Laser cleaning without a safety barrier: is 1,550nm the solution?
Woodrow Scientific's 'retina-safe' lasers have been used to effectively remove carbon from pistons
Engineers at UK firm Woodrow Scientific are developing laser cleaning systems using high-power pulsed lasers that operate at 'retina-safe' wavelengths, which could potentially be used without a safety barrier.
The technology, shared at the recent Industrial Laser Applications Symposium (ILAS) conference in Daventry, UK, could reduce the cost, time and safety challenges associated with setting up laser cleaning applications.
According to the firm, this can lead to wider market acceptance of laser cleaning across industry, and potentially into mainstream (public-access) applications.
Speaking at the conference, Tara Murphy, Senior Laser Applications Engineer at Woodrow Scientific, began by outlining the risks associated with the 1,064nm wavelength used commonly among laser cleaning systems on the market.
“At 1,064nm wavelength, light will penetrate the cornea and the lens, which will focus the energy onto the retina, potentially causing flash blindness or retinal burns and lesions.” she explained. “Even a single pulse from a 1,064nm cleaning laser can cause retinal damage”
To avoid this, PPE including goggles and skin protection must be worn within the nominal ocular hazard distance (NOHD) – defined under the laser safety standard EN 60825:2014 as the distance at which a laser beam has expanded enough that the level of accessible radiation no longer poses a threat to a person’s eyes or skin. Murphy noted that for a scanning 1,064nm laser with 100W average power, 10mJ pulse energy and 100ns pulse duration, the NOHD can vary from anywhere between 26.5m to 59.5m, depending on how the laser is scanned. This means the working area must always be enclosed by a safety barrier/curtain to ensure those in the vicinity are shielded from harmful radiation, and do not themselves have to wear any PPE.
1,550nm lasers: Reducing the hazard zone
Murphy and her colleagues are therefore currently exploring lasers at the 1,550nm wavelength (widely used in autonomous vehicle lidar systems) to see if they can bring down the NOHDs of cleaning systems to a more manageable level, which could dramatically increase the ease of setting up an application.
“At this wavelength, laser light that is incident on the human eye is mostly absorbed by the cornea,” Murphy explained. “While overexposure at these wavelengths can therefore damage the cornea, there isn’t the focussing effect of the lens onto the retina. This results in the safety standards treating it differently.“
While 1,550nm can still damage the eye and skin, according to Murphy: “As we scan the laser beam over a scanning area, it gives us a larger, time-averaged ‘effective divergence’. This means the NOHD decreases significantly – for example, it can become as low as 1.8m when the scan field size is 100mm x 100mm. When compared with a 1,064nm laser this corresponds to a ‘hazard zone’ up to 15-times smaller using the same laser parameters.”
Woodrow Scientific is optimistic that the first handheld 'retina-safe' cleaning lasers will be commercially available by the end of the year
And so while the user of the cleaning system will still need to wear goggles, anyone beyond 1.8m from the source in this scenario would be deemed to be in a ‘safe zone’ where no PPE is required, according to Murphy.
The caveat to this is that the system itself would need to ensure that the scanning pattern is maintained to ensure that the beam is always moving in the field uniformly – delivering no hotspots. Murphy assured attendees that this is indeed possible: “We have ways of monitoring the scanning in real-time,” she said. “Linking the scan-monitor to our safety interlocks, our tools will instantly shut down the laser if the scanner fails to maintain the required pattern.”
By combining such capability with proximity monitoring techniques by means of a ‘virtual safety barrier’ (enforced for example, using a lidar system that detects when someone steps within the NOHD and causes the cleaning laser to shut down), “we can unlock this technology to be used in open spaces and remove some of the laser safety concerns from potential users,” Murphy remarked.
But how do 'retina-safe' laser cleaning systems perform?
To demonstrate the cleaning capabilities of 1,550nm laser systems, Murphy shared multiple comparisons between two setups cleaning painted steel panels using the same parameters – with the only difference being the wavelength.
The two setups were used to subsequently clean panels coated with blue, grey and white paint of approximately 50µm layer thickness. It was found that performance and cleaning speeds were very similar with both lasers for each colour of paint (1,064nm was marginally better for grey paint, while 1,550nm was marginally better for blue paint). Murphy added that she has also had success with removing rust, carbon, oil and grease using a 1,550nm cleaning system, as well as a process developed to post-clean weld-seams and injection moulds, which she said gives remarkable results.
Paint removal is one of numerous applications 'retina-safe' lasers are being trialled on
Progress towards a commercial system is being made, with Murphy and her Woodrow Scientific colleagues being optimistic that the first commercial 1,550nm laser-cleaning tools, with their much reduced hazard zones, will be available by the end of the year.
“Performance in key application areas appears to be similar so far, but we have the added benefit of saving time and cost by not having to enclose an area,” she concluded. “Laser cleaning is an extremely powerful technology, especially within industrial applications. With retina-safe laser cleaning technology we can open up the market to the mainstream."
Images: Woodrow Scientific