By Tina Lahte
In the beginning, corrective lenses were simple. They consisted of a spherical curve on the front and a spherical or toric curve on the back. If the front curve was a +6 and the back curve was a -5 the power of the lens was +1. Lens power was simple math, +6 + ( -5) = +1.
More complex lenses, designed for presbyopes, included another spherical lens to add power. The additional power was in the lower part of the lens for near vision. Increasing the front curve in a defined shape or “segment” added the power. For the example above, if the front curve of the segment was +8, the power through the segment would be +8 + (-5) = +3. The wearer would have 2 “additional” diopters of power when looking through the segment, a 2 add.
Then things got interesting. Molded progressive addition lenses, PALs, were introduced. In a progressive lens, power changes across the surface to gradually increase add power as the wearer’s eye drops toward the near area. But, changes in wanted sphere power create unwanted cylinder power. In fact, according to Minkwitz, the change in unwanted cylinder power from the center line of the lens to both sides must be twice that of the wanted change in spherical power from distance to near. The challenge became what to do with this unwanted power.
Because molded progressive lenses are mass produced, each wearer within a specific Rx range wears the same design. Those designs were created for accurate power as read by a lensometer. The power would be accurate if the lenses within the frame, and human head and eye were flat, all parallel planes, and the eye never turned for side or lateral viewing.
Now the real fun has begun. Digital lenses are commonly available. These lenses are calculated and created individually. Digital lenses take each wearer’s prescription, and how the lens sits in front of their eye, into account when calculating power change. Powers that are perceived correctly by wearers will read differently on a lensometer. Lenses calculated this way are called compensated. The measurements affecting compensation are Position Of Wear (POW) measurements because they define the position of the lens as it sits in front of the eye. POW measurements include:
- Wrap angle, the angle at which the front of the frame curves to fit around the wearer’s face. Wrap angle is specific to the frame only. This measurement is taken in degrees and is measured without the wearer.
- Pantscopic tilt, the degree to which the lenses tilt down toward the wearer’s cheeks, and away from the forehead. This measurement is taken in degrees and is measured with the frame on the wearer.
- Vertex Distance, the distance between the back of the lens and the front of the wearer’s eye. This measurement is taken in millimeters and is measured with the frame on the wearer.
|Wrap Angle||Pantscopic Tilt||Vertex Distance|
Prism can also impact compensation, and laboratories use sophisticated software to calculate each individual job. These calculations are complex, making it difficult to form assumptions about the relationship between the prescribed and compensated Rx. We can be sure, however, that in a good compensated digital design, wearers will experience better vision, especially better peripheral vision.
Tina Lahti, ABOC, is director of national accounts for Indizen Optical Technologies (IOT) of America. She is a 30-year optical industry veteran with experience as an optician, store manager, educator and lens and technology representative. Ms. Lahti holds a bachelor of science degree in industrial and organizational psychology.