What is the geometry of prism eyeglass prescriptions? And why are they so expensive?

Factual Questions
1

I do not have a prism prescription, but I’m trying to understand my own prescription for astigmatism and ran across this term.

The sites I’ve been able to find talk about what type of vision defects are corrected by them but not what that really means. How can a prism lens correct double vision?

Also I’ve heard that prism correction can drive up the cost of lenses dramatically. I listened to a podcast about Luxottica and the host mentioned that he had a “complicated” prescription that costs about $1000. I don’t think he specifically mentioned prism correction but I assume that’s what he meant. I do not know how lenses are manufactured today but I would think you just punch the prescription into a computer and the manufacture is automated. I’m not sure why any one prescription would cost more than any other prescription.

2

This begins to explain.

… https://www.aao.org/eye-health/glasses-contacts/what-is-prism-correction-in-eyeglasses…

I have a traumatic brain injury from a car accident 6+ years ago. Following that I developed post-concussion vision syndrome, including double vision. A neuro-optometrist prescribed first temporary ‘fresnel’ prisms for my eyeglasses, when those improved my vision all my eyeglasses since then have had prisms. My depth perception is far better with prisms, which makes any number of thing safer and more successful, like driving, parking, walking, reaching for a cup of coffee and keyboarding. Far fewer headaches and less cognitive fatigue too.

Once I figured out to order my progressives with prisms glasses from Costco they became much less painfully expensive. Worth the price in any case.

The geometry of prism lenses? I have no idea, HS geometry was 55+ years ago~I’m attributing it to magic and wizardry. Bending light rays is enough for me.

3

That is essentially the answer. Due to a slight muscle weakness in my right eye, I experience double vision, which has gotten worse as I’ve grown older, and especially when my eyes are tired. Without my glasses I see one image above the other. The prism in that lens bends whatever I’m seeing so that it lines up with that eye. I’m told it can be surgically corrected, but doesn’t last, so why bother. Besides, the idea of someone poking around in my eyes gives me the willies.

4

Is the double vision in one eye, or is that the images from the two eyes do not converge?

5

I assume the latter since XOldiesJock mentioned muscle weakness. I, on the other hand, have monocular diplopia (multiple images in one eye), caused by irregularities on the surface of the cornea. That’s not correctable by lenses. It can be improved by scleral contact lenses, which cover the cornea, but I find them uncomfortable and have stopped wearing them.

6

The cost isn’t based on ease of manufacture but on a) what the market bears b) the initial cost of developing the technology and c) how big a market there is. I have an eye condition that affects about 165,000 Americans, and most of those can get good vision with a contact lens I can’t wear. (Glasses don’t help.) I wear specialty, custom-made contact lenses that only a few thousand people need. A small company in Texas makes them. They’re almost $500 for a pair lasting 3 months. I gladly pay it because I spent 7 years legally blind and can now see 20/25. I’m what the market will bear.

I suspect the same is true for people like BippityBoppityBoo who need prism eyeglasses.

I hope this isn’t too far off-topic.

7

By any chance, would that be in Coppell, Texas??

I’ve worn prisms since about the second trimester in utero. I have eye alignment problems despite three eye muscle surgeries to correct them.

Yep: the prisms bend the incoming light. They’re prescribed based on whether (and by how much) your eyes deviate in the vertical plane or in the horizontal plane (or a combination of both):

Another good explanation:

As @BippityBoppityBoo noted (sadly), trauma can cause problems with fusion – the ability to properly ‘fuse’ the images coming from the two eyes.

But four other things can also weaken fusion: stress, intoxication, illness, and fatigue.

For a slightly deeper dive for any with interest…

Back to the Basics, Part 1: Prime Yourself to Prescribe prism

There’s also something called the near triad (9pp PDF).

As we look at something near (close-up), we not only change our focus (accommodation), but we also pull our eyes closer together (converge).

There’s a normal relationship between:

  • Accommodation and convergence, and
  • Convergence and accommodation

For some people, this is the only place where their vision gets banged up (ie, the ratios above are abnormal), so sometimes reading glasses with prisms are the answer.

For others, their eyes are simply out of alignment and it matters little whether it’s near or far vision; they need prisms (or, sometimes, surgery).

For others still (raises hand), their eye misalignment is different in every direction in which they look, called incomitant strabismus.

The prism trick for those lucky souls is to prescribe the prisms that result in them being “ortho in primary gaze,” meaning: get their eyes to be as straight as possible when they’re looking out toward infinity and straight ahead.

The more ingredients you put into eyeglass lenses, the more margin of error that the outcome isn’t exactly as desired. Prisms can also add a LOT of thickness (where that shows up depends on base in/out/up/down) and weight.

New technology (ie, high index of refraction) lenses have vastly improved this issue, but it still exists. Depending on the overall prescription, it can also dramatically limit the patients choices in frames that will work with the lenses.

8

Indeed it is~I need to be able to read to have the will to live. Being able to drive is high up there and progressives are worth the cost for both needs. I need a new refraction and prescription at a minimum every year. I also get two pair each time~my driving and being out of the house walking and shopping, etc. progressives and another pair (also with prisms) that are for indoors and extended reading/computer work where the correction area is pushed out to more of the lens so I can read for more hours before my eyes say “nope, we’re tired for the day. Try again tomorrow. Not even the fanciest prisms are gonna do it, we eyes and weary bonked brain are done”.

Since the brain injury, even after vision therapy and snazzy whiz-bang lens returned my ability to read, drive, park, walk without falling and reach for a coffee cup or glasses without knocking it over, stress (like you have 24/7 trying to live with a brain injury) and fatigue (like you have when your brain has to work ten times as hard to process absolutely everything after a brain injury) can really sabotage the quality of your vision as the day goes on. I can tell what time of day it is by how well I can still see to read which size print. By 4 or 5pm, I can’t read things like prescription labels or the ingredients on food labels. I just have to shrug and pivot to a different activity like reading on my iPad were I can blow up the print size to inch tall letters if I need to.

Thank you for those links~I found them very interesting and I learned much from them.

9

All I know is that they’re manufactured or near Houston, so I don’t think it’s Coppell. I’ll have to ask the optometrist next time I go in.

10

I used to have prism lenses and they cost only a bit more. I had double vision and they corrected it (more or less). Then I had a botched cataract operation leaving me with little central vision in one eye. I still have the double vision, but my brain ignores it. So no depth perception, but I live with that. It doesn’t seem to interfere with my driving. Or, really, almost anything else.

11

As someone else who lost macular vision in one eye (from a retinal detachment after cataract surgery) about eight years ago, my depth perception has gotten better over the years as my brain learns to extract depth information from different clues. I can now hit a ping pong ball where I couldn’t the first couple of years post detachment. I still have trouble with close up operations - it takes me many tries to thread a needle.

12

In spite of often wondering about this, I had never heard the terms “monocular diplopia” or “scleral contact lenses” before today.

I don’t have diplopia (double vision). My day vision is less blurry with concave/ cylindrical prescription lenses. And when I look at a point light source at night, without glasses, I see a constellation.

Do all short-sighted astigmatic people see point-sources as constellations? if so why does nobody ever mention it? The whole WWW and nobody thinks it’s worth mentioning?

If not, would scleral contact lenses work better than glasses?

13

That sounds like what I have. I don’t think it’s a general characteristic of astigmatism, but of corneal irregularity. IANAO, so there may be other things that can cause it too, but my ophthalmologists have said in my case it’s because of my crappy cornea. Glasses do nothing to correct it. You would need contacts to cover the cornea and provide a smooth first surface for light entering your eye. The difference in index of refraction between the contact and your cornea is much less than the difference between the air and your cornea, so having a contact in front of the cornea should make an improvement. With glasses, whatever correction the glasses make, the light then has to pass through the air between the glasses and your cornea, so the corneal irregularity disrupts the light no matter how good the correction of the glasses is. I hope that makes sense; let me know if you want more info. I’ve been dealing with this for decades.

14

You can put the center of one lens higher or lower. Normally, that is a bad thing. Standard lenses are designed to minimize the effect of looking through the edges, and you don’t want the two eyes to see different size things.

It makes the lens thicker at one edge than at the other and is prescribed as some kind of ‘prism’ adjustment. I don’t know if it’s the same as a normal prism: it may be a “complicated” lens.

I think the problem it’s designed to address can also be adjusted by optical surgery.

15

I’d just like to point out that, if you want to apply a uniform amount of prism to an eyeglass, 3M sells flexible vinyl prismatic sheets that will stick to your glass or polycarbonate eyeglasses with just a little moisture. And they’re removable and replaceable. You can cut then to fit your glasses with razor blade, X-acto Knife, or a pair of scissors. And they’re available in a variety of “powers”:

I bought them to use for an optics project I was working on, not for my eyeglasses, and they were a quick-and-dirty way to add prismatic offset to a system without buying and mounting another element. The prisms are “fresnel” prisms, so they don’t actually add much thickness to your glasses.