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Optics with a twist
High index lenses can be the crème de la crème of lenses, but only if you treat them right. Here are the basics behind the high index lens phenomenon and tips for bringing out the best in this design
By Karlen McLean, ABOC, NCLC

The benefits increase substantially as well with improved optics; thinner, flatter, lighter weight; and less magnification/minification with an overall improved cosmetic appearance. Defined design is what's behind the high index lens scene and what makes high index lenses work.

REFRACTIVE INDEX

Refraction is caused by a light wave bending when it enters a medium where the speed differs. The index of refraction of any optical medium is the ratio between the speed of light in a vacuum and the speed of light in the medium. Ideally, all optical lenses would converge all light rays from a point in the object plane to the same point in the same image plane, forming a clear image. However, every type of
optical lens has a different refractive index. As refractive index increases, so can optical aberrations, which converge the different rays to different points.

One way lenses are defined is by their index of refraction, which categorizes them into ranges. This helps define the indices, although as the index bar gets higher, those categories may be redefined. Current designations are:

Standard Index: Crown glass (1.52), plastic hard resin (1.50).

Mid-Index: Lenses with index of refraction greater than 1.523 and less than 1.60. Optical centers can be ground to 1.5mm and some to 1.0 center thickness. Trivex has a 1.53 index while polycarbonate has a 1.59 index.

High Index: Lenses with index of refraction 1.60 to 1.66.

Hyper-Index (or super high index or ultra high index): Lenses with index of refraction 1.66 or greater. This category includes 1.67, 1.71, and 1.74.

AVOIDING ABERRATION

High index lenses are thinner and lighter and traditionally feature aspheric and/or atoric designs that boost visual performance as well as the visual appeal. Images courtesy of: (top to bottom) Hoya, Essilor, and Seiko Optical Products of America

There are six classified optical aberrations: Spherical aberration, coma, oblique astigmatism, curvature of field, distortion, and chromatic aberration. Chromatic aberration is the most commonly addressed aberration with high index lenses.

When the eye moves away from the optical center of any lens, it can result in chromatic aberration, which appears as peripheral color fringes or distortion. This is because lens materials refract or bend wavelengths of light at different rates (dispersion).

Dispersion helps separate colors in a prism and can cause chromatic aberration. Because the dispersion rate of high index lenses is higher than standard glass or hard resin lenses, chromatic aberration can be more apparent. Visual symptoms include peripheral color fringes and blur.

Abbe value is the standard parameter to measure relative dispersion of a lens material. Higher Abbe value means lower dispersion, thus lower chromatic aberration.

ASPHERICITY ISSUES

High index lenses usually feature aspheric and/or atoric design to help increase visual performance as well as cosmetic benefit. In addition to providing more cosmetically appealing lenses in plus or minus powers, mid index aspheric lenses can be ground to a 1.0mm or 1.5mm center thickness on most powers, depending on Rx, design, and performance demands on the lens.  

As a rule, finished and semi-finished lenses should not be supplied in the same pair of eyewear, since finished lenses can use flatter curves in plus powers, while semi-finished lenses may have a steeper front curve for easier processing.

Each base curve requires different amounts of asphericity in different Rx ranges for better peripheral vision.

Aspheric lenses require that the optical center (OC) be ground at the geometric center of the lens blank for optimal performance. The general rule is to fit aspheric lenses 3 to 5mm below the pupil center.

Take monocular pupillary distance (PD) using a digital pupilometer. Determine OC by having the patient look into the distance holding their head normally. Dot the center of each pupil on the demo lens with a marker.Drop the lens OC 3 to 5mm below the pupil center; this is fitting height. Power is verified at the OC.

Frame Fitting

For the best high index lens results, fit the best frame for high index lenses. Start by selecting a small frame. A small frame can help reduce or eliminate chromatic aberration as well as keep the lenses at their thinnest and lightest weight. Select a frame with minimal decentration; the patient's pupil should be at or near the center of the frame. Pre-adjust the frame, including face form, pantoscopic tilt, and bridge and temple adjustments. Proper face form—fitting the front of the frame to match the curve of the face—helps reduce backside reflections and also reduces peripheral vertex distance change. Pantoscopic tilt on most frames should be six to 10 degrees. Proper pantoscopic tilt ensures a close fit and the best vision by balancing the vertex in the 90 degree meridian, especially important with multifocal lenses for a wider field of vision.

The thinness of high-index is substantially greater than regular lenses. Image courtesy of Optima

Processing High Index

High index can have some unique processing quirks. Follow these quick tips for easier, better high index results.

Problem: Lenses edge smaller than calculated.

Solution: Adjust the edger by taking down the sizing 1/10 to 2/10 mm at the start.

Problem: Incorrect bevel placement.

Solution: Decentrate at the blocker rather than the optical center and let the edger set decentration. This helps reduce flexing. If the lens does flex, it will flex evenly and will still be read accurately by the internal bevel placement mechanism.

Problem: Coating pulls away when lenses are edged.

Solution: The problem is likely excessive clamping pressure and inflexible blocks. New edgers feature precise clamping pressure and flexible blocks, but older edgers may need to be adjusted to the lens surface instead of the machine. Use blocking tape on the front surface as a cushion.

Problem: Lens coatings craze during tinting.

Solution: Separate neutralizer from tints. Water-based tints should be heated to 205 degrees. Since neutralizer is oil-based and gets hotter (215 to 200 degrees) than tints at the same setting, it can craze lenses. Isolate neutralizer and heat it to 195 to 205 degrees for safe high index submergence.