Dispensing case study: Significance of MAR and thinner lenses when resolving Cx concerns

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by Kanimozhi Amudan FBDO, Specsavers Wheelers Hill, VIC

Background

Cx: 68-year-old female returned with recently purchased glasses after experiencing difficulties

Original dispense:
Pair A: Distance with standard CR39 hard-coated SVD lenses
Pair B: Reading with 1.6 index SVN UltraClear SuperClean

Prescribed Rx:
RE: +4.00 / – 0.50 x 75         ADD: +2.00
LE: +3.25 / -0.50 x 115         ADD: +2.00

Concerns

  • Issues were linked solely to distance glasses.
  • Was experiencing reflections during the day and night as well as halos around light sources.
  • Had difficulty with night driving due to double images of lights being perceived.
  • Sharp view of her eye was appearing temporally on the lenses, but disappeared when she moved her eyes to see the image.
  • Further discussion revealed she found the glasses heavy on her face and felt the lenses made her eyes look magnified.
  • Through observation, it was also noted that the size of the frame was large relative to her face and was compromising her comfort when wearing them.

Discussion

When a lens is used to form an image, some of the incident light is reflected at the lens surface. This reflected light reduces the brightness and contrast of the image and can cause unwanted reflected images. The customer’s concerns extended from the fact that she had no anti-reflective coating over her distance vision glasses.

Ghost images of objects in front of the spectacle lens are formed when rays bounce back and forth within the lens or bounce from the cornea to the spectacle lens. The sharp image of the customer’s eye was formed by reflection from the back or front surface of the spectacle lens. The temporally perceived image vanished when she moved her eye to look at it due to vignetting between the pupil of the eye and the spectacle eye wire.

When addressing the weight of the lenses as well as the magnification they created, the lens material and form were considered. In contrast with the lenses in the customer’s original distance glasses (CR39), high-index lenses are made with flatter curves and from materials capable of bending light to a stronger degree than indexes with a lower rating. Because of this, they require less physical material to produce a prescription.

The combination of an aspheric design with high-index lens materials creates a lens that, as well as being noticeably slimmer, thinner and lighter, will reduce magnification.

Credit: All About Vision

Solution

A frame with a smaller eye size was chosen to help reduce lens diameter and weight. 1.67 aspheric lenses were discussed, with reference to their being thinner and lighter in weight and in reducing peripheral aberrations and magnification. The properties of the 1.67 require it to have a MAR as standard, eliminating ghost images, preventing reflections, and making the eye more visible.

To ensure the best possible vision and reduce magnification, the frame was fitted with a shorter vertex distance to increase the field of view. Due to the fact that oblique aberrational error would increase as the customer looked further away from the optical centre, vertical centres were measured in forward gaze and the rule of lowering them 0.5mm for every 1 degree of pantoscopic tilt was applied.

Learning outcomes

High-index lenses can reflect up to 50% more light than regular plastic lenses, which can be troublesome with driving at night and low light conditions, however, anti-reflective coating eliminates reflections of light, allowing 99% of available light to pass through the lenses. Therefore, high-index lenses are often combined with anti-reflective coating.

Aspheric lenses are mathematically enhanced to produce improved peripheral optics and reduce aberrations. As a magnifier, an aspheric lens enhances image quality and minimises distortion throughout the viewing area, reduces distortion at wide angles, and improves corner resolution.

From a customer perspective, certain issues she was experiencing only came to light through further discussion at the point of a remake. The opportunity to resolve them was a valuable one, and forms the basis of a purposeful training session with the team to consider better methods of identifying a greater number of customer needs at the initial enquiry stage.

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