By Tom MacMullin
Wear your eyewear when working with your laser! How many times have you heard that? Your Laser Safety Officer (LSO) can, and should be, your best friend and advocate. Your LSO can base selection of LEP on OD and wavelength, with a further understanding of the unique environments and individuals – to make better choices that are safer and user-friendly as well. (Reference ANSI Z136.1-2014 220.127.116.11.2 Factors in Selecting Full Protection Eyewear.) Are you and your LSO working together? Remember, your eyes are in their hands.
Let’s start with the most fundamental question: why do you even need laser eyewear? Our premise is that stricter adherence to the guidelines found in the ANSI Z136 standards would likely result in construction of more laser safe environments. Add a wall; put your laser in an enclosure; isolate laser work from other processes. If your laser lab does not need laser protective eyewear, then you will not worry as much if someone just doesn’t like to wear goggles.
If you include laser eyewear in your laser safety program, perhaps you will consider the coolest technologies on the market. Coated products, for which multiple layers of thin metallic films are applied onto a lens surface, fit this bill with the promise of high protection levels and excellent visible light transmission. However, some coatings are not durable and may scratch off. Other coatings do not provide adequate “angle of incidence” protection; that is, the coating layers must protect against stray radiation that impinges the lens at an angle away from the perpendicular. Current standards require 30 degrees of protection and some products are available with protection up to 40 degrees. If your product selection overcomes these concerns, you should also recognize that adoption of coated laser products creates two-legged mirrors — be sure that everyone in that room is wearing eyewear!
Safety at What Cost?
Perhaps you hear staff complaints that laser glasses are too heavy. This is where polymer eyewear comes in. Most of these products incorporate some form of polycarbonate lenses but other plastics may be used. The advantages in addition to weight include generally lower costs and an ability to shape the product into more comfortable or more form-fitting designs. The downsides: polymer lenses may carbonize if subject to a direct hit by a laser with high power density and then can be quickly penetrated by laser radiation. Also, dyes used in polymers may exhibit photochemical bleaching, a phenomenon in which the intensity of the laser radiation impinging on the dyes exceeds the ability of the material to absorb and dissipate the energy. The effect is to open a temporary window through the lens for the duration of the high intensity exposure. If your environment poses such risks, you may be able to overcome these phenomena by switching to filter glass products. The latest ANSI Z136 standards highly recommend glass eyewear for high-powered lasers.
There are other common drawbacks to polycarbonate laser eye protection. Plastic filters designed for multiple wavelength coverage are often very dark and may distort color recognition in the visible range. The effects are to worsen trip hazards or to camouflage indicator lights. Some LSOs specify two or more pairs of eyewear for complicated environments: we try to discourage this inclination whenever possible. Laser operators will neglect that second pair and try to align a laser without eye protection. Or, worse, they will switch operating modes (think of all the wonderful colors you can get with a tuneable dye laser!) and will not switch out the eyewear.
Through the Looking Glass
As much as we love coated eyewear products, we really love filter glass. Costs are usually reasonable. We can often cover multiple bands with a single filter or a combination of filters in a single spectacle. Visible light transmission is usually pretty good; if not, we will select a filter that enhances colors appropriate to a work environment. Glass filters generally provide superior thermal stability when compared to plastic filters and polycarbonate in particular. Under high heat, however, glass will tend to splinter or shatter due to heat distortion. Glass filters should be treated or coated to hold the pieces together in the event of catastrophic failure due to a direct hit by the laser. It’s interesting to note that even a cracked glass filter provides some laser protection if the pieces remain intact. Look for the ANSI Z87 impact rating on your laser safety eyewear.
Filter glass eyewear, including some combinations with coatings, are the best solution for femtosecond (ultrafast) lasers. Many technicians rely on the “M” ratings provided by testing to the EN207 standard. It is our experience that this rating is typically adequate for picosecond pulses and is not appropriate for ultrafast lasers. Our practice is to have our customers test the filters in their environment with their specific femto laser operating conditions.
Just One More Thing
What else can go wrong? You’re ANSI Z136 laser safe, right? Did you forget you need dust protection; you will likely then select a goggle. Maybe you need splash or full face protection; select a face shield or a wide and tall polycarbonate wrapping style. Perhaps you have poor overall eyesight? Filter glass laser eye protection can incorporate a prescription in the lens; polymer styles are available with clip-in or fit-over features for your vision correction lenses. Hot, sweaty workplace? Consider anti-fogging agents or coatings. Maybe the extra burdens are more serious. Laser goggles with UV protection or glare reduction required for laser welding are available. We also now offer laser protection in combination with light duty X-Ray protection typically found in hospital operating rooms.
In the end, rely on the advice of your LSO. The LSO has your best interests, and your eyes, in their hands.
Tom MacMullin is President and General Manager of Kentek Corporation. For more information, visit www.kenteklaserstore.com.