EYEGLASSES WITH PRISMS
Prisms are a pyramid shape lens with a base at the bottom and an apex (tip) at the top.
We can take advantage of their shape because of the way light travels through the prism.
When light hits the prism it gets bent (refracted) towards the base of the prism and so when you look through it, all the space and objects appear shifted over to the opposite direction - the apex.
If you orient the prism so that the apex is at the top of the lens, then you look through it everything looks shifted upward.
If you orient the prism so the apex is at the bottom (6 o'clock position), then things will look shifted downward.
If the apex is oriented to the right, then things shift to the right.
And so on...
WHO NEEDS PRISMS IN THEIR EYEGLASSES?
The main reason why people get double vision (diplopia) is because their eyes are misaligned in some fashion. One eye might tilt up higher than the other (Hypertropia), or cross/wander outward (Strabismus). Let's take the case of hypertropia where one eye tilts up higher. When this occurs, the brain will compute objects and space coming from the higher eye as lower (yes, it's backwards...).
So take what we already know with prisms. We know they shift everything over in the direction of the apex. So all we have to do, in the case of hypertropia is grind a prism into the glasses lens to compensate for how much everything is displaced when the eyes are misaligned.
Hemianopsia or Spatial Neglect From Stroke:
Sometimes strokes to the occipital, temporal, and/or parietal lobe can cause visual loss to one side for each eye. If you have 'right field hemianopsia', then the right half of the visual field is gone in not only the right... but the left eye too. You just don't notice it as much with the left eye because of the overlap achieved by the left visual field of the right eye. If you have Spatial Neglect, you haven't lost vision on one side necessarily, you are just ignoring space over to one side. Usually this occurs from a stroke as well.
Whatever the condition, losing or ignoring space or objects to one side can be life altering because of how much you'd miss things in those areas.
For these type of problems, we use what is known as "yoked prisms". Yoked prisms are prisms that are oriented in the same direction on each lens of your glasses.
So if you've lost your right field on both eyes, what we want to do is shift things from the blind field (in this case the right side) over to the seeing field (left side). So all we have to do is to grind a prism in each lens with the apex oriented to the left. This will help you pick up information from the right side earlier when navigating, and easier with much less need for head movements.
Binocular Vision Disorders
Some people have difficulty keeping their eyes aligned and synced together as a pair. In these cases, the eyes don't look misaligned to the observer, but there's simply a tendency for drift. As a result, depth perception can be impaired and the person will often have symptoms of eye strain, fatigue with intense visual tasks, headaches and even motion sickness or balance problems.
Prisms can often be used for binocular vision disorders to improve visual comfort and to help with eye teaming. We do not use these as a replacement for vision therapy though. The prisms compensate for the problem, but vision therapy works to cure or treat the underlying eye teaming problem.
WHAT DO EYEGLASSES WITH PRISMS LOOK LIKE?
Eyelasses with prisms look like any other types of glasses, but sometimes you will notice the edge on one side is a little thicker then the edge on the opposite side.
You can get prisms in most frame styles, lens materials and lens styles including single vision, bifocals and progressive lenses.