UNDERSTANDING PRISM PART 2: Verifying Prescribed and Unwonted PRISM

Jenean Carlton. ABOC. NCLC

A back-to-basics look at verifying that lenses don’t include unwanted prism

in part one of this article, we learned that lenses with prismatic correction are prescribed to help patients with strabismus—the failure of both eyes to simultaneously direct their gaze at the same object in space due to an imbalance of the extraocular muscles—achieve one fused image. When eye muscles don’t move together in harmony, blurred vision or diplopia can occur.

When inspecting finished eyewear that includes prescribed prism, it’s important to verify that the position and degree of prism has been fabricated accurately, including the degree and direction of prescribed prism, before dispensing glasses to patients.

It’s also essential to verify that the lenses don’t include any unwanted prism or have either vertical or horizontal prism imbalance, or even a combination of the two.

Unwanted prism can cause discomfort and visual problems for patients including blur, headaches, nausea, and even double vision. If the PD measurement is off, the result is unwanted horizontal prism (base in or base out). If the OC placements are inaccurate, the result is unwanted vertical prism (base up or base down). A slight amount of unwanted vertical and horizontal prism, based on the power of a lens, is considered tolerable by the American National Standards Institute’s current guidelines for prescription lenses, ANSI Z80.1-2010.

For single vision, multifocal, and progressive lenses, a vertical prism imbalance over 1/3 diopter is considered unacceptable for patients. Horizontal prism imbalance over 2/3 diopter is considered intolerable.

Check with your laboratory for a copy of the ANSI Z80.1-2010 standard or at least a summary of the document. A complete copy of the standard can also be ordered online by visiting webstore.ansi.org.

Prescribed prism is incorporated into a lens in two ways, ground-in or induced. Induced prism is created by decentering the optical center (OC) of a lens away from the patient’s visual axis.

The visual axis, also called the line of sight, is an imaginary line from a viewed object in the visual field to the fovea centralis of the retina. If the lens power is sufficient to induce the amount of prescribed prism, the lens can be fabricated with the OC decentered. This is known as prism by decentration.

If the lens power is insufficient to achieve the correct amount of prescribed prism by decen-tration, the prism must be ground into the lens. This is achieved by grinding the surface of the lens at an angle during surfacing.

THE IMPORTANCE OF PD

The interpupillary distance (PD) measurement determines the horizontal placement of the optical center (OC) of lenses. Unless prism correction has been prescribed, the horizontal placement of the OC of each lens should coincide with the monocular pupillary measurement for each eye.

The binocular interpupillary distance is the measurement from the center pupil of one eye to the center pupil of the other eye. Because a patient’s face is seldom perfectly symmetrical, the monocular pupillary distance is used when precise measurements are needed, as when fitting PALs or when fabricating single vision lenses with a moderate/high-powered correction. It’s possible to take a PD measurement with a millimeter ruler, but the most precise method of determining this measurement is with a corneal reflex pupillometer (CRP).

ANSI Z80.1-2010 Prism Tolerances

VERIFYING PRISM

A manual lensmeter, also called a lensometer or focimeter, is the most frequently used piece of equipment in the dispensary. While there are several automated lensmeters on the market, having the skills to neutralize lenses with a manual lensmeter will always come in handy.

A lensmeter measures the focal length of a lens and converts this measurement into diopters that are read on the instrument’s power wheel.

A manual lensmeter has more functions than this, including determining the cylinder component and axis of a lens, marking and indicating the optical center placement, as well as the prism reference point (PRP), which is the point where the amount of prism prescribed is equal to the amount of prism found. A manual lensmeter also allows users to detect areas of a lens that are warped or have some sort of aberration; establish the add power in multifocals and progressives; and determine the amount and direction of prism.

Becoming familiar with how to determine prism with a manual lensmeter is a skill that takes practice. The most common method of using a manual lensmeter to determine the amount and direction of prism incorporated into a lens is described here; readers are encouraged to consult the user’s manual of their particular instrument as well.

INSTRUMENT SETUP

THE RETICLE. The reticle is the imprinted image of circles and numbers that includes a protractor scale and orientation lines for each lens meridian. The reticle is used to detect the optical center of an ophthalmic lens and to measure and locate prism base direction.

The majority of lensmeters used in the U.S. have the first prism ring indicate one-half of a diopter of prism; the dashes inside this ring represent 1/3 prism diopter. Be sure to study your lensmeter’s manual to fully understand the prism value of the innermost ring of the reticle.

FOCUS THE EYEPIECE. The lensmeter’s eyepiece is made up of a series of lenses. Focus the eyepiece before attempting to neutralize lenses by turning the power drum to plano or 0 power, then turn the eyepiece to its most plus position. Without inserting lenses or a pair of spectacles into the lensmeter, inspect the reticle through the eyepiece.

While viewing the reticle, adjust the eyepiece by turning it slowly toward minus until the numbers and circles appear sharp and focused, stopping as soon as the reticle is clear. If you continue toward more minus at this point, you will stimulate your accommodation, which could render the reading inaccurate. (Accommodation is the adjustment by the eye for seeing at different distances. It is accomplished by the crystalline lens changing its shape through action of the ciliary muscle.)

If working in a large clinic where many people are using the same lensmeter, it is beneficial to mark your eyepiece to avoid repeating these focusing procedures each time a pair of glasses is to be neutralized. Simply focus the eyepiece and mark it with a colored grease pencil at the indicator dot to signify your personal adjustment. After doing this, you can quickly adjust the eyepiece to your mark when neutralizing a pair of spectacles.

POSITION THE FRAME. Position the frame on the spectacle table, making sure that both eyewires are evenly placed on the carriage. If one of the eyewires is slightly off the table or if one of the temples is on top of the power wheel, the lens power, prism, and axis readings may be incorrect.

CLAMP THE LENS IN PLACE. The lens holder is a device on the lensmeter that clamps down on the lens and holds it firmly in place. Use this device when obtaining the power of a lens to ensure the accuracy of the neutralization. Use caution when clamping a lens into place as some lens materials are soft, such as poly or ultra-high-index materials, and scratch easily. Because of this, lenses are often damaged in the laboratory or office during final inspection.

Examine the lens holder on your lensmeter for damage and take notice of the circular pad affixed to the clamp. Sometimes this rubber pad will dry out and scratch the lens surface.

VERIFYING GROUND-IN PRISM

Inspect the optical order or lab invoice to verify the amount and direction of the prescribed prism in a pair of spectacles.

Locate the optical center of the lens by finding the central point at which the crosshairs meet. After locating the optical center of the strongest lens, lock the lens into place with the lens holder.

View the optical center of the strongest lens through the eyepiece. Notice how the optical center corresponds with the markings on the reticle. The rings on the reticle are numbered to determine the degree of prism diopter. As an example, if the lens has 3 diopters of prism in the right lens, then the optical center of the lens would coincide with the #3 prism ring on the reticle.

The direction of a prism is determined by its base. The reticle is divided into four quadrants. To calculate the direction of the prism, it is important to know in which quadrant the optical center appears, as well as its relationship to the bridge of the frame.

Determine the prism direction by checking the relationship of the bridge to the OC. If the OC is ground nasally on the 180 degree line, and if you are verifying the OD lens, then the direction of the prism is base in (BI). This reading would indicate base in because the nasal area of the face is the most inward position of the right lens. If the optical center were instead ground in the temporal position, in the OD lens, it would be base out (BO) because this would be the most outward position of the right lens.

Mark the lenses with the ink-marking device and measure the PRP on the glasses. This measurement should coincide with the PD measurement indicated on the job order.

If needed, determine the prism ground into the left lens by repeating the steps. Be sure to not alter the position of the frame carriage when moving from one lens to another.

Use the prism compensating device (PCD) to inspect lenses with a high degree of prism correction. The PCD is located close to the eyepiece of the lensmeter and moves horizontally to determine horizontal prism (BI or BO). The device can also be locked in the vertical position to verify lenses with vertical prism (BU or BD).

Move the device into the appropriate position before inspecting lenses with prism correction, and lock the device in the 180 degree position when verifying lenses with horizontal prism. When inspecting lenses with vertical prism, lock the device into the 90 degree position.

Position the strongest lens on the lens stop and move the spectacle table to hold the lens firmly in place. While looking at the reticle through the eyepiece, rotate the silver knob on the PCD until the optical center of the lens is centered in the reticle.

The prism scale on the device indicates the degree of prism present in the lens. The prism direction is determined by the position of the device and the position of the bridge. Repeat the steps for the other lens. Mark the lenses with the marking device to ensure that the PD is accurate.

Most lensmeters are capable of neutralizing up to 20.00 D of prism power with the prism compensating device. The total power of the prism scale is 15.00 D of prism and the reticle accounts for 5.00 D of prism.

To verify prism over 15.00 D, move the lensmeter’s optical center by 5.00 D by turning the silver knob on the device. This allows for an additional 5.00 D of prism. If verifying a lens with 20.00 D base out prism, move the optical center of the lensometer in 5.00 D and then use the PCD. The prism scale would indicate that the prism power is 15.00 D base out, which would indicate a total of 20.00 D because we already accounted for the other 5.00 D by moving the OC of the lensometer.

To state this mathematically, 15.00 D − (−5.00 D) = 20.00 D. When using the prism compensating device, be certain to set it back to zero after each use.

VERIFYING INDUCED PRISM

Ground-in prism generally cannot be moved to another position on the reticle. For example, 2.00 D of BI prism cannot be made to read 4.00 D of BO. However, induced prism often can be centered or moved to different positions on the reticle. This type of prism is calculated after aligning the OC in the middle of the reticle, and then using Prentice’s Rule to determine the amount of induced prism depending on the power of the lenses.

Once the amount of induced prism has been calculated, it is necessary to determine the direction of the induced prism. Plus lenses are “against motion” convergent lenses; if you move it while viewing through a lensometer, the crosshairs shift in the opposite direction of your movement.

Minus lenses are “with motion” divergent lenses; if you move a minus lens while inspecting it with a lensometer, you see the mires move in the same direction as your movement.

Optical centers ground to match a patient’s visual axis have no prismatic effect for plus or minus lenses. However, a plus lens mounted in the left eyewire that has been ground with a narrow OC creates a base in prism effect. Conversely, a minus lens mounted in the left eyewire that has been ground with a narrow OC creates a base out prism effect. It can be helpful to draw the situation on paper to assess the placement of the OC and the direction of the induced prism.

Prescribed prism is incorporated into a lens by being ground-in during surfacing or by decentering the optical center of a lens to induce the correct amount of prescribed prism.

When inspecting eyewear with prescribed prism, verify that the position and degree of prism has been fabricated accurately. Verify that the lenses don’t include any unwanted prism that can cause discomfort and visual problems for the wearer. EB

This fun crossword puzzle explores prism, and how it pertains to vision correction. The answers to this puzzle will be listed in the January issue of Eyecare Business and online at eyecarebusiness.com.

across

2 The failure of both eyes to simultaneously direct their gaze at the same object in space.

6 _______ _______ prism is achieved by grinding the surface of the lens at an angle during the surfacing process. (Two words)

11 Mark the lenses with the _______ marking device.

12 PRP stands for the _______ _______ _______. (Three words)

13 The _______ is the imprinted image of circles and numbers that includes a protractor scale.

14 A lensmeter measures the _______ of a lens. (Two words)

16 It’s important to _______ the eyepiece before neutralizing prescription lenses.

18 The lensmeter’s _______ is made up of a series of lenses.

22 Use the _______ _______ ______ to inspect lenses with a high degree of prism correction. (Three words)

25 It’s important to verify that the degree of _______ has been fabricated accurately.

26 The adjustment by the eye for seeing at different distances.

28 The direction of a prism is determined by its _______.

29 Induced prism is created by decentering the _______ _______ away from the patient’s visual axis. (Two words)

30 Most lensmeters are capable of neutralizing up to _______ D of prism power.

31 This type of prism can cause discomfort and visual problems for patients.

32 The rings on the _______ are numbers to determine the degree of prism diopter.

down

1 This device is the most accurate method of obtaining a patient’s PD. (Three words)

3 The abbreviation for the American National Standards Institute.

4 This measurement determines the horizontal placement of the optical center of the lenses. (Two words)

5 It is essential to verify that the lenses do not include any _______ prism.

7 A device on the lensmeter that clamps down on the lens and holds it in place. (Two words)

8 The _______ PD is used when precise measurements are needed.

9 Used to determine the amount of induced prism depending on the power of the lenses. (Two words)

10 Locate the _______ _______ of the lens by finding the central point at which the crosshairs meet. (Two words)

12 Determining prism with a manual lensmeter takes _______.

15 When inspecting lenses with _______ prism lock, the PCD in the 180 degree position.

17 The prism _______ on the PCD indicates the degree of prism present in the lens.

19 _______ lenses are ‘against motion’ convergent lenses.

20 When inspecting lenses with _______ prism lock, the PCD in the 90 degree position.

21 When neutralizing lenses, start with the _______ lens first.

23 A manual lensmeter is also called a lensometer or _______.

24 ls called the “line of sight.” (Two words)

27 _______ lenses are “with motion” divergent lenses.