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