FEATURE | PHOTOCHROMICS PRIMER

Benefits
YOU CAN SEE

The what, why, and how behind photochromic lenses

BY JENEAN CARLTON, BA, ABOC, NCLC

cLEAR-TO-DARK PHOTOCHROMIC LENSES AUTOMATICALLY ADJUST THEIR COLOR based on the amount of UV radiation they are exposed to. When walking from an inside environment to outside, these lenses quickly change to a darker color to provide the wearer with more comfortable vision by filtering bright light.

Photochromic lenses can help reduce eyestrain and protect wearer’s eyes, as well as surrounding delicate tissue, from 100 percent UVA and UVB radiation exposure. Almost any patient who wears prescription eyewear can benefit from all that these self-tinting lenses offer. And if they don’t already know that, it’s your job to educate them.

The science behind photochromic lenses has advanced considerably since the early days, with manufacturers expanding the scope of variable-tint eyewear with every development. Here’s a primer on the basics of photochromic lenses for everyday wear.

TOP CANDIDATES

Simply stated, the best candidates for photochromic lenses in their everyday eyewear are patients who wear prescription lenses. Those with active lifestyles or jobs that bring them from indoor locations to outdoor locations frequently are especially good candidates for photochromic lenses.

As kids can have a hard time keeping up with one pair of glasses, much less a second pair of sunglasses, it makes good sense to fit children with photochromic lenses. This way, when going outdoors for school recess, or when playing with friends in the neighborhood, children won’t have to worry about changing their eyewear. Most important, parents will be comforted knowing that their children’s vision is protected from the damaging effects of UV wavelengths.

Outdoor and sports enthusiasts may also benefit from a photochromic sunlens as a second pair, as those lenses adjust their tint level to the light conditions.

SCIENCE IN ACTION

Photochromic lenses change color due to a shift in the molecular structure of the photochromic dye pigments. This chemical reaction takes place when the lenses are exposed to UV wavelengths from 300-360 nanometers, and to some degree in the presence of short visible light wavelengths around 420 nanometers. This change is referred to as “photochromism.” Photochromism is simply defined as a light-induced change in color that involves a reversible chemical reaction.

With photochromic lenses, we are able to see science in action as colorless, or slightly tinted, dye molecules convert to a darker color in reaction to the presence of UV radiation.

• CLOSED CONFIGURATION. When the lens is lighter in color or clear, or in the inactivated state, the dye molecules are said to be in the “closed” configuration.

• OPEN CONFIGURATION. When exposed to UV wavelengths, the structure of the dye molecules changes to the “open” formation—meaning the pigments then activate and become darker.

Once the UV radiation source is removed, the dye molecules revert back to the “closed” state, the pigments fade and return to their indoor color.

• TEMPERATURE VARIATIONS. Heat can also reverse photochromism and cause the lenses to lighten. This is why photochromic lenses have a difficult time achieving a sunglass category 3 depth of color in high-temperature climates. That being said, advancing technology is definitely adding positive movement to beating the heat hurdle. It’s important to note that modern dyes are less heat sensitive than earlier lens offerings. Manufacturers are constantly developing dyes that perform better in hotter climates.

Conversely, the lenses may achieve a very dark tint in colder climates as the molecules are more readily able to activate in cooler temperatures.

DEVELOPING DYES

Industrial chemists consider many performance goals when developing photochromic dyes. Color, activation and fade rates, stability of the dyes in the “open” or darkened state, fatigue rate, and temperature sensitivity are all important characteristics when developing dye compounds for optical lenses.

Selections are made from different dye “families” and are combined to create optical products that will meet all of the performance objectives of the manufacturer.

Each of the dye families offers unique performance characteristics. One may be less sensitive to higher temperatures but activate at a slower speed. Another may be unstable in the darkened state, which can lead to an undesirable color shift when UV wavelengths are present. Manufacturers go to great effort to develop the optimum dye combinations for their product offerings.

There are two important families of photochromic dye compounds used in the optical industry: spiroxazines and naphthopyrans. Spiroxazines offer fast activation and fade rates, resist fatigue, and have a long life span. Naphthopyran dyes are particularly important because they change quickly from the faded to activated state and provide excellent stability in the “open” formation.

The in-mass process involves mixing the photochromic dyes with the lens monomer before molding, so the dyes become part of the lens material. Image courtesy of Vision-Ease Lens.

All photochromic dyes have a finite lifespan. Ultimately the dyes will fatigue, meaning the speed with which the lenses darken and fade will gradually slow over time. However, today’s photochromic lenses contain dyes that can be expected to maintain peak performance for at least the lifespan of a standard prescription.

Though dye compounds are an important part of creating photochromic lenses, they are only part of the process. The lens materials, or lens matrix, that house the pigments are another important piece of the puzzle in developing these lenses. For the dyes to perform well, and change quickly from the colorless state to the activated state, they need to be incorporated in a flexible substrate that will allow the molecular change to easily take place. For this reason, manufacturers often develop specialized lens materials to help the dye compounds perform at peak levels when incorporated with lens monomers.

CLEAR OR NOT

Some photochromic lenses are clear indoors and others have a slight residual tint because the dye pigments activate to short wave visible light around 420 nanometers or because the lens design has a slight fixed tint. Regardless of lens, it’s important to discuss the inactivated color of the lenses with your patients.

If you know that a lens has a residual color when worn indoors, however slight, let your customers know while discussing lens options with them. Discussing this with patients early on in the lens selection process could save your practice the cost of remaking lenses.

The best way to know what the lens color will be in both the activated and inactivated states is to wear the lenses yourself. Ask your vendors or laboratory representatives to allow you to try out their lens offerings so you can provide your patients with information based on personal product experience.

When explaining photochromic lenses to your patients, use a sample pair of glasses to demonstrate how quickly the lenses change color when outdoors. Photochromic sales will soar if patients are escorted outside while wearing a trial pair of photochromic lenses. Patients can see for themselves how the lenses change in seconds to a darker lens when outdoors and how they quickly lighten once back indoors. Demonstrating product in this way will allow patients to experience the benefits of photochromics themselves before making the financial commitment to order lenses.

ADDITIONAL TREATMENTS

Once applied and cured within a substrate, photochromic dyes are a good deal tougher than they may seem. They cannot be scratched off or removed with solvents such as alcohol or acetone. Additional lens treatments such as anti-reflective (AR) coatings or absorptive tints don’t generally affect the performance of photochromic dyes.

The imbibition process involves applying photochromic dye to the front surface of the lens and then processing it (usually by heat) so that it penetrates the surface of the lens material. Image courtesy of Transitions Optical, Inc.

However, when adding an additional absorptive tint (such as those applied in an in-office laboratory), it’s important to first check with the lens supplier to make sure that the tint does not include a UV filter. If so, the lenses may not perform well as the dyes depend on UV radiation to activate.

CHOICES ABOUND

A common complaint from patients who want to reject the idea of an everyday photochromic lens is that they won’t darken in a vehicle. Because photochromic molecules are dependent on UV wavelengths to shift from their clear to darkened state, most photochromic lenses won’t activate inside vehicles because windshields and side windows contain a UV filter.

Some photochromic lenses, however, do indeed darken slightly (though they won’t get sunglass-dark), because they are activated by visible light as well as UV. Do your homework, and know which lenses are best suited to each particular patient.

If a patient opts for a photochromic lens that does not activate in a vehicle or the patient needs a sunglass-dark lens when driving, some ECPs recommend a sunglass clip-on.

This can be a good solution to solve glare problems when driving, particularly if the clip-on is also polarized.

One photochromic brand also offers a lens that has variable polarization along with variable tint. Though it won’t activate in a car, it offers a choice for patients who may want variable polarization AND variable darkening when outside.

Choice is a key word when discussing photochromic lenses. There are more choices than ever—not just within brands or “families,” but also between brands. Each offers its own benefits and differences, with a variety of pricepoints represented.

INFORMING PATIENTS

Most patients are aware of photochromic lenses through consumer-oriented marketing and articles. So be prepared when they ask you about them. Let your patients know that you are up to date on advanced lens offerings by informing them about the features and benefits of photochromic lenses.

Take advantage of the many resources that are available from manufacturers, including marketing tools, in-office demonstrators, and signage. Get the conversation going, and talk to every patient about photochromics.

Photochromics are a good choice for all patients, regardless of age, because of their many attributes: they automatically change to a darker color in the presence of UV wavelengths, thereby filtering bright light and providing wearers with more comfortable vision when outdoors; they protect eyes from 100 percent UVA and UVB wavelengths; and, though not intended to replace sunwear, can provide the convenience of visual comfort without changing glasses.

Photochromic Lenses: Benefits You Can See

Most patients will benefit from the comfort, convenience, and UV protection offered by photochromic lenses. This puzzle focuses on the science, features, and benefits of photochromics.

JENEAN CARLTON, ABOC, NCLC

ACROSS

1 The Lens ___________ is another important piece of the puzzle when developing photochromics.

5 The lenses may achieve a very dark tint in ___________ climates.

7 Photochromics may have a slight _____________ tint because the dye pigments activate to short wave visible light.

9 Parents will be comforted knowing that their children’s vision is _______ from UV rays.

11 It makes good sense to fit _____________ with photochromic lenses.

13 All photochromic dyes have a _____________ lifespan.

15 With _______________ the pigments are infused into the front surface of the lens and are embedded into the material.

17 In the __________ formation, the pigments activate and become darker.

18 _____________ are dependent on UV wavelengths to shift from a clear to darkened state.

19 _______ dyes provide excellent stability in the open formation.

24 Manufacturers often develop ________ lens materials to help the dyes perform at peak level.

26 _____________ can also reverse photochromism and cause the lenses to lighten.

27 Chemists consider many __________ goals when developing photochromic dyes.

DOWN

2 Once the UV radiation is _________ the dye molecules revert back to the closed state.

3 When the lens is lighter in color, the dye molecules are in the _______ configuration.

4 Each of the dye families offer unique performance _____________.

6 A thin ___________ of dyes is laminated between layers of polycarbonate.

8 Dye molecules convert to a darker color in ___________ to the presence of UV radiation.

9 The best candidates for photochromics are patients who wear ________ lenses.

10 It’s important to discuss the _______ color of the lenses with your patients.

12 __________ are one of the dye families used in the industry today.

14 Use a sample pair of glasses to _________ how quickly the lenses change when outdoors.

16 Most photochromic lenses won’t activate inside a vehicle because _________ and side windows include a UV filter.

18 ____________ is defined as a light induced change in color that involves a reversible chemical reaction.

20 Dye ___________ are combined to create optical products that will meet the objectives of the manufacturer.

21 Photochromics change color due to a shift in the _________ structure of the dye pigments.

22 Photochromic lenses help reduce ____________ and protect wearer’s eyes from UV wavelengths.

23 Often photochromic dyes are incorporated into a resin lens ____________.

25 With the _____________ method, photochromic dyes are homogeneously mixed into the monomer while in a liquid state.