Q. A Trade Secret
In a recent article you wrote for Eyecare Business, you mentioned a new plastic, 1.71 high index lens. I asked my lab about it, but they hadn't heard about it yet. Can you provide more on this new index? Who makes it, what are the benefits, what is the availability, etc.?--Alan Smith

A.
We may have been premature in mentioning a plastic 1.71 index lens, but not intentionally. The product comes from Hoya Lens and was heavily promoted during New York's Vision Expo show. With a 1.71 index, this is the highest refractive index available in plastic. Everyone who has seen the lens has been impressed. There has been a temporary lull in the marketing of this lens, resulting from certain difficulties in laboratory surfacing of this new material. Altering the refractive index of a plastic lens material requires significant changes in the molecular structure of the material.

As American labs began processing the 1.71 material on a production basis, some relatively minor problems arose. Hoya reports the problems are being addressed and any surfacing problems should be overcome shortly. In the meantime, the lenses are being processed in Hoya's lab in the Northeast.

Stay tuned. We'll let you know when this new lens material is readily available.

Q. A Most-Asked Question
When verifying bifocals in our lensometer, why is the add power always 1/4 diopter stronger than requested? Our lab tells us not to worry, but we wonder if it really is an issue. In particular, the add is consistently off in plus powers.

A.
The add power of all multifocal lenses should be measured with the segment side of the lens against the lens stop of your focimeter (lensometer). For modern lenses, this is typically the convex, front surface. The reason for this is that manufacturers mark the add powers of bifocal segments based upon the difference between the front vertex powers of the distance portion of each lens and its multifocal segment. The front vertex power is the power measured when the front surface is against the lens stop. When you measure the distance Rx, on the other hand, you are measuring the back vertex power since the back surface is against the lens stop.

The same lens will differ between front and back vertex power measurements, and add powers measured using the back vertex powers will typically have more plus power than add powers measured properly.

Although the wearer actually uses the back vertex power of the multifocal segment, lens manufacturers cannot control the back vertex add power since it varies as a function of lens thickness. For consistency, the front vertex power is used -- since the difference between front vertex powers is not affected by lens thickness or surfacing.

For thin lenses (like low- and minus-powered lenses), there is little difference between front and back vertex power measurements. Consequently, flipping a minus lens around to measure it from behind won't affect the add power reading as much. Plus lenses, however, should always be measured with the front surface against the lens stop since they generally have appreciable center thickness. In addition, some automatic lensometers might compensate for this effect, so refer to your automated lensometer manual if you are in doubt.--Darryl Meister, ABOM, SOLA Optical Technical Marketing

Q. The Magical World of Color
What can labs do to help their customers' color-blind patients? I know of some filters and BPI's Monochrome 600 tint, but wondered if there was any new technology being developed or currently available? Thanks, we all love your column.

A.
Over the years there have been several attempts to improve color vision with filters, the X-Chrom( contact lens being one example. Filters can often enable a color-deficient patient to pass a color vision test, but this does not indicate that the person has better color vision.

Color vision tests. such as the pseudo-isochromatic plates (e.g. identify the number embedded in the colored dots) are designed so that the "number" and the "background" are the same brightness and therefore only distinguishable by color. The "color vision filters" cause a brightness difference between the brightness difference between the number and the background; therefore the patient can see the number based upon a brightness difference, not a color difference. For every color combination that such a lens enables a person to distinguish, it also takes away a color combination that they could otherwise see but can't see with the filter. Such a filter will cause colors to appear different (even to a normal color person). However, that do not enable anyone to identify more colors.

There is a newer attempt to create a color filter for color deficient patients. In formation can be obtained at www.colormaxlens.com. Of course, we should look closely at any new technology that might be able to provide better vision for some people. However, we must also ensure that we recommend products shown to be effective by scientific study. Given the history of color vision filters, we need to be particularly cautious.--James E. Sheedy, OD, PhD, director of professional development, SOLA Optical EB

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