By Published On: 27 May 20163.9 min read

Prism lenses may be prescribed by your optometrist for your eye glasses for double vision (or diplopia) depending on the eye condition you may have. Are you wondering how prism glasses work?

Here is some important information about prism lenses you may find useful:

  • Prism lenses are made from thin pieces of the optical material used in prescription eye glasses.
  • Their apex is thinner and their base is thicker. With their light bending properties (or index of refraction), the light passing through is slowed down more through the base than through the apex. This results in the light changing direction down towards the base when it exits the prism. The amount of change will depend on the difference in the index of refraction of the lens material and the air. The amount of change also depends on the shape and thickness of the lens – the thicker the base, the more the light is bent.
  • Higher index materials allow the production of ultra-thin lenses that improve appearance and comfort of eye glasses.
  • Lenses with consistent thickness cannot change the direction of the light although they can still slow down the light.
  • Prism can be ground into lenses during manufacture or, a prism effect can also be achieved by moving the centre of the spectacle lens off the visual axis. This is easy to do with higher spectacle prescriptions but lower prescriptions usually require the prism to be ground into the lens.

When are prism glasses prescribed?

Prism lenses are often prescribed to eliminate eyestrain and double vision, also called diplopia. Here are some instances when your optometrist may prescribe them:

Crossed Eyes  / Strabismus

  • This condition may be referred to as a turned eye or a strabismus.
  • When you have this condition, your eyes can turn in (esotropic) or out (exotropic). They may also turn up or turn down. The eyes may even turn vertically and laterally at the same time.
  • This may result in double vision which can affect your lifestyle and mobility.
  • When you are born with this condition, you may not develop double vision because the brain can shut off the vision from one eye, so only one image is seen. When this happens, the lazy eye never gets to develop its vision fully and can be permanently affected.
  • If double vision presents later in life it can be debilitating and cause headaches and nausea.
  • Prism lenses in eye glasses can address this condition. The lenses will not appear as triangular, but will be a bit thicker on one edge and thinner on the other, like a thin wedge. They may be ground into the shape of the lens or repositioned in the optical centre to induce prism.
  • The amount of prism prescribed may be built up slowly as the brain adjusts to how the two eyes work together again.
  • Certain medical conditions like diabetes and stroke can result in double vision where the amount of double vision may fluctuate. Thus, the prescription of your glasses may need to be changed frequently to adjust the amount of prism.
  • Fresnel lenses or temporary press-on prisms may be used when change is expected, but optical quality tends to be poor.

Heterophoria

  • This condition refers to the tendency of the eye to turn although it mostly pulls itself back in alignment with the other eye in order to avoid double vision.
  • This results in headaches, fatigue while reading and eye strain.

Traumatic Head Injuries

  • If you suffer from these injuries, your optometrist may prescribe yoked prisms. These prisms have equal power for both eyes and shift the visual field of both eyes to the same extent.

Double Vision can be serious…

When you begin to exhibit this symptom, you should see your optometrist immediately to have your eyes checked. Eye glasses with prism lenses may be prescribed to address your problem.
In some instances, however, you may have a serious medical condition such as diabetes, a brain tumor, high-blood pressure or thyroid problems.  Developing cataracts can also cause a type of double vision, but is easily treatable. Your optometrist will be able to diagnose what’s causing your double vision and refer you to a specialist as necessary.
In the case of double vision of recent onset, your optometrist will usually perform a field test to map your peripheral vision and determine if your visual pathway is functioning as it should.

Prescribing prism to correct eye muscle imbalances needs to be done accurately. At The Eye Practice, complex eye-glass prescriptions are what we do best.  Call us on (02) 9290 1899 or BOOK an APPOINTMENT ONLINE.

This article was originally published in August 2012 and has been updated for accuracy and relevance. 

Comments

16 Comments

  1. Frank McCallum 19 September 2020 at 4:17 am - Reply

    Thanks for the information

    • Jim Kokkinakis 22 September 2020 at 8:39 pm - Reply

      Glad it helped Frank. If you have any questions do not hesitate to ask.

  2. Stephen Price 25 November 2020 at 4:05 am - Reply

    Hi
    I was prescribed prism lenses after my distance glasses went out of focus after a time wearing them.
    They were great for a few months, but now are again going out of focus after an hour or so.
    Any reccommendations?
    Steve Price

    • Jim Kokkinakis 26 November 2020 at 11:21 am - Reply

      Difficult to comment without knowing why and what magnitude of prism was prescribed. I would suggest that you revisit your practitioner to be reassessed and go from there.

  3. Jane 3 December 2020 at 7:43 pm - Reply

    Hi , I used to wear varifocals but have just been prescribed as needing prisms in near sight to correct muscle weakness and slight double vision. I also need to correct my distance sight , that doesn’t require prisms. Do I get distance vision glasses or varifocals? My optician is being very non commit all and says it’s trial and error. I plan on wearing my prism glasses for looking at computer/phone reading. Would greatly appreciate any advise. Many thanks Jane

    • Jim Kokkinakis 8 December 2020 at 11:00 am - Reply

      Jane
      Assuming your currently varifocals are still adequate for distance vision, you will need a separate set of computer / reading varifocals. These will allow you to see up to a distance of about 1 metre (just beyond normal computer distance) and as close as your phone. These glasses will have the prism you need for better reading. When you are out and about and your reading demand is low, it is then best to swap glasses to the normal varifocals.

      If your current varifocals are not good enough for distance vision and very basic short term reading tasks eg reading a menu in a cafe, then they too might need updating.

      Hope that helps.

  4. Matt Stevenson 8 December 2020 at 3:02 am - Reply

    I recently got prescribed prism glasses to correct double vision because I had serious eye pain, especially when looking at a computer screen for a longer period of time. My doctor wasn’t sure if I should wear the prism glasses all of the time though? I can see far objects just fine with the prism glasses but generally only need them when looking at close objects. Thanks for the advice!

    • Jim Kokkinakis 8 December 2020 at 11:16 am - Reply

      Matt
      Glad to help. Personally I would suggest wearing the glasses only for reading if your distance vision is clear and comfortable. The other thing to consider is what type of prism your glasses have. If it is base in prism, often supervised convergence exercises can make you more independent of these glasses. If you issue is vertical or base out horizontal prism, this is more difficult to treat with eye exercises. Each case is somewhat unique.

  5. Kaden Sumsion 10 December 2020 at 3:00 pm - Reply

    Hi Jim, your article was very helpful to read!

    I am wondering what your thoughts on combining a sphere +1.0 SPH prescription along with a horizontal prism base in? My doctor just prescribed this and I was a bit surprised because I can see perfectly fine without glasses, I just have eye strain from preventing close objects from doubling. Im curious if that’s common to prescribe those together?

    • Jim Kokkinakis 11 February 2021 at 2:39 pm - Reply

      Kaden depending on your eye muscle imbalance this prescription could be appropriate but it depends on results from a complete and comprehensive eye examination as everyone is unique.

  6. Rossco 21 January 2021 at 11:19 am - Reply

    I have permanent double vision post brain stem stroke. I have tried prisms and also 1 lens long distance 1 short vision. Any ideas what else can be done as still no joy.

    • Jim Kokkinakis 11 February 2021 at 12:00 pm - Reply

      Rossco every case is unique. Where are you from? Best to see an optometrist that specialises in post trauma eye muscle disorders.

  7. Rosie 19 February 2021 at 3:46 pm - Reply

    I was diagnosed with hyperphoria and prescribed a prism. It helped tremendously, but now after two years they say I don’t need it. I am afraid to be without it.

    • Jim Kokkinakis 9 March 2021 at 12:13 pm - Reply

      Rosie
      I assume you fear that your vision will become worse if you do not use the prism. The reality is, all that will happen is that you will experience eye strain or maybe even double vision. All you will need to do is keep the old glasses aside to go back into if these problem manifest themselves. You will not suffer any further degradation of your vision.

  8. Shannon 19 March 2021 at 2:24 pm - Reply

    Hi Jim. I am about a month in with my prism glasses. I am far sighted almost 20/20 so I don’t wear them continuously but I need the prism because my eyes are misaligned. My right eye goes in a little bit more. I wanted to know if not wearing my glasses will make my left eye go outward more. Because i think it might be, or it could just be me :( help

    • Jim Kokkinakis 25 March 2021 at 2:04 pm - Reply

      Even though I cannot be 100% sure without seeing you, it is possible that you are aware of the eyes misaligning more without the glasses.

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