CONTINUING EDUCATION, 1 CE Credit � $9.99, 1 Hour, General Knowledge, Level 1, Release date: October 2007, Expiration date: October 31, 2012

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THROUGH 
THE LENS

Shades of Color:
The Way Photochromics Work

Photochromic lenses are lenses that change from lighter to darker due to exposure to ultraviolet coatings. The ultimate in comfort lenses, photochromics help the eye care professional provide clear vision indoors and out to his or her patients. Although not a replacement for sunwear, photochromics can fill most of the basic visual needs of the patient.

Putting the Glass into Glasses

The first photochromic lenses were invented by Corning in 1964 and were marketed under the term PhotoGray. Shortly afterward, PhotoBrown was introduced. Using a process called in-mass technology, glass lenses change by the activation of a combination of silver chloride and silver bromide crystals that were added to the lenses while the material was in a liquid state. The early lenses had an approximate 20% tint to the lens that darkened to about a 50% tint. The result was a lens that, although comfortable in a lot of lighting conditions, was still too light in bright light. Corning�s next step was a lens that would get as dark as a sunglass as the amount of sunlight increased. The result was the PhotoSun single vision lens that was developed in 1971. It was too dark in its unactivated state to be used indoors or at night, however.

In 1978, the PhotoGray Extra lens was placed on the market. Combining the technology of the PhotoGray and the PhotoSun, it was as clear as the PhotoGray at 20% in the unactivated state and as dark as the PhotoSun with full activation at 75% tint. Today, there is the PhotoGray and PhotoBrown Thin & Dark. A thinner and lighter lens, it can be ground to a 1.5mm center thickness in the United States.

Benefits to glass photochromic lenses include:

  • Better scratch resistance

  • Enhanced optics

  • UV protection

Drawbacks to glass photochromic lenses are:

  • Weight

  • Lighter colored bifocal segments. This is because the fused segment does not contain the photochromic crystals and any change in color occurs on the underlying lens.

  • Uneven color patterns in higher prescriptions. The thicker portion of the lens is a darker color than the thinner portion of the lens.

  • Loss of lightening ability as the lens ages (this is not the case with the Thin & Dark lenses).

The World of Plastic Lenses

Photochromics come in many different types of plastic substrates including CR-39, mid-index, high-index, and Trivex. What matters in the functioning of the different types of photochromic lenses is not the material itself, but the process that the lens goes through to become photochromic. The main ways in which a photochromic lens gains its color changing capabilities is through the imbibing process, en-masse process or a coating process.

Imbibing Process

Imbibing is when the photochromic substance is penetrated into the lens substrate. It was originally developed in the 1970s by American Optical for their glass Photolite lens and then improved upon by Transitions Optical in the 1980s. The first successful plastic photochromic lens was released by Transitions in 1991.

How imbibing usually works is that a liquid photochromic solution is sprayed on the front surface of a finished or semi-finished lens. The lens is then heated to enable the solution to penetrate the surface up to 200 microns. The lens is then hard coated to prevent the photochromic chemicals from becoming degraded from oxygen. The photochromic compound in the solution varies depending on the lens substrate that is being used. The process takes approximately 18 hours.

Imbibing is commonly used in standard index and mid-index lenses. It is not used in harder lens materials such as high index, polycarbonate, or Trivex because the photochromic solution cannot penetrate as deep.

En-Masse

En-masse is used with lens substrates that can uniformly dissolve photochromic dyes. These materials include low to mid-index resins. Once the resins completely dissolve the photochromic dye, the resin is then cast into a lens. The reason that the plastic lenses do not have the ring effect that is found in glass lenses is that the surface photochromic molecules of a plastic lens are all that is activated. This allows the lens to darken evenly.

How long the changing capability lasts will depend on the material used, the amount of photochromic molecules placed into the resin and the amount of UV exposure. The average lifespan is one to two years. Lenses that are manufactured using en-masse processes are Corning�s SunSensors, Rodenstock�s ColorMatic Extra and Kodak InstaShades.

Coating Process

The coating process is used by many lens manufacturers, such as Vision Ease, for the lens substrates that do not take the imbibing or en-mass processes well. Manufactured in a clean room to prevent dust particles from depositing on the lens, a clean semi-finished lens goes through a coating process.

Typically, a primer coat is applied to the lens to act as �glue� between the substrate and the photochromic layer. Next the photochromic layer itself is placed on the lens. Although the layers were traditionally applied as a dip coat, they are more commonly applied as a spin coat today. Spin coating enables the photochromic molecules to spread evenly across the lens surface ensuring even coloring. After the coats cure on the substrate, the lens is then hard coated to protect the photochromic particles.

Into the Darkness

All plastic photochromic lenses are activated by UVA and UVB rays. Glass lenses are also activated from UV, but this is in addition to the changes that occur from visible light. How dark the lens actually gets however, depends on many factors:

  • The intensity of the light � the brighter the light, the darker it gets

  • Temperature � the hotter the temperature, the less effective the darkening molecules

  • Age

  • The type of tempering process that glass lenses go through

  • UVA spectrum changes that occur naturally throughout the day

  • UVA changes that occur from location to location � example, mountains have less UVA than the beach so lenses will be lighter in the mountains

  • UV blocks � items such as windshields and visors have UV screens incorporated in them, as a result, the lens will not get as dark

Once a dark lens is removed from a UV source, the bleaching process begins. Bleaching can take anywhere from a few seconds to a few minutes, depending on the lens substrate, photochromic molecules, temperature and light sources. On average however, it takes a lens two to three times longer for a lens to become fully clear than it does to color.

To coat or not to coat? That is the question.

In the past, Anti Reflective coating was not recommended for use with photochromics. The older AR coats contained UV blocker which inhibited the darkening process. This is not the case today. It is not only safe to place AR coatings on a photochromic; it is beneficial to the wearer because it minimizes the mirror effect of a dark lens. This is not the case with other coatings. When it comes to most coatings, less is more when it comes to photochromics.

Photochromics provide the best in UV protection and visual comfort to the patient. With the wide range of materials and styles available, the visual needs of most patients can be fulfilled with photochromics.

Photo: Courtesy of Transitions Optical, Inc.

Carrie Wilson
BS, LDO, ABOAC, NCLEC

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