DEC 2013

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Your monthly guide to staff training outside the box

Eyes / Lenses / Fitting Lenses / Free-Form / Frames / Sunwear / Patient Solutions / In-office / Standards

LENSES, Lens Power and Prism

Prism Imbalance

Patients do not easily tolerate vertical prism differences, often called vertical imbalance. Individual tolerance varies, but a difference between the right and the left lens of one prism diopter (1 prism) or more of vertical prism can cause asthenopia (eye strain), adaptation problems, reading difficulties and even diplopia (double vision). Patients with single-vision lenses and vertical imbalance usually learn to turn their heads to read through the lenses at a point closer to the optical centers.

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Prisms and Progressives, Is What You Order, What You Really Want?

By Palmer R. Cook, OD

Clinical Tips for Complicated Ordering

  1. Lenses incorporating prism should be anti-reflective because AR will improve their performance, especially with relatively lower powers.
  2. Prism always exacerbates the negative effects of chromatic aberration especially when the prism is in the distance portion of the lens. The more prism, the more problems with chromatic aberration—always and especially with people who work outdoors.
  3. Shorter corridors shorten the drop from the Distance Reference Point (DRP) to the Near Reference Point (NRP), so they require lower power slab offs.

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Prism Pearls

By Opticians Handbook Staff

The ability to turn eyes in (converge) and yes, turn slightly outward (diverge), is the reason that the ANSI standard for PD and prism allows up to 2/3 prism diopters of "tolerance." However, it's virtually impossible for people to move their eyes differently up and down. The yoked muscles make the eyes move equally up and down when needed. As a result, the ability of the eyes to move to images that are not at the same point vertically is less than horizontal. That's one of the reasons for the vertical prism tolerance of only 1/3 prism diopter.

It seems unusual then that a PD is taken for every Rx when optical center (OC) height is almost never. That's because your lab will automatically place the OC height at the frame's midline for single vision and bifocals. For progressives, the OC height (if no prescribed prism or prism thinning) or more accurately, the prism reference point (PRP) is always located equal distances below the fitting cross and varies by manufacturer. The PRP is located 0 (at the fitting cross), 2 mm, 4 mm or 6 mm below the fitting cross.

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Chromatic Aberration: The End of the Rainbow

By Palmer R. Cook, OD


PRISM DIOPTER—The displacement of an image formed when the line-of-sight passes through a prism. If the angle of displacement is one prism diopter (1 prism), the image of an object at 1 meter will be displaced by 1 cm. The displacement is in the direction of the apex of the prism.

PRENTICE'S LAW—The power of the lens in diopters times the distance from the optical center to the point at which the line-of-sight passes through the lens equals the prismatic effect in prism diopters. The direction of the base of the prismatic effect is always toward the blue fringes.

OPTICAL CENTER—The point in the lens through which light passes undeviated.

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  • Usually horizontal prism is either base in (BI) in both eyes or base out (BO) in both eyes.
  • Usually vertical prism is downward in one eye and upward in the other.
  • When the prism indicated by the Rx varies from what is "usually" done, a call to the prescribing doctor may help you avoid making an error in ordering.
  • When horizontal prism—base in (BI) or base out (BO)—is specified, try to avoid decentration as much as possible by careful frame selection.
  • When vertical prism, base up (BU) or base down (BD) is specified, avoid using a frame with a deep (large B measurement) lens shape.

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Taking a Careful Look at Patients Who Return

Patients who return shortly after receiving new eyewear frequently offer comments ranging from mild displeasure to outright anger. The common denominator among these unhappy folks is usually one or several unmet expectations. Patients who purchase new eyewear expect to look good, see well, be comfortable and perceive fair value. If you think in terms of these eyewear expectations, you are already on the road to resolving the issues that trouble them.

Complaints or Clues
Off-axis errors tend to cause floors and tabletops to appear tilted. Floors that appear to be humped up or scooped out when the patient looks downward at a few feet in front of them is usually caused by lateral prism, either base-in or base-out. These kinds of complaints can be important clues that can help you resolve your patient's difficulties more quickly.

A Simple Test Identifies Potential Causes
The optical center of the lens represents the point at which light rays pass through undeviated. The optic axis passes through this optical center at right angles to the lens surfaces. Unless prism has been prescribed, the very best lens performance occurs when the patient's line-of-sight coincides with the optical axis of the lens.

If you want to assure the best lens performance, locate the optic axis rather than simply marking the optical center. To locate the optic axis, use a direct ophthalmoscope or penlight at a distance of 20 inches or so from the patient. The patient should be directed to look at the light, and you should observe the two reflections of the light (from the front and back lens surfaces). By moving as you keep the light directed toward the patient's pupil you will find one location at which the two reflections are superimposed to appear as a single reflected point of light. Once located, you need to note the relationship of that reflection to the patient's pupil and the orientation of your own line-of-sight to the patient's facial plane.

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—Palmer R. Cook, OD

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