy meter, over which the material to be tested is placed. A measurement is taken, recording the way in which the temperature of the sensor increases when the textile is illuminated by the laser. The safety condition any textile must meet is that the temperature increases slowly enough for the wearer to both realise that they are being burned, and have time to remove the hand from the laser without any skin damage taking place. The criterion also states that the time between the point at which the wearer first experiences pain and the point at which actual skin damage begins to occur cannot be less than four seconds.

While several materials were tested by LaserVision, the material most readily able to meet these demands was a textile based on fibres of silicate glass. The material has the added bonus of being cut-resistant too. While more high-tech fibres such as polyaramids were tested, Froehlich states these lack the thermal conductivity necessary for the user to be aware of the heat. In future, the gloves may make use of several different layers of material. While there exists the possibility to extend this laser-safe clothing to cover other parts of the body, the requirement for the clothing to be in contact with the skin (in order for the wearer to feel when they are getting burnt) may mean that it is some time before laser-proof lab coats are a reality. In addition, Froehlich states that the laser gloves are a product entering an entirely new market, and as such, the company does not yet know how great a demand currently exists for such equipment.

Other safety products

As mentioned, the various classes of laser have various safety requirements imposed upon them, supplementary to the PPE used by the operator. Class 3 and 4 lasers must have both key-switches and interlock systems, in order to ensure that only authorised persons can use the laser, and the latter to ensure that the laser is killed should a door open unexpectedly, or should some other dangerous event occur. UK company Lasermet has made a business out of the need to meet the safety standards, providing interlocks, shutdown systems, curtains, enclosures, warning signs, and off-the-shelf labels, suitable for the 30+ combinations of laser class and power that might require a warning sticker to be added.

Paul Tozer, managing director of Lasermet, says that the company has installed interlock systems, including door signs and cut-off systems, in more than 1,000 research institutions and universities throughout the UK; the infrastructure of laser safety is just as important as personal equipment, and doubly so in a busy or shared environment.

Staying safe

When push comes to shove, it is always going to be the responsibility of the individual to ensure that he or she is adequately protected, and that proper procedures are followed. In an increasingly litigious society, however, employers must be aware of what their responsibilities are, and in particular, an awareness of the various safety standards that are applicable should be sought.

For more information, Electro Optics recommends that laser users consult EN60825-14, which is the EU’s guide to using lasers safely.

IEC laser classifications

Class 1 – Safe under all conditions of normal use. This includes high-power lasers within an enclosure.

Class 1M – Safe for all conditions of use except when passed through magnifying optics, such as microscopes and telescopes.

Class 2 – Safe because the blink reflex will limit the exposure to no more than 0.25 seconds. Visible-light only (400-700nm). Limited to 1mW CW, or more if the emission time is less than 0.25 seconds or if the light is not spatially coherent. Staring into beam may damage eyes.

Class 2M – Safe because of the blink reflex if not viewed through optical instruments.

Class 3R – Safe if handled carefully, with restricted beam viewing. Visible CW lasers are limited to 5mW. For other wavelengths and for pulsed lasers other limits apply.

Class 3B – Hazardous if the eye is exposed directly, but diffuse reflections from matt surfaces are not harmful. CW lasers from 315nm to far infrared are limited to 0.5W. Pulsed lasers of 400-700nm have 30mJ limit. Other limits apply to other wavelengths and to ultrafast pulses. Protective eyewear is typically required. Must be equipped with a key switch and a safety interlock.

Class 4 – All lasers with beam power greater than class 3B. Can burn the skin and cause devastating and permanent eye damage as a result of direct or diffuse beam viewing. May ignite combustible materials and thus may represent a fire risk. Must be equipped with a key switch and a safety interlock.