There are several ways that we can classify prism. There is wanted or prescribed prism, and there is unwanted prism. In either case, prism has effects that can be seen/experienced with vision in one eye, or monocular effects, and there are effects that require the use of two eyes, or binocular effects.

When we discuss the visual effects of prism you will need to keep in mind whether we are discussing an effect that requires only monocular vision or that requires binocular vision. You will also sometimes need to remember whether the prismatic effects are intentional or in error.

MONOCULAR EFFECTS OF PRISM

This is the effect that is wanted when a refractionist prescribes prism in glasses. Usually prism is prescribed because the wearer has binocular vision and there is a need to help the eyes work together. We will discuss this later. There is a side effect that occurs when prism and lens power are in the same lens. This side effect can be experienced with either monocular vision or binocular vision.

What happens when you look through a plus lens at a straight line that is not going through the optical center? At the point (#1) where the line is closest to the optical center there will be a little prism, and the line will be displaced a little bit. At a point a little further away from the optical center (#2 and #3) there will be a little more prism, and the line will be displaced a little more than it was for point #1. Near the edge of the lens (#4 and #5) the point is further still from the optical center of the lens, and the line will be displaced more than it was at #2 and #3. See what the effect of power (which is created by progressively increasing amounts of prism) does? It distorts the image of the straight line. This effect does not occur when there is prism alone without power, as in the lens that you are using.  It is a side effect of lens power.

Imagine that a person was wearing that +5.00 lens over the right eye, but when looking straight ahead the person was looking through a point to the right of the optical center. What would straight lines look like to the wearer? Right. They would be curved.

We (some of us) routinely place the optical center of glasses below the wearer's pupil. This means that people with plus prescriptions are looking through base _____ prism. People with minus prescriptions are looking through base ____ prism. If both lenses have about the same power this does not create a problem. Does it?

It does not create a prism imbalance problem if the powers and therefore the prism amounts are about the same. But it sure does create distortion for the high prescription wearer. Look at what happens if you look through the top of a minus lens.

(PS - the missing words above are 'down' and 'up' respectively. Did you get them right?)

The minus lens creates base up prism which moves the image of each point on the ceiling down -- but the points that are the furthest from the optical center move more than the points that are closest to the optical center, creating curved lines. Same for the floor -- points move down away from the wearer. The illusion is that the floor slopes down away, as if the wearer were standing on a pitcher's mound.

For the plus wearer who is looking through base down prism the lens creates a distortion that feels like standing inside a bowl -- the floor slopes up.

Whether the wearer is using one eye or two, excessive base up prism creates the feeling of standing on a mound. Excessive base down prism creates the feeling of standing in a bowl. Excessive base in or base out prism creates the impression that the walls or the doorway curve. Those of us who have been wearing glasses for years have gotten used to this distortion and do not notice it. A new wearer of a strong Rx will notice it! A contact lens wearer who is forced to wear glasses for the first time in years will notice it! A monocular wearer who's glasses were made badly because the dispenser figured that the monocular pd is not necessary for a person with vision in only one eye will notice it -- unless some one started doing this to him/her years ago . . . "Wear it for two weeks and get used to it . . ." is a valid statement ONLY if you have determined that you cannot do better for the wearer.

The human brain is a remarkably adaptable organ. It can get used to almost anything. That does not mean that it SHOULD get used to our mistakes.

OK, I will get back down from my soap box.

Read Optical Formulas Tutorial page (86- 87 / 107-108).

BINOCULAR USES FOR PRISM

When the two images fall on corresponding portions of the retina the brain can fuse the two images into one image and the person has stereoscopic vision. If they do not work correctly together the patient will either have diplopia (double vision) or will suppress the vision in one eye. Neither diplopia or suppression is good if there is a way of allowing the images to fuse.

If the turning is only a tendency, instead of always being turned, we say it is a -phoria of whichever type. Typically, the person with a -phoria has enough fusion ability to overcome the muscle imbalance most of the time, and has trouble only when tired or possibly taking medication. So a -phoria is a turning that can be overcome with fusion. If the eye is permanently turned, the person has a -tropia of whichever type. In this case, there may be some type of paralysis which results in the person never being able to fuse the two images (without prism).

The person who has a muscle imbalance is said to be strabismic.

If the eye which is strabismic has usable vision the Doctor has two options to present to the patient: attempt to correct the problem, or give prescribed prism to allow fixation and binocular vision. Correcting the problem may take the form of eye drops, patching, eye exercises or surgery. (Surgery is usually the last option.) Adverse prism may be prescribed in certain circumstances (during eye exercises, for example) to aid in training/strengthening the weak muscle. In this case the apex is placed over the weak muscle in order to increase stimulation of it in the same way you might exercise with weights to help strengthen an arm or leg muscle. If the eye is essentially blind adverse prism might be placed over the eye to cause it to appear to be in a more acceptable position for cosmetic purposes.

Relieving prism may be prescribed as a permanent solution, if other methods of correction have not worked, in order to move the image to where the eye is looking, thus restoring fixation and binocular vision. In this case, the base of the prism is placed over the weak (or paralysed) muscle, sending the image in the direction of the gaze.

Disassociating prism is used during an eye examination to interrupt fusion and create double vision. The refractionist may do this in order to determine if the new Rx's give the same clarity in each eye. Typically the refractionist may disassociate the images so that one image is higher than the other and then say "which image is clearer, the upper one or the lower one?" If the two images are not equally clear the refractionist may change the Rx for one eye so that the two images are equally clear.

Measuring prism may be used during the eye examination to determine the strength of a muscle or group of muscles. It is used in progressive amounts to measure the ability to fuse.

Diplopia refers to double vision, for whatever reason. Diplopia may occur because of the inability of the fusion process to cause the eyes to track together. Diplopia may be the result of prism in the glasses (either intentionally in the Doctors office or unintentionally in poorly made glasses.) Occasionally, diplopia will be the result of an accident or head injury, hopefully resolving its self in time. Diplopia should always be brought to the Doctors attention.

CREATING WANTED OR UNWANTED PRISM

Suppose the Rx reads -5.00 sphere 2.5 BI OU. The refractionist wants the glasses made so that at the point in front of the pupil that the wearer will be looking through there will be 2.5 BI. How do you do this?

You can pick up the telephone and call in the Rx with the prescribed prism, and the lab will charge you extra for the prism. Or if you work in the lab you will punch the prism into the lab computer and it will include it in the surfacing calculations. But when all is done and you check out the glasses there will be an optical center that you can find in the focimeter -- it just won't be where the wearer is going to be looking. Where will the optical center be?

Look at the diagram above. These are minus lenses. On minus lenses the prism ____ is where the optical center is. So the optical centers will be OUT - toward the temples of the glasses. The optical center will be decentered out from where it would have been if there had been no prism prescribed. The 'glasses pd' or the distance between the OC's will be more than the wearer's pd. How much?

We have a power of -5.00, so D = 5.
We want a prism of 2.5, so P = 2.5.
We need to find out how far from the oc 2.5 is, so we need d.

(In the blank above you should have said "apex". Did you? Is the reason clear?)

In the next part of the lesson we will do more of these. I just want to make some points here. So just keep reading.

Suppose the Rx reads
    OD +8.00 DS 3 BD
    OS +8.00 DS 3 BU

The refractionist wants the glasses made so that at the point in front of each pupil that the wearer will be looking through there will be 3 BD in the right eye and 3 BU in the left eye. How do you do this?

These are plus lenses. On plus lenses the prism ____ is where the optical center is. So the OD optical center will be DOWN -- below the pupil. The OS optical center will be UP -- above the pupil. How much?

We have a power of +8.00, so D = 8.
We want a prism of 3, so P = 3.
We need to find out how far from the oc 3 is, so we need d.

The OD oc will be 4 mm below the wearer's pupil, and the OS oc will be 4 mm above the wearer's pupil. Right?

Right.

(In the blank above you should have said "base". Did you? Is the reason clear?)

This is how you get prism that has been prescribed, or wanted prism.

Unwanted prism is produced the same way: by not being careful of where the oc is placed in the glasses, or by placing the oc in the correct position but having the wearer look through some other area of the glasses.

So if the Rx is -5.00 DS OU and I make the glasses with a pd that is 10 mm to big, I will have given the wearer 2.5 prism in each eye that was not intended. Or if I make the +8.00 DS OU Rx with the OC's 8 mm apart vertically, I will be giving the wearer vertical prism that the refractionist did not intend. The first of these examples unfortunately happens every day. The second example is an exaggeration, but in smaller amounts it also happens every day.

Something else that happens every day: People who wear bifocals purposely look below the optical center of the glasses every time they read. We will deal with that one in another course.

CANCELLING AND COMPOUNDING PRISM

What happens when prism base down is placed in front of the right eye? The image is displaced toward the apex, which is up, so the right eye rotates up to see the image.

What happens when prism base up is placed in front of the left eye? The image is displaced toward the apex, which is down, so the left eye rotates down to see the image.

What happens when both of these things happen at the same time? The eyes rotate in opposite directions. They disassociate. The eyes are forced to diverge. If it is unintentional there is diplopia or suppression of one eye. There is vertical imbalance. The amount of vertical imbalance is the sum of the two amounts of prism. If the right eye had 2 diopters BD and the left eye had 2 diopters BU there would be a total of 4 diopters of vertical imbalance. There would be 4 diopters of disassociation or 4 diopters of divergence or 4 diopters of vertical imbalance..

Prism in each of the two lenses that causes each eye to rotate in the same direction as the other eye is cancelling prism. If the amounts are the same then the prisms completely cancel each other. If the amounts are different then the weaker prism partially cancels the stronger prism.. Prism in each lens that causes each eye to rotate in opposite directions is compounding prism. The amounts have to be added together to determine the total amount of imbalance caused by the combination of prisms.

For cancelling prism, if the amounts are unequal, take the difference and place it at the eye and in the direction of the greater amount. For compounding prism, add the amounts and show for either eye. Cancelling prisms will still give the monocular distortion discussed earlier even if they cancel completely. If there is enough residual prism the cancelling prism can also result in diplopia, as will compounding prism.

Please read pages (81-87 / 99-104) in the Optical Formulas Tutorial. This should all be starting to make some kind of sense to you. The parts that we did not do in this lecture should also make sense. At this point just read and follow the examples; do not do the exercises yet. We will do similar problems in the second part of this lecture.

When you are finished with this click here to discuss ANSI Standards for prism, and prism in glasses.