Shades of
Color:
The Way
Photochromics Work
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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.
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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:
Drawbacks to glass photochromic lenses are:
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Weight
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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.
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Uneven color patterns in higher prescriptions. The
thicker portion of the lens is a darker color than the
thinner portion of the lens.
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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:
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The intensity of the light – the brighter the light,
the darker it gets
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Temperature – the hotter the temperature, the less
effective the darkening molecules
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Age
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The type of tempering process that glass lenses go
through
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UVA spectrum changes that occur naturally throughout
the day
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UVA changes that occur from location to location –
example, mountains have less UVA than the beach so lenses
will be lighter in the mountains
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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.
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