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Module 31 |
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Module 31: |
Measuring Prism in Glasses |
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Section 2: |
Vertical Prism | ||
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Module 31 |
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Module 31: |
Measuring Prism in Glasses |
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Section 2: |
Vertical Prism | ||
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Measuring vertical prism correction
When measuring vertical prism in a pair of glasses, knowing the patient’s viewing position through the lenses is not as critical as it is when measuring horizontal prism. In fact, the total prism correction (right lens prism diopters + left lens prism diopters) can be found without marking the pupil center positions on the lenses. This comes in handy if you are in a hurry and simply want to confirm the total prism correction. You would, however, need to make the marks in order to find the precise prism correction in each lens.
Recognizing the presence of prism in a pair of glasses
Each time you read a pair of glasses on a lensometer you should be checking for the presence of a prism correction. Don’t ever assume that there is no prism correction in the glasses of a new patient. Many times patients will not volunteer this information and it may not be obvious from the history or from previous exams. As discussed in the last Section, the presence of horizontal prism can be recognized because the lens has to be shifted away from the normal patient viewing area in order to center the intersection of the mires in the lensometer target. For example: Normally, a lens placed on the lensometer stage, such that you are taking a reading just above the center of a flat-top bifocal line, will result in mires centered in the lensometer target, as pictured below.
If, however, you find that the intersection of the mires has shifted significantly away from the target when reading the lens in what should be the proper position, then you know that there is a prism correction present, as pictured below.
The key to recognizing the presence of vertical prism in a pair of glasses is to leave the lens stage in the same position when checking each lens. The lens stage adjusts up and down. For example: The intersection of the mires is centered in the lensometer target and the right lens prescription is measured.
Now align the left lens on the lens stage of the lensometer, but do not move the lens stage up or down from the original position.
Notice if the intersection of the mires is centered in the lensometer target; or is the intersection above or below the lensometer target? If it is above or below the lensometer target, as pictured above, then you know that a vertical prismatic power is present.
Measuring the vertical prism correction
At this point the total vertical prism correction can be determined simply by noting the direction of the shift in the left lens and by noting the power as measured by the diopter markings on the lensometer target. Remember that the intersection of the mires was centered in the target when we measured the right lens.
Our example above shows a vertical shift of the mires. The intersection of the mires is lined up with the mark that is between the "1" and the "2" measuring circles on the target. This indicates a prism power of 1.5 D, base-up.
If you want to know the individual prism correction for each eye, you will need to mark each lens at the point through which the patient views, as discussed in Section 1. If you are in a hurry, and the patient wears a lined multifocal (such as a flat-top), then you can usually assume that this point will be directly above the line, in the middle of the seg, as pictured below:
Once you have the lenses marked, measure each lens with the mark in line with the lens port of the lensometer. If base-up prism is present, the intersection of the mires will be above the target of the lensometer. Rotate the scale (using the knob on the eyepiece) to a vertical orientation and measure the diopter power from the scale.
Now measure the left lens. Line up the mark on the left lens without moving the lens stage up or down. If the mark is above or below the port, then you know that either you marked the lens incorrectly, or the patient’s glasses are not properly fit to her face. Remark the glasses or adjust the fit as necessary.
Assuming the lens is marked correctly, measure the prism correction in the left lens. If the prism correction in the right lens is base-up, then either there will be no vertical prism correction in the left lens (the mires will center in the target), or there will be base-down prism in the left lens. The reverse is also true. Base-down prism in the right lens will mean that there is either no vertical prism in the left lens, or there is base-up prism in the left lens. Add the right and left vertical prism corrections if you want the total vertical correction. For example: 2 D base-down OD and 1 D base-up prism OS is equal to 3 D total vertical prism correction. The orientation for the total correction can either be base-down OD or base-up OS. Glasses are not intentionally made with base-up prism in both eyes or base-down prism correction in both eyes. If the glasses measure this way, it usually means that the patient is viewing (or you are measuring) either above or below the intended vertical optical center of the lenses. When measuring a pair of glasses in this situation, you will need to adjust the lens stage so that either the mires of the left lens, or the mires of the right lens, line up vertically with the lensometer target.
Vertical prism correction that is off the scale
Occasionally you may encounter vertical prism correction that is off the diopter scale of the lensometer.
Just as you did when measuring horizontal prism, you will need to use an auxiliary prism to bring the image back onto the scale. If the prism direction being measured is base-down, the auxiliary prism will need to be aligned base-up in front of the lens port. A 4 D loose prism usually does the trick.
The loose prism will bring the image back onto the scale. Remember that you will need to add the power of the loose prism to your new scale reading.
I n the example here, the 4 D value of the loose prism is added to the scale reading of 3 prism diopters to give a prism reading of 7 D base-down.
Test Yourself
Mr. Jones wears flat-top bifocals. When checking his vision, you notice that his line of site is just above the add line, in the middle of the seg. When reading the right lens on the lensometer you line up the lensometer port just above the seg line, in the midline of the seg. Below is pictured the image you see when reading the lens power.
Now you shift the glasses over to read the left lens, without moving the lens stage up or down. The left lens is aligned such that the lensometer port is just above the middle of the seg line. The image below is what you see when looking into the lensometer.
Is there a prism correction in the right lens? How about the left? If there is a prism correction, what is the direction and power in each lens?
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