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Module 31 |
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Module 31: |
Measuring Prism in Glasses |
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Section 1: |
Horizontal Prism | ||
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These Modules discuss how to measure prism correction with a manual lensometer. There are may good auto-lensometers that do a fine job of measuring prism corrections, but the same principles apply. If you do not align the glasses properly in an auto-lensometer, you will not arrive at the correct measurement. Therefore, those of you with auto-lensometers will benefit from this discussion. Introduction Don’t you love it on a busy day when you get ready to read that new patient’s glasses and he says to you; "By the way, those glasses have prism in them." Don’t despair. Here are some tips to help you through those trying times. Every time you try to evaluate a patient’s prism correction you could potentially be dealing with three different prism situations. I like to call them Intended Prism, Measured Prism, and Effective Prism. Intended Prism: This is the prism correction that is written on the patient’s glasses prescription. This is what the doctor intended to give after the EOM evaluation. Measured Prism: This is the amount of prism correction that is measured by the person evaluating the glasses, using a lensometer. Effective Prism: This is the amount of prism correction that is at work on the patient’s eyes, depending on the fit of the glasses. Hopefully these will all be the same. Unfortunately, many times they are not the same, adding to the confusion. No wonder prism corrections can be a nightmare. For these three types of prism to be equal, the glasses have to made to the prescription, the glasses evaluator has to measure the glasses properly, and the patient has to be looking through the correct positions in the lenses. Does this scenario sound familiar? A patient new to your office has a prism correction and wants new glasses. The patient is comfortable with the present prism correction (no double vision), but a record of the last prescription given is not available. The prism is measured and a new glasses Rx is given, presumably keeping the prism correction the same. The patient returns with the new glasses complaining of double vision. In this situation it would be nice to know what the intended prism was, but that is not usually the problem. The problems come most often from the Measured Prism and/or the Effective Prism. In this Module, we are going to find out how to properly measure horizontal prism, vertical prism, and then combination prism corrections. Effective prism If you have tried to read prism on a manual lensometer, or on an auto-lensometer as well, you know that the prism amount that is read depends upon the position of the lens against the lensometer port. In other words, the prism power depends upon what part of the lens is being read. Likewise, the prism power that has an affect on a patient’s eye depends upon what part of the lens the patient is looking through. In order to determine the effective prism power for a given patient, you will need to pay attention to how the patient is wearing his glasses. Sometimes a patient will have double vision with prism glasses because he is not wearing the glasses properly, or because the glasses were not fit well. Obviously, in such a case the patient should be instructed on the proper alignment, and/or the fit should be adjusted. This is the first thing that should be checked/evaluated/adjusted when a patient is complaining of problems with a pair of glasses having a prism correction. When preparing to measure the prism in a pair of glasses, the glasses should be aligned to a proper fit on the patient’s face. Once the glasses are properly aligned, use a felt tip marker (such as a Sharpie) to mark the position on each lens that lines up with the center of the patient’s pupil. This mark can later me removed with lens cleaner or alcohol.
It is not easy to be precise about these markings, particularly if there is a large vertex distance. Also measure the patient’s PD. The horizontal distance between your marks should not be different than the PD. If it is, adjust your marks accordingly.
Many adults with prism in their glasses will be wearing flat-top bifocals. The proper vertical alignment (high on the nose, or lower) of single vision lenses and progressives is not always obvious, but the proper alignment of a flat-top bifocal, or trifocal, is definite. With a flat-top multifocal, the patient should be viewing directly above the segment line, near the center of the line.
A shortcut
If the glasses have a flat top multifocal, it is possible to take a short-cut, because you already know that with proper alignment the patient should be looking directly above the line on the seg. In this case you can simply measure the patient’s PD and use your ruler to place the marks with equal spacing just above the seg line.
Remember, you are not necessarily making a mark at the optical center of the lens, you are making a mark at the point in the lens that the patient’s pupil lines up with. Reading the prism power at these marks will give you the Effective Prism power. This, of course, is also the Measured Prism power. If you measured the prism without the benefit of these markings, then your Measured Prism power may be different than the Effective Prism power, further complicating the situation. If the optical shop did their job correctly in making and fitting the glasses, and if you did your job correctly in measuring the prism, then the Intended Prism, the Measured Prism, and the Effective Prism will all be the same. When you are skilled at reading prism in glasses, then measuring the prism can be eliminated as a source of error. All that remains is to determine if the measured prism matches the intended prism (the prescription) and the effective prism (the prismatic effect of the glasses on the patient's eyes).
Measuring horizontal prism correction How would you know that there is prism power in a pair of glasses? The most obvious tip off is from the patient history. Is there a prism correction written on a previous Rx, or is there a reference to a prism correction in the notes? Is the patient complaining of double vision now, or has the patient complained on previous visits of double vision? You might make it a habit to ask new patients if they have ever experienced double vision. Aside from the history, the lensometer can tip you off to the presence of a prism correction. Prism is revealed by the lensometer when the mires cannot be centered in the lensometer target or when the lens has to be shifted away from the normal viewing area in order to center the mires in the target. This phenomenon will become evident as you learn how to measure prism corrections. As previously discussed, in order to accurately measure the horizontal prismatic effect of a given pair of glasses on a particular patient, the points on the lenses through which the patient sees must be marked. If this first step is ignored, your measurement will be accurate only by chance.
Procedure
1. The glasses are positioned on the lensometer so that the mark on the lens lines up with the lensometer port.
When you look into the lensometer eyepiece, you will notice a target or reticule similar to the one pictured below. It will have degree marks and a diopter scale that can be rotated using a wheel on the eyepiece head.
2. Focus the single line mire and the double line mire just as you would when measuring the lens power. Notice where the two mires intersect. If the lens is spherical, the mires will form a focused intersection.
If the lens is cylindrical, the single line mire and the double line mire will not be in focus at the same time.
The lensometer can be focused half way between the two mires of a cylindrical lens to form a blurry image of the intersection.
If there is no prismatic effect in the lens, this intersection will be in the center of the target of the lensometer. If there is a prismatic effect in the lens, this intersection will not be centered in the target of the lensometer. Below: This is the image of a spherical lens with no prism correction. The center of the intersection of the single and triple line mires is centered in the bulls-eye of the eyepiece target.
Below: This is the image of a cylindrical lens with a prism correction. The single and triple line mires do not focus at the same time and they have been focused at a point in-between. A blurry image of the intersection is displaced to the left of the bulls-eye of the target, indicating that a prism correction is present. The mires are seen at an angle because of the oblique axis of the cylinder.
3. The distance of the intersection from the center of the target is a measure of the strength of the prism. The farther the intersection is away from the center, the greater the prism power is. The diopter scale can be rotated to facilitate measuring the diopter value. In the example below, you can see that the intersection of the mires lines up with the target circle marked "2", indicating that there is a two diopter prismatic power.
4. The direction of the displacement of the intersection from the center of the target is an indication of the orientation of the base of the prism. In the example above, the prism base direction would be "base out", if we are measuring the right lens. If we are measuring the left lens, the prism would be "base in".
Above: base-in displacement Below: base-out displacement
What is the correct prism reading on the right lens pictured below?
We first notice that there is a prism correction because the intersection of the mires is displaced to the right from the bulls-eye of the target. Since we are measuring a right lens, the displacement is nasal, meaning the prism is base-in. The diopter value is 1.5 because the intersection aligns with the line that is between the 1 diopter circle and the 2 diopter circle. We know that there is only horizontal prism correction present because the displacement is along the 180 degree line. What if there is so much prism correction that the intersection is off the scale? This situation does arise. The prism may be so strong that the intersection cannot be located, no matter how the lens is placed. The image in the eyepiece may look something like this, with perhaps only the triple line mire in view.
When dealing with high diopter prism corrections, a loose prism may be needed to bring the intersection back onto the scale. Start with a 5 D loose prism and place it in front of the glasses lens, over the mark, with the base of the loose prism opposite from the base direction of the prism being measured. In this case we are measuring a left lens. The single line target and the intersection are off the scale to the right, indicating a large base-out prism correction.
Place a 5 D prism with the base inward over the mark on the glasses lens. Some lensometers have a holder for placing a prism from a trial lens set.
This will bring the intersection back onto the scale, as pictured below.
The auxiliary prism power must be added to the scale reading. In the example, the intersection is on the 3 D line, so we add 5 D to 3 D to get a total prism correction of 8 D.
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