Explore eyecare and the world around it….

Prism Correction

Prism can be a little complicated and hard to understand. I will assure you that when you are done reading this article, you will get quite the understanding regarding prism correction.

First off…What is prism?

Prism can be used to correct vision for an individual whose eyes are not perfectly aligned as with, for example, a patient with strabismus. When the eyes are not aligned, the right and left eye see different images resulting in blurred or double vision. Sometimes the brain can even “shut off” one eye, in an attempt to remedy the vision, resulting in monocular vision and loss of depth perception. Prism can often be used to align the images seen by both eyes, so the eyes can fuse or see the same image, restoring visual clarity and depth perception.

Prism, like lens power, is also measured in diopters (Δ), but measured differently. One diopter of prism is equal to the prism required to divert a ray of light 1 cm from its original path, measured at a distance of 1 m from the prism.

As important as the amount of prism, is the direction of prism. The prism must displace viewed objects in the proper direction to achieve the desired visual correction. Prism direction can be specified in two ways, either using the prescriber’s method or the 360 method.

The prescriber’s method specifies the direction if the prism in terms of the base, using base-up, base-down, base-in, and base-out (base-in referring to the direction of the nose and base-out referring to the direction of the temple). Often prescriptions will include a combination of directions to achieve the proper resultant prism. For example: 2 Δ base-in and 1 Δ base-up.

Labs however, use a 360° or 180° method of describing base direction. Using the 360° method, when a lens is viewed from the front, a prism with a base direction to the right (base-in for the right eye and base-out for the left) becomes 0°. Likewise, a prism with a base direction to the left (base-out for the right eye and base-in for the left) becomes 180°. Base-up then becomes 90° and base-down 270°. Using this method, prism directions other than base-in, base-out, base-up, and base-down can be specified at a single angle e.g. 2.7 Δ base 64°. The 180° method is similar, however, as the name suggests, only 180° are used, consequently, an up or down direction must also be specicfied.

Prism specified in using the presciber’s method consisting of multiple base direction components can easily be converted to the 360° or 180° methods by using a prism chart or simple trigonometric formulae.

Source: Laramy-K

∆ is the symbol for Prism





Prism by Decentration

To better understand why there is induced prism, the cross-section of a plus lens can be likened to two prisms base-to-base, as the lens is thicker in the middle and thinner at the edges. Likewise, a minus lens can be likened to two prisms apex-to-apex, thinner in the middle and thicker at the edges.

This induced prism can actually be used to the advantage of the lab when prism is called for in a prescription. If the lens power is sufficient, to induce the prescribed prism, the lens can simply be cut off-center to achieve the required results. This is known as prism by decentration. If the power is insufficient, however, the prism must be cut into the surface of the lens.

A simple equation can be used to calculate the prism induced by decentration. Prentice�s rule states that prism in diopters (Δ) is equal to the decentration distance (c) in centimeters multiplied by the lens power (D).

Prentice�s Rule

Δ = cD

Source: Laramy-K


Comments on: "Prism Correction" (25)

  1. I have been working in the eye care industry and I have a horrible time trying to read prism thru a lensometer. Can you explain this or is there a site that would explain it. Also how do I know if it is going up and down or in and out and how many diapoters.

  2. I have a question regarding prism. My last prescription had .75 BU on one eye, and .75 BD on the other. I noticed after my latest exam last week that the optomotrist wrote the prescription for 2.5 BU in one eye only. Won’t these glasses make my vision extremely wonky when they arrive in two weeks?

  3. @Jean
    I am getting ready for work no so I really cannot get into the explanation of how to read prism because it can get lengthy. When I get home, I will explain it for you! Thanks for visiting!!

    It seems like your prism correction has changed. It can do that. Just means that one eye needs prism correction. Since the prism correction went up in that one eye to 2.5 Diopters and totally diminished in the other eye, your eyes will have to adjust to that new RX. So, you might get a little headache and feel a little off, but that is normal. You brain is trying to adjust to something new. You will be fine after a few days of wearing them. Hope this helped! Thanks for visiting!!

    Best Regards,

  4. Thanks – I’ll let you know how it works out when my glasses arrive.

  5. @ Greg

    You are welcome. Please do let me know how your new glassess turn out.


  6. No problems with the new glasses. Some minor adjustment time was needed, but that happens with new glasses every time……….


  7. hi, i’m a new guy who working in a lworkshop,one of the customer provide the info below :
    R:-6.50/-1.00*9 prism:2 BU
    L:-5.50/-1.25*150 prism:2BD
    for reading use only.

    my question is :
    how i going to cut the lens ? normally i cut the lens by just follow the PD ,do i need to calculate the prism in before i cut?if yes,please teach me what to do with this order

  8. @Greg
    Glad the new glasses are working out for you!!!

  9. @Justin
    Normally you have to calculate the prism in any job before you start grinding them. What kind of equipment do you use? After I know what kind of generator and finishing system you use, I can more into how you should calculate the prism.
    Thanks for the question and glad to have you here!!!


  10. i’m using the DIA e6.10s automatic lens edger,so what calculation should i apply in this case?

  11. I am not familiar with that edger so I am not sure how to tell you to calculate the prism. Maybe try http://www.optiboard.com….it is an optical message board…I am sure someone there can help you. Sorry and I wish you luck!

  12. Joan Beaton said:

    Sphere cylinder Axis prism add

    OD +250 sph BO 3 +250

    OS +200 sph BO 3 +250

    I got this prescription filled mid-June. Now i am having trouble especially driving…seems that my eyes are pulling. Better if I cover the left eye. The lights of night driving seem VERY bright and confusing.

  13. Hi Joan!
    Thanks for stopping by!
    If you are having trouble, go back to your doctor. It might be that your prism correction has changed. Also, I would recommend getting Anti-Reflective coating on your glasses to reduce the glare for night driving. I have the same thing happen to me when I drive at night. I always get Crizal Alize on my glasses. Sharpview is also another great option and is a little cheaper than Crizal.

    Best Regards,

  14. widad is'haq said:

    if i have patient with vertical and horizental prism (base-in& base-down). how i will calculate the amount of prismatic power?

    best regards,

  15. I just recently got glasses for the first time (I’m in my mid-thirties). They have no correction but have some prisms. I’ve had the “not fused” vision for as long as I can remember and anytime I mentioned it before the eye doc basically blew me off because I would say that if I tried to focus I could get the double to go away. It seems now that I probably never could get the double to go away, but I could get the really bad double (happens if I let my gaze go) to go away.

    Anyway, I got 3.0 Out (both lenses) for my first pair. I wasn’t sure they were working very well. I went back and he set me up with 5.0 Out (both lenses) It seems like these might be worse. The lady at the eyeglasses place (my doc does not sell glasses, only an eye doc — he is an M.D.) said that with this much prism it might take a while to get used to. I was hoping to only have to wear them when doing paperwork (I can’t fill out the forms without closing one eye because I see too many boxes to put numbers and letters in).

    Is there a way to test them out on my own? Specifically is there a diagram or graphic or something that I can print out or look at on a computer screen that can help me tell if I am over-prism-ed now, and/or was under-prism-ed before. It seems wasteful to go get a 4.0 out pair just to test it out. (This re-lens was free as a “doctor error” but you only get that once.)

    Thanks for any help. This seems to be a pretty hard thing to find good info on.

  16. HI Brian!! I apoloize for not responding sooner, but I do work a lot and only have one day off.
    I really do not think that there is any way that you can test out your prism correction on your own. What I would suggest is going back to your doctor and explain this to them or get a second opinion with another eye doctor. You do have a lot of prism so I would not just go by just one doctor. That is just my opinion. Sometimes it takes many visits to get prism correction right, especially with a high amount of prism. Keep me updated on what you find out. I really would like to know th outcome!
    Thanks Brian!!


  17. Hi, I’m at work and want to cut the lens with 0.5 BD using Essilor Kappa. How do I centre the optical centre of my lens? Do I move it 0.5 down or 0.5 up?

    I appreciate your help:)

  18. Louis Gardella said:

    I am a 53 year old male that has been having extreme difficulty over the past three months not being able to focus my eyes! What happens is that my eyes just start moving around especially while driving,so much so that I have to squint and pull the car off the road! I have been wearing glasses since I was 5 years old,my lenses use to have prism in them but it was removed in 2002.I need to find out how to resolve this as it is getting to the point where I cannot function anymore.

    Louis Gardella

  19. Hello, I recently had an eye exam and was told I will need prism glasses because I have a 3.5 diopter ‘lateral displacement’. I’ve worn glasses for about 15 years (I’m 37) and I’ve never had this problem before. I was wondering if later displacement is normal? The Optometrist mentioned that displacements greater than 5 diopter can be a sign of an underlying neurological problem but because mine was 3.5 I should be fine. So, I was wondering if lateral displacements are rare and if I should consider having additional testing such as a neurological exam.

    Many thanks

  20. My son was told by an optometrist that though he has 20/15 vision his eyes were not teaming and he had less peripheral vision than was ideal. He prescribed prism lens as a means of correcting this condition. My son wore them briefly but grew resistant since he didn’t notice any difference when he wore them and thought they slightly obscured his vision. This optometrist predicted that our son’s school work and athletic performance would be effected if he didn’t correct this problem. Recently we’ve seen a drop off in both and are now wondering if we should revisit the correction of his vision. Can anyone comment on the affectiveness of prism glasses? Could one’s vision really be a factor in a drop off in performance even if they have 20/15 vision? Thank you for sharing your thoughts and experience.

  21. My dad has gotten new glasses and they are giving him a dull headache and his vision is not as good as with the old pair. His old prescription has prism correction and the new ones don’t. His overall vision has not changed greatly. Could this cause his problem with the new glasses

  22. Hi there! I have just been reading all the posts here- it’s all really interesting.. i have just collected my new glasses (with prisms in). i have astigmatism and slight short sightedness and also need prisms. I kinda assumed the prism was in both lenses but this may not be the case at all.

    Anyway.. they told me i might get a funny ‘pulling sensation’ but i’m experiencing something much weirder… something i wouldn’t call just a ‘pulling sensation’.

    When i look in front of me, everything gradually starts to slope away so it feels like i am at the top of a ramp and everything drops down away from me. I couldn’t walk down the garden steps without each step looking like is was sloping diagonally downwards, away from me. Also, as I walked through a carpark the ground felt as if it was getting higher and higher until the floor appeared to be half a meter from my face, i felt like i had sunk into the ground up to my waist! Really odd. I am trying to keep them on in the hopes that the side effects will wear off. But i wanted to make sure this is normal and not something to worry about. Also.. once my eyes adjust will it be just as weird when i DON’T wear them? I wouldn’t like to be tied down to wearing them because every time i take them off the world would start to seem warped as my eyes re adjusted!

    Thanks so much for your help!


  23. I have monocular vision. I had to increase my prescription and my prism increased from 2 to 6 on each eye. I am right eye domininant, although I alternate between my two eyes to see. I got my new glasses and now the glasses make me use my left eye more and cause my right eye to turn inward. This taxes my muscles behind my right eye, which are good and the muscles hurt and the eye is irritated. Any suggestions? I have went back to my opthamologists and told them the prism is misplaced in the right lens, but they ‘don’t believe’ me?

  24. Hi

    A 4th nerve palsy in the left eye, as a result of Neuro Surgery in 2008 has caused double vision. The double vision had an element of tilt, which has recently been addressed via eye muscle (squint) surgery. I still experience double vision when looking straight down and down to the right. The expectation is that a prism in glasses will be able to line up the images.

    To my question: Can prism be fitted so as it only comes into play if looking straight down or down and to the right?

    Any info would be helpful

  25. All made easier with the best equipment and the best people..All good comments and great questions.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: