Weigh up your options before buying eyewear

06 Mar 2007

Deciding which laser-safety eyewear is the best option for your experiment can be difficult. Tom MacMullin looks at what’s on offer and details the criteria to consider before choosing.

Some of the frequently asked questions that we hear regarding laser-safety technologies suggest that even the most sophisticated laser users do not usually understand the basics of protective eyewear. For example, some common queries that often complicate the eyewear buying process include: Can I get a plastic version of this glass eyewear? Is polycarbonate eyewear as safe as filter-glass eyewear? Where does the laser radiation go if it hits my eyewear? Can I choose the colour of my filters?

Knowledge transfer

Eyewear technologies
Let’s briefly review what a laser filter is designed to do. A laser emits radiation at a specific wavelength that may or may not be within the visible spectrum. Some lasers, particularly tunable lasers, emit radiation at several wavelengths simultaneously. A laser filter must protect the eyes from a particular set of wavelengths but, at the same time, allow at least some normal ambient light to pass through to the eyes. This means that one filter alone cannot block all the wavelengths while still allowing the user to see their work. Although the laser filters described in this article are primarily related to eyewear, similar science applies to filter-glass windows and other laser work-area viewing products and optics.

There are two basic types of laser-protection filter technology: absorption and reflection. With absorption, the energy of the laser is captured by the protective medium and transformed into heat, which must then be dissipated by the surrounding materials. Absorbers can be formulated for selective wavelength attenuation and for broadband or multiple wavelength attenuation. Reflection means that laser radiation directed at the filter bounces off in a different direction, often in a broad scattering pattern. Reflective filters can also be designed for one or several wavelengths.
• Absorption: filter glass. Filter glass is traditionally used in protective eyewear to absorb laser radiation. Products are sometimes referred to by the colours of the final product, for example blue glass, green glass, and orange glass. Each filter contains an element, or mixture of elements, that is known to absorb laser energy at particular wavelengths. Two approaches involve introducing ions of heavy metals or rare earth elements and colloidal colorants into the glass.
• Absorption: polymers. Polymers impregnated with dyes and other materials will reproduce the absorptive behaviour of mineral glass-laser filters. The polymers used in laser safety are typically polycarbonates, but other materials, including nylons and acrylics, may be used. As is common in many industrial plastics processing technologies, the dyes are mixed directly into the polymer during the moulding process, extruded into concentrated pellets of the desired end-product material and later diluted for product moulding, or impregnated into the surface of the polymers. Plastic laser-safety compounds can be moulded into a much wider variety of sizes and styles compared with filter-glass products.
• Reflection. Reflective laser-safety coatings come in the form of thin-film coatings, metallic-film coatings, dielectric films, or dielectric interference coatings. Multiple layers of specially selected materials are applied to a substrate under vacuum conditions. This creates a custom-designed interference pattern that reflects only the desired wavelengths and allows the remaining light to pass through. The laser energy is reflected not only at the surface, but at the layers built up near the surface of the substrate. The coatings used in laser safety are primarily applied to glass, but similar coatings on polymers are an eventuality.
• Combination technologies. Some laser-safety products comprise several technologies, the most common include coatings on filter glass; laminations of multiple filter-glass substrates; and laminations of filter glass and polymer substrates. Lamination of multiple protective layers is often the best way to build a product that protects against multiple wavelengths and unusual combinations of wavelengths.
• Patient eye protection. Completing the inventory of laser-safety technologies is the growing family of opaque patient eyeshield products. These are technically absorptive products and are usually entirely opaque through the extended laser spectrum from 380–11,000 nm. These products are often metallic or contain a metal layer and are increasingly available as disposable products for use in dermatological laser and intense pulsed-light applications.

Making your selection
Be aware that there are trade-offs involved when selecting any laser-safety eyewear technology. The table on p26 summarizes our filter-technology recommendation algorithm and lists criteria to consider when selecting eyewear. One common trade-off is that filter glass will be heavier than polymer products, but usually provides better visible light transmission (VLT).

Coated substrates are selected when multiple-wavelength protection is required, but these products tend to be among the most expensive and require the most care. The physical properties of polycarbonate make eyewear based on this technology suitable for all-day wearing and for moulding single-lens products with a wide field of view.

Before finalizing your selection of laser eyewear, consider your work environment and not just the laser system. High VLT products are best for low-light environments. Impact resistance is necessary in production environments and should be considered in all workplaces, whereas UV protection and glare reduction may be needed for welding. Larger products and wrapping eyewear products provide additional splash protection for medical applications and many of these customers prefer lightweight polycarbonate eyewear. The technology you select will impact these performance characteristics.

Because laser-safety eyewear is a personal protective device, a review of the failure modes and failure characteristics of products made from the various technologies may assist you further in your choice.

Failure modes
Glass filters generally provide superior thermal stability when compared with plastic filters, and polycarbonate ones in particular. In high temperatures, 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 a catastrophic failure due to a direct hit from a laser. It is worth noting that even a cracked glass filter provides some protection if the pieces remain intact.

Polymer lenses may carbonize if they are subjected to a direct hit by a laser with a high power density and can then be penetrated. The dyes used in polymers may also 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.

A consideration for coated products is the angle of incidence. The coating layers must protect against stray radiation that impinges the lens at an angle away from the perpendicular. Current standards require 30° of protection and some products are available with up to 40° of protection.

To summarize, laser-protective filter technology selection depends not only on the application, but also on the characteristics and relative trade-offs of each technology. A careful review of your work environment and eyewear-performance expectations against the various outlined criteria will enable you to shop smartly and effectively for laser protection.

• This article originally appeared in the February 2007 issue of Optics & Laser Europe magazine.