Conventionally, progressive addition lenses have been designed with two power reference locations: a distance reference point (located at the center of the distance verification circle) and a near reference point (at the center of the near verification circle). The distance reference point is equal to the base curve of the lens, while the near reference point is equal to the base curve plus the add power of the lens. Lens designers have ensured that these two points remain relatively spherical, and that surface astigmatism (produced by the progressive change in surface power) is prevented from encroaching into these regions. This was done to ensure that dispensers and optometrists could readily verify the lens with typical vertometers and vertometers, these spherical points allow dispensers to verify the distance and add powers without unwanted cylinder (from the surface astigmatism) affecting the reading.
Although lenses designed like this provide accurate power readings at the distance and near reference points, they don’t necessarily deliver the exact intended prescription to the wearer. Because of the effects of oblique refraction through the lens, pantoscopic tilt, prism, and vertex distance, the power actually experienced by the wearer with the lens in position may differ from the power measured by a conventional lensometer and vertometer. Consequently, determining the optical power provided by a progressive lens in the “as-worn” position is a more accurate measure of what the wearer actually sees. However, this can not be done with conventional lensometers or vertometers without special modification – or by optical analysis using ray tracing software.
It is possible, using asphericity, to optically optimize the distance and near reference points so as to provide an optical power that matches the desired prescription when the lens is actually in position. Unfortunately, designing a lens to provide the desired optics in the “as-worn” position generally results in a difference between the doctor’s prescribed prescription and the power measured by a conventional lensometer or vertometer. The distance and near reference points may be designed with a small amount of cylinder, or a change in the sphere power, out of necessity – in order to compensate for the as-worn power of the lens. This added cylinder and/or sphere power makes it difficult to determine whether or not the lens was made accurately. (That is, whether or not the powers were accurately manufactured and/or surfaced.) This is further compounded by the fact that this small cylinder may combine with the doctor’s prescribed cylinder, producing a crossed cylinder effect. When the prescribed cylinder is also small, this crossed cylinder may have a different axis and cylinder power than the prescribed cylinder.
So, the option becomes: A) Keep the distance and near reference points relatively spherical to ensure ease of verification and quality assurance, or B) Optimize the distance and near reference points to provide a more accurate optical power in the as-worn position. It is also possible to compromise between both; the distance and near reference points can be left relatively spherical, while the region surrounding them is aspherized for as-worn optimization. So… Given these alternatives, what would your opinions be?
Best regards,
Darryl
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