One of the common questions we see is what options are available for thinner lenses. I’m going to be up front that the answer is complex. A good optician is essential here and will help you understand how all these pieces fit together. In fact, this answer is based largely on a comment on the Little Four Eyes facebook group written by Wesley Scott, an SC licensed, American Board of Opticianry master-certified optician who was initially trained by the US Army to make glasses in an ophthalmic lens lab and who currently works for an optical lab software company. All that is to say that he knows a lot about ophthalmic lenses. Many thanks to Wesley for letting me adapt this to a post. That said, any inaccuracies in this post are entirely my fault as I try my best to simplify a complex question.
[photo of a pair of wire-rimmed glasses from the top showing the thickness of the lenses and a girl wearing the glasses]
There are a few things that can affect the thickness of lenses:
- the prescription
- the material of the lens
- the curve of the lens
- the size of the frames, and how they compare to the patient’s face. Remember, you want a frame where the patient’s eyes are well centered in the lens.
The higher the prescription number (whether it’s plus or minus), the thicker the lenses are going to be at some point.
Lenses that correct hyperopia (+ prescriptions) will have the thickest point in the middle of the lens.
Lenses that correct myopia (- prescription) will have the thickest point on the edges furthest from the middle
Shape of the glasses:
The larger the lenses are in glasses, the thicker they are going to be (this is especially true for glasses for myopia). Again, you should talk with an optician about specifics of which frames will work best, but in general, smaller oval-shaped glasses will mean thinner lenses.
Polycarbonate is customarily used for kids’ glasses because it has been around a long time. Trivex is a relatively newer material for lenses. Both are highly impact resistant and provide full UVA/UVB protection, which means both are good choices for children. Trivex lenses will be thicker than Polycarbonate for the same prescription. On the other hand, Trivex is a lighter material than Polycarbonate. Because Trivex lenses are thicker, there’s not much difference in weight between the two for the same prescription. Trivex is somewhat clearer than Polycarbonate.
There are also high-index materials. “Index” here refers to the “index of refraction” which describes how much light bends when passing through the material, the important thing to note is that the higher the index, the thinner the material. Hi-Index 1.67 lenses are not as impact resistant as Polycarbonate or Trivex, but they are more impact resistant than plastic lenses (CR-39) or glass lenses. In fact, before there was Polycarbonate or Trivex, glass and plastic lenses were used for children’s lenses. In very high plus prescription, 1.67 will still be impact resistant, and much thinner than Polycarbonate or Trivex, and so may be a more appropriate choice (again, talk with your optician).
The way the lenses are curved will also make a difference. Traditionally curved lenses will have a steep curve for higher prescriptions. There is a newer technology in lens curve though, called aspheric lenses. Aspheric lenses will provide full strength correction but with a flatter curve. Aspheric lenses are especially recommended for high hyperopic prescriptions.
Other lens options
There are other options that an optical shop may or may not recommend for your child. These are optional and will not impact the thickness of the lenses (but will impact the final cost):
Scratch-resistant coating: Some lens materials (Polycarbonate in particular) are soft and prone to being scratched easily. A scratch-resistant coating will help prevent some scratches. Please note the word “some”! Lenses with scratch-resistant coatings are not scratch proof and can still be scratched.
Anti-reflective coating: Also known as “anti-glare coating”. This coating will reduce the reflection of light from the outside of the lenses (that’s what causes glare on lenses).
Photochromic coating: Often referred to as “Transitions” which is a brand name of this type of coating. Photochromatic lenses will darken in the sunlight.