By Bill Heffner

While plastic materials are very common in most areas, there are still a few specialized areas where glass offers some unique benefits. These areas include things like x-ray protection and contrast-enhancement. Due to the specific needs in each of these areas, the lenses need to be able to block certain wavelengths, while still being robust enough to function as safety eyewear. The main problem with these areas is that they tend to be very specialized. Usually when things are 'specialized' in eyewear it means that they are expensive and that it can take a while to get them. Since they are so specialized, these products tend to have a very limited range of availability, or are made on demand. This is because there isn't the constant demand there to make it worthwhile to keep large inventories or have expansive lens ranges.


Unfortunately, this isn't the type of x-ray glass you might be hoping for. While these aren't quite up to the level of Superman, they are used to block x-rays. Outside of the realm of eyewear, this type of glass is often used in windows. This is usually found in settings such as X-Ray or CT scanning rooms, airport x-ray screens, and many other places. One popular product that's used for this application is Med-X Glass from Corning.

The unique thing about this glass is the exceptionally high lead and barium content. Going directly from the information sheets, we see that the material is about 52 percent lead and 17 percent barium. That certainly seems like a lot, and it is – but you need to remember that the product had to be able to block x-rays. While there are other manufacturers that provide x-ray shielding glass, they all have similar properties. This glass also has a very high refractive index, which ranges from 1.76 to about 1.80. Due to the high lead content, the lenses also have a very high density.

The one main thing to remember about x-ray lenses is that while they may be glass, they don't necessarily behave like glass. Again, due to the high lead content, they will still scratch as easily as plastic. This means that patients also need to be aware of what they are wearing, and to be sure they are careful with the lenses. The upside to this is that since these are for radiation shielding purposes, they aren’t a pair that the patient should need to wear for hours on end.


The Amethyst Contrast Enhancement (A.C.E.) lens has become more popular over the last several years. Originally, it was designed to be used by aircraft pilots. It was intended to increase contrast and make differentiating objects on flight instrument panels easier. While the lens can still serve this purpose, it's not the area in which it has achieved the most popularity. It has become very popular among both amateur and professional glass workers. The lens itself is usually an amethyst color, but some manufacturers offer other colors, such as green.

Didymium used to be the preferred material for this, as it was able to block out the yellowish light found at the 589nm wavelength, often referred to as the ‘sodium flare’. For eyewear, this has largely been replaced by A.C.E., as it is able to achieve the same results. These lenses, however, are really for the sodium flare itself, and are not intended to be protection against ultraviolet and infrared wavelengths. For glassblowers, the specific protection that’s required depends on the type of glass used, as soda-lime glass has lower radiation hazards than glasses that have higher melting points, such as borosilicate. The former is often used by bead makers, who need the A.C.E. eyewear to help them see what they are working on, as the sodium flare makes it very difficult to see what they are working on. When someone is working with a glass such as borosilicate, lenses that block the sodium flare, ultraviolet, and infrared will be used.


When you get into the realm of specialty lenses, the number of suppliers of the lenses tends to shrink drastically. When we look at the x-ray lenses, they are available in single vision, flat-top 28 bifocals, and traditional progressives. If you want anything other than a single vision lens, however, you are likely looking at a week or two of lead time before you can get a pair of semi-finished lenses. This is usually due to the relatively low demand for specific progressive lens prescriptions and the relatively high cost of the lenses due to the material costs. A.C.E. Lenses have an even more restricted availability, as they are typically only available as single vision lenses. Even then, they tend to only be available in a few base curves.

This limited semi-finished lens availability is a prime example of the benefits of free-form. Instead of needing to carry an inventory of progressive lenses, having only single vision is all that's needed. From there, we can fabricate the desired prescription on the back of the lens. This means the turn-around for these products can go from weeks to days, as there is no need for a pair of lenses to be custom-cast for every add power.


This isn't just the case for a progressive lens, but also for bifocals. By using the same free-form calculation technology, we can fabricate a free-form blended round seg on the back of a single vision lens. While there are some differences between a lined bifocal and a round seg, it's nothing that a patient can't adjust to. This is especially true when there's really no other lens option, or the other options are simply too expensive or take too long to be feasible. By using computer-controlled calculations, we are able to not only make this blended bifocal on the back of the lens, but we can also control every aspect of it. This means we can make the addition area as large or as small as needed. This is very powerful when we are looking at the ability to create task-specific lens designs in specialty materials.

One other important thing that we need to think about when we talk about specialized lenses is analyzing the lens designs that are being used. While the material of the lens may be geared for a specific task (such as glass blowing), can we have a lens design that is geared for that as well? Since the lenses will be worn exclusively when working with glass, we know that there's very little need for distance viewing, but a great need for intermediate and near viewing. This makes it similar to the visual needs of a computer/office centric lens. Similarly, if we have x-ray lenses that are being used in a dental office, we know the visual needs are going to be similar in terms of being intermediate/near zone heavy.

While these are some of the more 'fringe' uses of glass, they are perhaps the ones that benefit most from the latest advances in technology. As I mentioned, some of these products can take weeks if not months to get a finished pair of lenses. By using free-form, we not only bypass expensive custom-cast orders, but improve prescription accuracy and quality. Free-form lens designs also make it possible for us to match the best possible lens design for the task at hand, whether it’s a progressive lens design that focuses on near, intermediate, distance, or some customized combination thereof. Being able to create these specialty lenses with free-form technology serves to make them cheaper, more accessible, and more usable.


This is the fifth in a six part series exploring the many uses of glass lenses, dispelling myths around glass and helping to show glass as the premium product it is. The following topics will be discussed in the November/December: Innovations in Glass Technology—Thinner, lighter, better.

Bill Heffner, aka Other Bill, is the director of IT, marketing and sales for FEA Industries. Other Bill is a fourth-generation lab executive following in the footsteps of his father, grandfather, and great-grandfather. Other Bill has worked at FEA in an on and off capacity for most of his life coming aboard full time in September 2009. To reach Bill with comments on this article email him at [email protected]







Labtalk June 2020