MATERIALS
Thinner for strong prescriptions
The College of Optometrists says high-index lenses are thinner than conventional ones, which matters most with a strong prescription. Aspheric designs are thinner and lighter too.
Match a feature
THE TECH IN A PAIR OF GLASSES
What your lenses
are really doing.
A spectacle lens is more than a curved piece of plastic.
Most lenses today are lightweight plastic, often with thin coatings on top: one to cut reflections, one to resist scratches, sometimes a chemistry that darkens in sunlight. Here is what each layer does, and what it does not.
LAYERS you cannot see
Match a feature- Fewer reflections
- Thinner for strong prescriptions
- Tougher surfaces
- Small layers, clearer view
THE SHORT ANSWER
Three jobs,
three kinds of tech.
Lens material decides thickness and toughness.
Coatings deal with reflections and scratches.
COATINGS
Fewer distracting reflections
An uncoated glass surface reflects about 4% of light. Anti-reflection coatings cut this, which the College says helps most with computer work and night driving.
See the numbers
PHOTOCHROMICS
Lenses that darken outdoors
Photochromic lenses darken in ultraviolet light and clear again indoors. Behind a car windscreen they may not darken as much, because the glass absorbs most near-UV.
Living with themGeneral information only, not medical advice. For advice about your own health, speak to a doctor, pharmacist or other qualified health professional.
FOUR THINGS TO KNOW
Look after
the layers.
Coatings are thin, and every lens type has a trade-off. These points come straight from optometry and lens-testing guidance.
Treat coatings as resistant, not proof
A hard coat makes plastic lenses harder to scratch and longer-lasting, but the College is clear they are not scratch proof.
Expect photochromics to age
Some older photochromic materials degrade over time. If your lenses no longer go as clear as they used to, the College suggests a new pair.
Do not count on them in the car
Windscreens absorb most of the near-UV that darkens photochromic lenses, so they may stay lighter while you drive.
Keep up eye examinations
Ready-made readers are fine as spares, but the College recommends an eye examination at least every two years after 40, whatever glasses you wear.
TWO NUMBERS WORTH KNOWING
Light bounces,
lenses get tested.
One figure on reflections from a bare surface,
one on how impact resistance is checked in the US.
≈ 4% per surface
Light reflected at each uncoated surface
Edmund Optics explains that about 4% of light is reflected at each air-glass interface of an uncoated substrate, leaving only about 92% of the light transmitted through both sides.
REFLECTION · OPTICS REFERENCE
50 inches
Drop height for the steel-ball test
US regulation 21 CFR 801.410 says eyeglass and sunglass lenses must withstand a 5⁄8-inch steel ball, about 0.56 ounce, dropped from 50 inches onto the lens without fracturing.
IMPACT TEST · US FDA RULE
The FDA's guidance adds that plastic lenses are not necessarily impact resistant just because they are made of plastic, and that re-edging or resurfacing a treated glass lens can significantly reduce its impact resistance.
A QUICK FEATURE MATCH
What do you
use them for?
Pick a situation to see which lens feature the College of Optometrists links to it. The bar shows how directly the guidance ties that feature to the situation.
Based on the College of Optometrists' public guide to glasses. General information only: your optometrist can advise on your own prescription.
WHY COATINGS WORK
Cancelling light
with light.
An anti-reflection coating is a film roughly a quarter of a wavelength of light thick. Light bounces off both the top and the bottom of the film, and the two reflections are timed to cancel each other out.
A single layer works best at one colour of light, so modern coatings stack several layers to cover more of the visible range. Edmund Optics describes a standard visible-light coating that reduces average reflection to 0.4%.
Questions
Questions people ask
Are anti-reflection coatings worth having?
The College of Optometrists says they eliminate distracting reflections from lens surfaces and are particularly helpful for computer users and night driving. They also make the lenses look less noticeable and can help disguise thickness.
What does high-index mean?
It describes a lens material that bends light more strongly, so the lens can be thinner for the same prescription. The College says this matters most for people with a strong prescription.
Why do my photochromic lenses stay light in the car?
They darken mainly in ultraviolet light, and car windscreens and windows absorb most of the near-UV that triggers them. They may not get as dark in the car as outdoors.
Are plastic lenses automatically impact resistant?
No. The US FDA's guidance says plastic lenses are not necessarily impact resistant just because they are made of plastic. In the US, lenses must pass a drop-ball test set out in 21 CFR 801.410.
Can a scratch-resistant coating stop all scratches?
No. The College of Optometrists says scratch-resistant coatings protect against damage and prolong lens life, but are not scratch proof.
Sources
- College of Optometrists, Look After Your Eyes: Glasses
- eCFR: 21 CFR 801.410, Use of impact-resistant lenses in eyeglasses and sunglasses
- US FDA: Impact-Resistant Lenses, Questions and Answers (guidance)
- Edmund Optics: Anti-Reflection (AR) Coatings
Updated