Jessica Rowbury looks at the technology used to protect the eyes from laser beams
Protective eyewear is a must when working with lasers, as a direct or reflected laser beam, even at lower power output levels, can cause ocular injury. Although not designed for directly looking into the laser, a pair of safety goggles that meet safety standards should protect against accidental exposure for a minimum of five seconds, or 50 pulses. But it is not always the safety that is the main concern of customers, as eyewear suppliers are finding that style and comfort are becoming just as important for ensuring that users keep their protective glasses on.
According to David Bothner, marketing director for NoIR LaserShields, there are two primary methods in which laser safety filters are based. ‘One uses an absorptive technology, the other uses reflective. You can also have a hybrid of the two,’ he said.
Goggles are typically made out of glass or polycarbonate materials, which are infused with organic dyes designed to absorb laser radiation. To provide additional protection for very high power lasers, a reflective coating can be applied to a dye-infused glass filter.
In order to give the correct protection for the laser, absorptive dyes are designed to ‘tune’ the absorption of the laser radiation to the desired wavelengths. For safety eyewear manufacturers, the aim is to create a filter which allows as much light as possible through, so it doesn’t become too dark for the user. ‘We look to build filters with very narrow band absorbers, designed for specific wavelengths − so there is a very high absorption at the laser wavelength and very high transmission on other wavelengths,’ explained Bothner. ‘The goal is to come out with a very faintly coloured, very high visible light transmittance filter that provides protection just at the wavelengths required.’
Ideally, the absorption band of the filter would be within a few nanometres of the laser wavelength, but it can be challenging to produce narrow band filters, added Bothner: ‘For a green laser at 532nm, the optimal filter would have very high absorption values between, say, 525nm and 535nm, and then pass everything else,’ he said. ‘That is very hard to do with dyes, so commonly you’ll have filters that are broader than that − so it will be 500-532nm.’
Allowing as much light as possible to penetrate through the goggles is especially difficult in the wavelength range visible to humans; between 380nm and 780nm. ‘Some lasers – especially these new wideband lasers that are essentially white light sources − are very difficult to protect against; you have to provide eyewear that will provide broadband protection in the visible,’ said Russell Bailey, who is responsible for the laser safety product line at Pro-Lite Technologies. ‘We make sure that [customers] know that if they’re going to be using these sorts of lasers then it has to be a bright environment, because if you need to protect against the whole visible then you need a very dark filter.’
But there are options available that provide protection for a wider range of wavelengths, such as the Laservision Eco L-07, provided by Pro-Lite, which protects from 180-545nm, and 630-1,210nm. ‘These give you 10 per cent light transmission − it’s not extremely bright, but it’s not much worse than wearing dark sunglasses,’ said Bailey.
For some applications, more than one pair of glasses can be used in order to provide full protection, Bailey added, which also allows for greater visibility. ‘Alternatively, [customers] may have several different wavelengths across the visible, then we will discuss the applications and generally speaking customers will be using different lasers at different times, so they can wear one pair of glasses when using laser one, and change glasses when they are using laser two,’ he said. ‘In this way, each pair of glasses retains as high a level of light transmission as possible.’
As lasers are being improved and developed, so are the filters. There is ongoing research addressing absorptive dyes and how they can be improved to provide the best protection, particularly in the visible wavelength range. ‘Laser users will not wear something that is prohibitively dark. So the goal always is to find dyes or design filters that maximise visible light transmittance while providing at least the required but if not higher protection values,’ commented Bothner. ‘We have relationships with dye suppliers, and as new lasers come out, we look for new dyes to cover those, or we’re looking to always improve the dyes that we have for existing product lines.’
As well as making glasses that are penetrable to light, a big focus for eyewear suppliers is to provide glasses that are as comfortable as possible for the user. ‘We’re very conscious of the design, because the best protective eyewear is that which the user wears. You could have a great pair of goggles, but if they are not comfortable and the user chooses not to wear them, then that is the worst case scenario,’ noted Bothner. ‘We have a dozen or 15 different frame styles to try to serve the facial structures and the style requirements of the users − we are always developing new frames to try and get something that people will like to wear.’
Wearability is so important, because sometimes, just getting customers to keep glasses on can be a challenge, according to Bailey: ‘The hardest thing is making people aware that they need to be wearing glasses. There are so many people that when aligning their laser, will lift up their eyewear to check that everything is in the right position. This is the worst thing you can do, and that is the main difficulty − stopping people from doing things that put themselves at risk,’ he noted. ‘This is why we run laser safety awareness courses in conjunction with Laservision several times a year, so as to promote a better understanding of safety with lasers.’
Plastic glasses are generally the most popular choice, as they are generally less expensive, lighter and more comfortable: ‘[Plastic lenses] can generally fit or be put into more comfortable frame styles with more curved lenses, fancy looking frames and so on,’ Bothner explained.
However, when it comes to high power lasers, glass lenses provide a higher level of protection, according to Bailey: ‘Plastic filters have improved greatly in recent years but are typically used against low to medium power lasers due to the more limited power handling of polycarbonate. Glass filters tend to be more robust and better able to withstand direct exposure to the beam,’ he said. ‘For the most intense laser beams, the best protection is provided by a reflective, dielectrically coated glass filter.’
For added protection, glass frames can also be reinforced with metal, Bailey added, to help comply with laser safety standards which require the frame to be able to withstand the same level of radiation as the filters, as well as prevent the laser beam from reaching the eyes from the sides.
When selecting laser eyewear, it is often a balancing act between cost, comfort, and level of safety. ‘All customers are concerned with cost and also the comfort and fit of eyewear,’ explained Bailey. ‘Some customers need more “hand holding” than others when it comes to understanding the cost and how to go about choosing glasses with the right level of protection. We [Pro-Lite] tend to lead the customer towards what they actually need rather than what they want.’
The blocking ability of a filter, known as the optical density, should not be the only means of deciphering the level of protection a pair of glasses will provide, according to Bothner. ‘Optical densities are important, but you must also look at what laser damage threshold the filter has,’ he explained. ‘Some very short pulsed, high energy lasers deliver a lot of energy in a very short amount of time, so the filter might have a very high optical density, but might have a low damage threshold − that is not a safe situation.’
It is therefore recommended that laser eyewear is tested in the working environment before the goggles are put to use, according to Bothner: ‘You need to always have, I believe, a risk assessment done by a laser safety officer, where all of the factors − the working environment, the beam characteristics, the laser energy, and the eyewear and other safety controls − should all be evaluated.’
Although crucial, eyewear should be considered after other safety equipment, and should in no way be relied solely upon, when using high-powered industrial laser systems, Bothner commented: ‘For high-powered industrial laser systems… if you’re relying on goggles to protect your eyes, I think it should be the last resort,’ he stated. ‘Industrial systems typically have engineering controls built in to provide a good deal of safety, and the operator ideally doesn’t have a direct exposure risk… The best care would be that the user is behind a wall with a viewing plate.’
The Glaser Jailer from Lasermet is a new active laser safety filter window which can typically be incorporated into the door panel of a laser safety cabin. This allows users to see inside the cabin where multi-kilowatt lasers are being used for processes such as cutting, welding and cladding, while remaining safe outside.
The window has an integrated active laser safety system that connects to the laser interlock controller, so if the laser beam inadvertently strikes the window, this is identified by the interlock controller and the laser safety input is isolated within 50 milliseconds. ‘The laser, therefore, stops firing and any personnel outside the cabin in the vicinity of the window are kept safe,’ explained Phil Jones, marketing manager for Lasermet. ‘Once the window has been struck by the laser, an engineer needs to identify the cause of the strike and resolve the issue so that the window can be replaced and the system reset in order to continue firing the laser.’