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Protective power

Kilowatt-class lasers are becoming more common and readily available for manufacturing, and with higher powers come more stringent safety precautions. At its latest technology open day in January, Trumpf UK was demonstrating an 8kW disk laser, using it to cut 40mm thick stainless steel. The process could be seen through a protective window.

It’s not just complete systems where the power levels are rising; higher laser power in general is now much more affordable than it was in the past. ‘Even in laboratories, scientists might have a 1kW laser on an optical table,’ stated Paul Tozer, managing director of the UK branch of laser safety equipment supplier Lasermet. ‘In that sort of situation, there’s a serious safety issue that needs to be dealt with.’

Lasermet has built active safety enclosures that fit on top of optical tables to contain the beam. ‘With a 1kW laser on that sort of size optical table you need to be looking at active guarding, because it will burn through 2mm of steel quite quickly because of the small beam
diameter,’ Tozer said.

The decision on whether to use an active or passive safety system generally revolves around power and the size of the enclosure. (A passive system is able to dissipate the laser energy, whereas active protective equipment for higher powers contains a light detector linked to an interlock system, so that if the beam hits the enclosure the laser shuts down.) As a rough guide, Tozer said that active protection is required for more than 5kW of laser power for a large enclosure, and more than 1kW for small enclosures. The size of the enclosure determines, along with other parameters, the size of the beam when it hits the walls.

Lasermet makes both passive and active enclosures. The Laser Castle is its passive enclosure, while the Laser Jailer is the active component for high power lasers. It also supplies the Glaser Jailer an active guarding window.

An active system will theoretically protect against any power level. There are still considerations to be made though, even with active protection, as Tozer noted: ‘With really high powers, you have to consider the passive guarding you’ve got behind the active portion, because if it takes 200ms to shut down a laser then you’ve got to contain the laser beam for that period of time. With a very high power laser of 20kW or 30kW with a small beam diameter, then you could have issues with containing that beam. All these factors have to be considered when designing the enclosure.’

There are many variables when it comes to laser protective equipment, and while safety standards must be met, the design will influence the cost of a system. Pro-Lite Technology runs laser safety training courses at its Cranfield, UK facility to ‘ensure that laser users don’t make a mistake and under protect’, remarked Ian Stansfield, a director at Pro-Lite, ‘but by the same token they don’t spend a fortune on protective equipment – that the user protects where it makes sense.’

Pro-Lite runs the courses in conjunction with Uvex Academy – Pro-Lite distributes products from Laservision, a German company and part of the Uvex safety group. The courses are designed to teach a range of people, from laser safety officers to maintenance employees in industry who might have to clean or do other tasks around lasers.

‘When designing a laser safety enclosure, first we work out what is needed according to the standards,’ stated Stansfield. ‘The protection level is based on power density for CW lasers or energy density for pulsed lasers. We take a minimum output power that we expect to see in the system, and work out the distances for enclosures. From this we can specify a certain protection level according to the standard.’

Safety windows

The level of protection given by laser safety windows or eyewear is measured to a certain extent by its optical density, which reflects how effective a material is at blocking the light. According to the EN 207 standard (for more on safety standards, see box), safety eyewear and windows have to have a high enough optical density to stop the light, and the material has to remain intact for five seconds and 50 pulses without damage.

Polycarbonates are the preferred material for protective windows for lower laser powers. However, as power increases, mineral glass is used as it is better at dissipating the heat.

Laservision is developing higher power protective polycarbonates for windows and screens. ‘There are polycarbonates with really high optical densities, of 11 for instance, which is great, but a 2kW CO2 laser can cut 18mm thick armour plating, and will melt polycarbonate material very quickly,’ Stansfield explained.

Laservision is now releasing high power polycarbonates, which are competing with a lot of the mineral glass, according to Stansfield. Mineral glass windows are limited in size, which traditionally have maximum dimensions of 100 x 200mm or an A4 paper size. In addition, these sorts of sizes of windows are expensive. ‘That’s quite restrictive when people are building systems where they want to view what’s going on inside an enclosure,’ said Stansfield. ‘They want bigger windows without compromising on the protection. Laservision is launching these high-power polycarbonate materials for just such a purpose.’

Tozer at Lasermet warns of the danger of using polycarbonate windows for high-power systems. ‘There are a lot of people using polycarbonate windows where really they’re not appropriate for a multi-kilowatt laser,’ he commented. ‘Above around 300W then a plastic window is really not suitable to protect against a direct beam. Once higher than around 3kW, then a glass window isn’t any good either – you need an active window.’

Lasermet’s Glaser Jailer is an active guarding window, while Pro-Lite supplies a large active polycarbonate window, which has been tested with a 16kW fibre laser. Pro-Lite’s window is made up of two polycarbonate sheets and a central sheet that acts as a light guide connected to a detector, which shuts the laser off. The window can be made to dimensions of around 500 x 840mm. ‘There’s a big shift in the customers we’re seeing wanting to use this specialised polycarbonate material [from Laservision],’ said Stansfield.

In addition to the protection level of windows and eyewear, there are also considerations concerning visible light transmission. Some protective materials might have a high optical density, but the user can’t see through them. ‘The application has to be considered,’ said Stansfield. He gave the example of doctors using laser safety goggles will need good colour recognition when viewing a patient. 

Reflections are also dangerous when using lasers. ‘The biggest problem is that reflections are really difficult to quantify,’ Stansfield said. ‘Is it going to be diffuse reflection spreading light in all directions, or have the effect of focusing the beam?’ Reflections might not be so much of a problem in an enclosed system, but some machines contain cameras that can be damaged by reflected light. ‘We often put protective windows on cameras to guard against this,’ Stansfield added.

‘You can’t dismiss reflections because they’re unquantifiable,’ he continued. ‘It might only be 0.1 per cent reflectance, but on a 16kW laser it’s still significant. Half a watt is going to burn your skin.’

Tozer commented that safety equipment for high-power fibre lasers is a big growth area for Lasermet. ‘The price of lasers is coming down, which means companies are buying higher power systems that can process things quicker,’ he said. And upgrading lasers to higher power versions also might involve different safety equipment. ‘A 4kW laser welder might have a passive enclosure, but if the company upgrades the system to an 8kW laser then you need to be thinking, “is a passive enclosure good enough?” We’ve been involved in retrofitting active guarding to existing passive enclosures for situations like that where people have upgraded the power of their laser,’ Tozer concluded. 

Active laser clothing

Lasermet has been developing active clothing and an active laser faceguard for use with high-power handheld lasers. The company will demonstrate its technology at TWI, a research institute based near Cambridge, UK.

TWI has built a 5kW laser for nuclear decommissioning (see Energy article on page 22 for more on this) which sometimes is used as a handheld device. ‘We found a way of incorporating our Laser Jailer [active] system into clothing,’ commented Paul Tozer, managing director of the UK branch of Lasermet. ‘We’ve also developed a safety-rated wireless protocol for the clothing, so that you don’t have to have a trailing cable. It’s easy to use wireless communication, but there’s nothing that exists on the market that’s safety-rated to the highest performance levels.’ Lasermet has developed a wireless safety protocol that achieves performance level E under EN 13849-1, the highest rating there is.

‘Active guarding systems or any interlock system needs to achieve performance level E under EN 13849-1, which is what our Laser Jailer product is rated at,’ Tozer said. Performance level E (PLe) to EN 13849-1 is the standard that governs safety-related control systems. That would apply to any interlock system, including active guarding systems, active windows and active clothing. ‘We’ve developed our wireless communication protocol to meet PLe, which I think is unique,’ he added.


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