First of all, I have an M.D. degree but I am not an Ophthalmologist, nor skilled in optics. Have posted here before an the Mod did not delete my posts. I need input from you professionals because of a problem in producing and detecting one base in O.U. prisms in my lenses.

Current Rx:

-1.25 -0.50 40
-0.75 -1.25 82
Add 2.25
1 BI O.U.


A well-known outfit (purposefully not naming) has tried making the glasses twice, and they didn't seem correct. When I had 0.5 BI in both lenses, I could look above the lens and see two blurry images, side by side, while through the lens they fused. With these two attempts by this "club", any vertical structure appears as one, both above and in the lens. Now that two optometrists prescribed 1.0 BI, I still no not sense prisms. On the cover-uncover eye test, the newly uncovered eye jerks medially, with or with these two attempts to get the correct Rx.

So I brought the glasses to ten optometrists. Eight said there were no prisms. One said no prisms, but I told him they must be there, he went back and agreed they were maybe 0.5 BI, not 1.0 BI. The other doc said one eye had an up and out prism, but not BI.

Today I brought the remake, in which the grinding lab insisted the prisms were there, to an Optometrist near the Club. She found no prisms by a refractometer, but then marked my lenses for IPD and went into her office, looked through a monocular device, and visualized crosshairs. She use a knob on the left, lined things up, and she found some BI prisms, totaling 1 BI, not two BI.

She explained there are two ways to get prisms:

1. Grind them in

2. Offset the lens

and she said this Club uses #2, not #1. She refracted me again and said I do need 1 BIN in each lens.

The lens material is "Ovation".

The manager of the grinding company who does these lenses said the prism is ground into the lens, and the offset method is not used. The lab produced a print out with each pair documenting "1.00 BI". I thought it strange that the 1.00 was exact to the hundredths place and in both eyes. Seems like the values were faked.

Even a Lenscrafters® lab could find no prisms in these two redone glasses.


QUESTIONS:

1. Why have so many optometrists been unable to detect these prisms ?

2. Is it true there are two methods of making prisms in lenses ?

3. I sense no prisms in these remakes, so if I can't see a split & fusion as before, can there still be prisms in these lenses ?

4. If there are prisms, why have so many optometrists & labs insisted no prisms are present ?

5. Why didn't the Humphrey find prisms in this pair ?

6. Is that monocular device with crosshairs reliable to detect prisms, or is it measuring something else ?

7. How does a human test his/her glasses for 1 BI O.U. (2.0 total) for prisms without consulting a professional ?

8. Will the offset technique be undetectable by the Humprey machine ? But still be clinically useful for the wearer ?

9. Can prisms actually be, or actually and accurately reported as exactly 1.00 in each eye, when sph and cyl and axis are not reported to nor accurate to the hundredths place ?

10. Each of the ten optometrists came up with different readings.... none were close to each other on the supposed Rx determined by the laboratory. Is Optics such a science and equipment/user so variable that one cannot expect consistency from place to place ?


Yes, I have eye fatigue with prolonged reading and when tired, it feels better to close one eye.

I put ten hours of investigation into these two repeat sets.

Thank you in advance for your help, and ask any other questions if it helps you assist this perplexing issue. A well-known outfit (purposefully not naming) has tried making the glasses twice, and they didn't seem correct. When I had 0.5 BI in both lenses, I could look above the lens and see two blurry images, side by side, while through the lens they fused. With these two attempts by this "club", any vertical structure appears as one, both above and in the lens. Now that two optometrists prescribed 1.0 BI, I still no not sense prisms. On the cover-uncover eye test, the newly uncovered eye jerks medially, with or with these two attempts to get the correct Rx.


This is what's interesting. First, you should do as Harry suggested - go to a real optician.Most ODs (not all of them) use an automated lensometer that does a poor job of detecting prism in progressive lenses, such as the Ovation (a brand of lens,not a material) A skilled optician using a manual lensometer is more likely to detect the actual amount, and yes, prism can be measured that precisely, though only digital ones read to that small an amount. Bear in mind that the ANSI tolerance is more than a few thousandths.
But..it seems that your dual vision is only intermittent, as you state above. With some glasses, looking over them produces a split image while other times it doesn't.Looking over them removes the glasses as a culprit, since sometimes your vision fuses well, other times, not so well. Hence the differences in amounts of prism. Probably it increases when you are tired, thus producing seemingly incompatible results. Were the exams done at different times of the day? The over the lens comparisons also?

Resolving the Complexities of Prism

By Corey Roveri, ABO-AC, NCLE-AC

Goto RuiQi Optics to know more.

Learning Objectives:

Upon completion of this program, the participant should be able to:

1. Explain how triangular prism deviates light and displaces and image
2. Describe how plus and minus lenses change the vergence of light by deviating light toward the prism base inherent in ophthalmic lenses.
3. List the correct steps to verify prism in an ophthalmic lens.

Faculty/Editorial Board

With more than a decade of experience in the optical industry, Corey is an instructor of Ophthalmic Technology at the College of Southern Nevada. He is an ABO/NCLE Level III speaker, holds a Bachelor of Science degree in Business Administration, and currently serves as the Second Vice President of the Opticians Association of Nevada.His experience managing an independently owned optical practice has allowed him to help solve some of the most complex cases; from infants to adults, high diopter prescriptions, patients with craniofacial abnormalities, and high value prismatic RX’s.

Credit Statement

This course is approved for one (1) hour of CE credit by the American Board of Opticians, ABO, Ophthalmic Level 3. Course # STWJHI087-3

Every professional optician working in the field will eventually have to deal with Prism - sooner rather than later. Prism in ophthalmic lenses may be prescribed by an optometrist, ophthalmologist, or refractionist to treat systemic and ocular medical conditions affecting vision. Still, more frequently, it is an unintentional byproduct of incorrect measurements or decentration errors during the finishing process.

A Recap of the Basics

Before a deep dive into Prism, there are a few essential things to remember:

• A triangular prism used in ophthalmic lenses is wedge-shaped with no parallel sides.
• Every Prism has a base (the thicker, flatter edge).
• Every Prism has an apex (thinner and pointed).
• Ophthalmic lenses (whether plus or minus) use the prismatic effect of deviating light toward prism bases to change the vergence of light entering the eye.
  • Plus, power lenses have convex curves with prism bases at the Center, prisms stacked base to base.
  • Minus power lenses have concave curves with the prism bases out. They can be depicted as prisms stacked apex to apex.
    • When light enters either of these mediums, it is slowed down, bent (refracted), and dispersed into its component colors.
• Prisms are measured and prescribed in diopters, however, individually, they do not contain any focusing power and therefore cannot correct myopia, hyperopia, astigmatism, or a combination of these conditions.
  • Therefore, a prism will not cause an image to be minified or magnified.
  • A prism will not disrupt the convergent or divergent characteristics of the lens.

A prism works by displacing the image toward the apex while simultaneously deviating incident light rays toward the prism base, as illustrated in Figure 1.

This amount of displacement is also dependent on:

• The distance between the Prism and the object being viewed.
• The power of the Prism (in diopters).
  • This is determined by the difference in thickness between the base and apex.
• General rule of thumb calculation:
  • If a one-diopter prism is one meter away from an object being viewed, it will displace light by one centimeter (10mm).
  • A two-diopter prism will deviate light by two centimeters (20mm) when viewing an object one meter away, and so on.

Optical Center, Major Reference Point, and Prism Reference Point

The Optical Center (OC) is found where the prisms used to form an ophthalmic lens meet (apex to apex or base to base). It is the only point on a lens where light is not being deviated as it passes through the optical medium. It may be helpful to picture the optical Center in front of the patient's visual axis when Prism is not prescribed. A depiction of a minus lens showing the apex-to-apex prism configuration both vertically and horizontally illustrates the location of the patient's visual axis (pupil) centered behind the lens OC in front of the eye can be seen in Figure 2.

The term Optical Center may be replaced by Major Reference Point or Prism Reference Point when an optometrist or ophthalmologist has prescribed Prism for a patient. This MRP or PRP is the point at which the correct refractive power and prismatic correction are present and accurate. The MRP is the only position within a lens in which the user will not experience any unwanted or induced prism. The Optical Center and Major Reference Point will be in the same location and thus interchangeable if Prism is not prescribed as part of the patient's RX.

Note that a patient will experience a prismatic effect any time their gaze shifts or deviates from the Optical Center, Major Reference Point, or Prism Reference Point of the lens. This would also include instances in which the optician has placed an order at a given pupillary distance and/or OC height, but during the final inspection, the measurements differ from what was ordered. In this case, the lenses would be inducing an unwanted prismatic effect.

Verifying, Identifying, and Calculating Prism

Lensometry

Final inspection and verification play a vital role in the eyecare process. As classified medical devices, prescription (and safety) eyeglasses are meant to conform with uniform standards set forth by the American National Standards Institute. In turn, the lensmeter (figure 3) is an invaluable tool to an optician when verifying prescription eyewear. This tool allows the optician to mark the Major Reference Point of a lens with the precision necessary to correctly verify refractive power, axis, and Prism when present.

Please note it is always important to prepare the lensometer for use prior to any verification: Focus the eyepiece, verify the prism compensator is set to zero at 90 degrees, verify the power drum is set to 0.00 and the sphere and cylinder lines are sharp and clear, etc.

The lensometer's reticle is another key piece of the puzzle when verifying Prism. The reticle can be rotated around its axis by using the reticle adjustment knob (or chrome knurled sleeve) as pictured in figure 3 (A).

When measuring horizontal Prism, the reticle should be oriented along the horizontal meridian from 0 to 180 degrees. When measuring vertical Prism, the knurled sleeve should be rotated so that the reticle is oriented along the vertical meridian from 90 to 270 degrees.

The reticle contains a series of concentric rings used to measure the prism present within a lens at the MRP. The values of these rings vary by manufacturer; but most will have rings at one, two, and three diopters away from the Center of the reticle. Some manufacturers incorporate straight vertical lines (or dash marks) outside of the concentric circles marking four and five diopters respectively which can be seen in figure 4(A).

The observer may also notice the lines of the reticle protrude into the inner most circle. Depending on the value of the inner most circle, the tips/end points of these lines can be estimated to be 50% of the power of the inner most circle (Example: if the inner most circle is equal to 0.50DΔ, the end points within that circle are equal to a prismatic value of 0.25Δ). These points are shown in figure 4(B).

Also keep in mind that when viewing a lens through a focimeter, the Prism being induced is in the same direction as it is being viewed: Nasally – base in, temporally – base out, up – base up, down – base down, (what you see is what you get). Example: When viewing an OD lens through the lensometer reticle, prism base directions will appear as shown in figure 5.

Prentice Rule Formulas for Verification

Alongside the lensmeter, the Prentice Rule formulas play an integral part in the verification process regarding Prism. While the vertometer will allow you to have a good understanding of how much Prism is being induced and in what direction, the result simply cannot be as precise as the value provided by the mathematical equation.

Multiple variations of the Prentice Rule Formula:

• To find prism diopters (P) given lens power (D = +2.00) and decentration in millimeters (h = 3mm)
• P = (D x h) / 10
  • Solution:
    • P = (2.00 x 3) / 10
    • P = 6.00 / 10
    • P = 0.60 Δ

*This is often the most used formula when viewing and calculating induced Prism in a lensometer*

• To find the decentration amount (h) in millimeters given the lens power (D = +2.50) and prism value in diopters (P = 1.2).
  • h = (P / D) x 10
    • Solution:
      • h = (1.2 / 2.5) x 10
      • h = 0.48 x 10
      • h = 4.8mm decentered
• To find the dioptric power (D) of a lens being decentered given prism value in diopters (P = 1.8) and decentration amount in millimeters (h = 3mm)
  • D = (P / h) x 10
    • Solution:
      • D = (1.8 / 3) x 10
      • D = 0.60 x 10
      • D = 6.00 Diopters

Prism in Quadrants

In a laboratory setting prism may be notated in degrees which correlate to quadrants rather than base direction. This is known as the polar coordinate system. In this case, lenses are divided into four equal sections. When viewing a mounted pair from the front, the top right-hand quadrant is labeled as quadrant one, quadrant two being the upper left-hand corner, quadrant three as the lower left-hand corner, and quadrant four as the lower right-hand corner, a visualization of this concept can be seen in figure 6.

An example of a lab order form using the polar coordinate system is provided in figure 7.

Medical Conditions and Refractive Errors for Which Prism May Be Prescribed

There are a variety of ocular conditions for which an ophthalmologist or optometrist may prescribe Prism to a patient, they include but are not limited to:

Diplopia (Double Vision) – The patient views an object as doubled due to a lack of binocular fusion.

Strabismus (Ocular Misalignment) – Can be caused by a variety of factors, one being weakened or non-functioning rectus or oblique extraocular muscles. Especially in children, strabismus could cause a delay or create a hurdle for the all-important development of binocular fusion.

Aniseikonia (Unequal Images) – Most likely the result of refractive surgery. Most often the brain is receiving unequal ocular images from the eyes, causing fusion to be a challenge for the patient. Aniseikonia may also present itself in the form of meridional aniseikonia in which one eye perceives the image to be unequal due to a high degree of astigmatism in one meridian.

Anisometropia (Unequal Measure) – The anisometropic patient requires lenses of the same sign (either both plus, or both minus) for correction, but has a refractive difference between the two eyes of more than one diopter. This patient is hyperopic or myopic in both eyes, or hyperopic in one eye and myopic in the other.

Antimetropia (Opposite Measure) – A patient suffering from antimetropia has one hyperopic eye and one myopic eye and requires lenses with opposing signs (one plus lens, one minus lens).

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Both anisometropia and antimetropia could cause the retinal images to vary greatly from eye to eye, while also having the potential to induce vertical imbalance and create Prism at the reading level. These two conditions may ultimately stem from pathological or physiological issues. It is vital for binocular fusion that patients experience equal motility, acuity, and ocular images. Without these criteria being met, the patient may view a distorted image, have uncomfortable vision, or experience a delay of binocular fusion development.

Stroke – After suffering a stroke, the vast majority of patients suffer from vision issues or loss of visual field. They may be sensitive to light (photophobic), experience double or blurred vision, or have uncontrolled and / or rapid eye movements (Nystagmus). The patient may also experience visual field loss by causing the brain to neglect and omit visual stimuli from an area of the field. (Lazarus, )

Prism in Practice

Prism to Correct Visual Conditions

Many times, eye doctors prescribe Prism to a patient with the hope of realignment from strabismus, in an effort to eliminate double vision, or to improve the perceived inequality of an image. In the most general terms, prisms displace the image being viewed towards the apex. Prescribing doctors will use this displaced image to turn the eye by directing the apex and prismatic power, accordingly, thus focusing the patient's gaze on the image's new position. In theory, this would bring the monocular images together and assist the brain with binocular fusion.

Cancelling and Compounding Prism

Prism may be compounding or cancelling (subtracting), ensure to mathematically take this into account when considering final pass or fail determinations.

• Prism combinations that are subtractive or cancelling and will negate one another:
  • Base up / base up.
  • Base down / base down.
  • Base in / base out.
    • Example: If the OD lens shows one diopter of base in Prism and the OS lens shows one diopter of base out prism, these prismatic effects will negate each other, causing the resultant Prism to be zero or negligible.
• The following prismatic combinations are compounding and will increase the prismatic effect:
  • Base up / base down.
  • Base in / base in.
  • Base out / base out.
    • Example: if the OD lens shows one diopter of base up Prism and the OS lens shows one diopter of base down Prism, these effects would combine to an overall value of two prism diopters for the mounted pair.

Splitting Prism

If an optometrist or ophthalmologist prescribes Prism in only one eye, it is possible for the Prism to be split based on the principles of combining and cancelling prismatic effects. The eyecare professional should always check with the prescribing doctor or refractionist prior to splitting Prism. The overall concept applies mostly for cosmetic and weight balancing purposes when visualizing a finished set of mounted lenses.

Should a patient have six diopters of base out Prism, it would be much more logical from an aesthetic standpoint to split that Prism between the two eyes. The end result would be three diopters of base out Prism in both the left and right lenses. In this case the prescription appears more cosmetically balanced and provides better weight distribution for the wearer, especially those patients who prefer a frame with nose pads. From a weight and balance standpoint, it would be far less comfortable for the patient if the OD lens were significantly thicker and heavier than the OS lens. The weight of the OD lens would then be focused into a single pressure point such as a silicone nose pad.

There are guidelines which should be followed with regard to splitting Prism:

• Always check with the referring doctor prior to splitting Prism.
• Prism should be split equally in half between the two lenses unless otherwise specified by the referring doctor/refractionist.
  • Always follow the rules of cancelling and compounding Prism. Be sure to maintain the prescribed base direction for the original eye.
    • Contrarily, ensure that the other half of prescribed Prism is placed in the compounding direction.
      • If base out Prism was initially prescribed for only the left eye, the total amount of Prism should be divided by two, and the left and right eyes should receive an equal amount of base out Prism.
      • Keep in mind that compounding scenarios differ vertically. Meaning that if the right eye is prescribed base up Prism, the optician should ask the laboratory to place an equal amount of base down Prism in the left lens.

Progressive Lenses and Prism

Although not prescribed, Prism is an integral part of the mechanics of a PAL. Due to the prismatic effect throughout the lens, the patient's gaze will naturally follow the progressive channel rather than taking a straight-line path down through the Center of the lens. Many progressive lens designers have compensated for this by adjusting the position of the reading zone. Ultimately this will aid in convergence and allow the patient's gaze to track downward to naturally find the necessary zones of prescribed intermediate power for computer work, or near power at the reading level.

When discussing inherent Prism and PALs, the importance of precise horizontal monocular PD measurements cannot be overstated. If these measurements are inaccurate, it could cause the patient to experience an issue with finding the natural path through the progressive channel. This could also result in a displaced and less usable near vision area.

In addition to precise monocular horizontal pupillary distance measurements, it is also vital that the MRP height be accurate. "An inaccurate major reference point height will cause one eye to track down the corridor ahead of the other. This means that the add power is not increasing equally for the two eyes." (Brooks & Borish, , p. 320)

Vertical Imbalance

Vertical imbalance may occur when there is a difference of more than 1.50 diopters between the left and right lenses at the reading level. The degree to which an individual patient is affected by vertical imbalance varies greatly, it is entirely possible to have separate patients with similar refractive errors or conditions perceive the imbalance entirely differently.

With single vision wearers, correcting the imbalance may be as simple as asking the patient to lower (or drop) their head so that their gaze is set through the optical centers of their lenses. As previously discussed, the optical Center is the point within a lens where no prism is being induced, thus eliminating the imbalance.

Correction of this imbalance proves more difficult for multifocal patients than single vision patients. When a patient's gaze converges down into the reading area, they may be viewing an object through two lenses which are of a significant power differential. In this case one eye will turn down further than its counterpart and is not ideal for synchronous binocular movement of the eyes. In certain cases, myopic patients have a better tendency to tolerate vertical imbalance than those who are hyperopic.

Slab Off

Also known as Bicentric Grinding, slab off is the most commonly used method to correct vertical imbalance at the reading level. Slab off can be ground into single vision lenses but is most often utilized when ordering multifocal lenses.

Bicentric grinding may correct up to six diopters of vertical imbalance and can be applied to a variety of different lens materials. In practice, it is most common for the slab off to be ground into the lens containing the least amount of plus or most minus power. To achieve this, laboratories grind base up Prism into one portion of the lens, thus guiding the eye's gaze downwards towards the apex. When completed, cosmetically it appears that a portion of the lens has been removed, thus garnering the name "slab off".

When receiving a prescription from a referring doctor, slab off may not always be notated. It is vital that the optician has a thorough understanding of what to look for and when to ask questions of the prescribing doctor or refractionist. We must dive deeper than what we see at first glance. Take into account the power in the 90-degree meridian, transpose the RX when necessary, and determine if power at the reading level varies by more than 1.50 diopters between each eye.

Reverse Slab Off

Unlike conventional slab off, reverse slab off is achieved by creating base down Prism in the lens containing the most plus or least minus power within the vertical meridian. Situations may arise where the patient is encountering more than six diopters of vertical imbalance which can be corrected by conventional slab off manufacturing. In this case, a good option to consider would be to use conventional slab off in one lens and reverse slab off in the other.

Other Methods of Correcting Vertical Imbalance

Contact lenses may be used as a means to correct imbalance in the 90-degree meridian because the MRP or optical centers move along with the eye, thereby reducing or entirely eliminating the prismatic effect.

Though less convenient, correcting vertical imbalance could be as easy as offering the patient two pairs of single vision eyewear. As previously mentioned in the vertical imbalance section, single vision eyeglass wearers can often correct vertical imbalance with a simple head movement by fixing their gaze through the optical centers of the lenses. A pair of distance vision only eyeglasses coupled with a pair of single vision reading glasses could be the solution. In this case, it is best practice for the pair of near vision only spectacles to have their optical centers placed slightly lower than that of the distance only pair.

Individual preference can vary greatly from patient to patient, making it vitally important for the optician to understand and be able to effectively describe all available options available.

Unintentional Prismatic Errors

As previously mentioned, Prism can be induced by improper measurements taken by the eyecare provider or by incorrect decentration (either vertically or horizontally with regard to fitting height or pupillary distance). While all lenses with a refractive power have the potential to induce Prism, those with higher refractive powers will mathematically induce much more Prism, much more quickly when decentered from the desired Major Reference Point.

Depending on the amount of Prism being induced and its respective direction, it is entirely possible for one or both lenses to be decentered and the pair still pass final inspection.

Troubleshooting Patients with Induced Prism

During dispensing, it is important to be able to identify what type of issue the patient may be experiencing with regard to their visual acuity. Patients may not be able to explain the distinct types of Prism, or that they feel like they might be looking through base down Prism, but their feedback is of the utmost importance. Most times, they are very general in their explanation, and it can be helpful to understand the different sensations being experienced when looking through diverse types of Prism. If a patient explains that they have clearer vision when only keeping one eye open, this is normally a sign of a difference (disparity) between left and right ocular images or a prismatic imbalance.

This can often be seen in practice when dispensing a newly manufactured pair of eyeglasses. Often times patients will want to compare and contrast the vision through each eye individually by closing or occluding one eye. We know that this is not necessarily the best-case scenario to evaluate binocular fusion. If the patient consistently describes monocular vision being preferable, mark the lenses up at the OC/MRP and double check to ensure no prismatic effect is being induced.

Patients who are experiencing excessive base down Prism (as seen in figure 8) may complain that:

• Objects being viewed vertically seem to be taller.
• The patient feels as if they are standing at the bottom of a hill.
• The patient feels as though they are constantly walking uphill.
• They describe the floor in a way which equates to appearing concave in nature.

Patients who are experiencing excessive base up Prism (as seen in figure 9) may complain that:

• Objects being viewed vertically seem to be shorter.
• The patient feels as if they are constantly walking downhill.
• The patient reports that they feel as if they are standing at the top of a mountain.
• They describe the floor in a way which equates to appearing convex in nature.

Patients who are experiencing excessive base in or base out Prism (as observed in figure 10) may explain the visual discomfort as:

• They are experiencing a "swimming" sensation.
• Their horizontal plane of vision seems low (towards the apex) or high (towards the base).

Patients often have a tougher time tolerating induced vertical prism rather than induced horizontal prism. This would be logical due to patient convergence coupled with the overall concept of vertical imbalance.

Conclusion

It is vital to the success of the patient that the optician be able to perform a myriad of tasks effectively. The eyecare provider should be able to take consistent and accurate measurements, to be able to verify the validity of those measurements after manufacturing, and to be able to troubleshoot and identify a variety of sensations related to prismatic effect. It is also extremely important that the optician be able to anticipate potential issues which may arise from a prescription which gives little to no direction.

It is not uncommon for an optician to call a referring office to confirm the presence of antimetropia, and that the use of opposing signs is appropriate (especially in the case of repeat customers who have had a significant change in prescription). The optician should be able to deduce when the prescription at the reading level has the potential to create vertical imbalance, and when slab off should be implemented to eliminate this imbalance. The ECP should also possess the knowledge and skill necessary to effectively split Prism without creating imbalance in any of the horizontal, vertical, or oblique meridians.

In summary, it is easy to see the importance of understanding the many facets of Prism, along with how it may impact a patient's vision and overall quality of life. To put it quite simply, their best vision can be dependent upon the knowledge, commitment, and precision of their optician.

Works Cited

Brooks, C. W., & Borish, I. M. (). System for Ophthalmic Dispensing. Butterworth-Heinemann.

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