|
High index lenses are often the lens of choice for high power prescriptions and rimless mountings. The higher the index the more efficiently the light is refracted, requiring less lens material to provide the same amount of correction as standard plastic or glass lenses.
However, that does not mean that high index is a panacea. Good fundamental opticianry still needs to be maintained in order to arrive at the best looking spectacles for your patient. That means watching sizing and frame shape. Keep the frame shape symmetrical with the A dimension within 2 mm of the ED and try to match their pupillary distance whenever possible.
High index glass
High index glass lenses come in 1.60, 1.70, 1.80 and 1.90 indices. High index glass has excellent optics and scratch resistance and it is thinner as the index increases. However, because of the high specific gravity of these lenses, the weight increases as the index gets higher (See “Specific Gravity” below). The 1.60 index lenses can be hardened and will pass the drop ball test for impact resistance, but the 1.70, 1.80 and 1.90 index lenses generally require chemical tempering. In some cases they may not meet ANSI standards and may require a waiver from the patient stating that he understands that the lenses will probably not be impact resistant. The 1.90 index lenses are not available in the United States any longer.
High index plastic
High index plastic offers the biggest benefit to minus prescriptions, where the edge of the lens is thicker than the center and therefore, easy for the patient to notice. Plus lenses can also benefit from high index materials but the benefit is not as obvious to the patient. However, many high index materials have an aspheric design, which gives them a thinner and flatter profile and reduces the magnification that conventional plus lenses are known to produce. Minus prescriptions and astigmatism also benefit from the high index and aspheric combination because of the lighter weight and thinner profile characteristics, but here again, the aspheric benefits are not as obvious to the patient.
Prescriptions above +/- 3.00 sphere benefit most from high index lenses. However, this rule will vary depending on the individual characteristics (amount of decentration, ED, etc) of each pair of spectacles. Following are general guidelines for materials and Rx powers:
1.67 index Sphere below +/- 6.00 and Cylinder below +/- 4.00
1.70 index Sphere below +/- 7.00 and Cylinder below +/-5.00
1.74 index Sphere +/- 8.00 or above and Cylinder below +/- 6.00
Abbe Value
The Abbe Value of a lens is a measurement of the dispersion of color in the lens as light passes through it. Lenses are basically constructed of two prisms, base to base in plus lenses and apex to apex in minus lenses. As white light passes through these prisms, it is separated into the components of the visible spectrum made up of wavelengths which correspond to the various colors of the spectrum. This separation of the colors after refraction is called chromatic aberration. The higher the Abbe Value the less chromatic aberration. High index lenses tend to have lower Abbe Values, thus producing more color dispersion.
Specific Gravity
Specific gravity is the ratio of density of a lens material to the density of water. The higher the specific gravity of the lens material, the greater its weight. The lower the specific gravity, the less the lens weighs. Specific gravity is measured in grams per cubic centimeter.
There is a large variety of indices, Abbe Values and specific gravity in lenses. The following table shows the components of some of those lens materials.
| Lens material |
Index |
Abbe |
Specific Gravity |
|
CR-39 |
1.498 |
56.8 |
1.32 |
| Trivex* |
1.53 |
44 |
1.11 |
| Polycarbonate |
1.586 |
30 |
1.20 |
| High Index 1.60 |
1.60 |
36 |
1.22 |
| High Index 1.67 |
1.67 |
32 |
1.35 |
| High Index 1.70 |
1.70 |
39 |
1.41 |
|
High Index 1.74 |
1.74 |
33 |
1.46 |
| Crown Glass |
1.523 |
59 |
2.54 |
| Glass 1.70 |
1.70 |
31 |
2.99 |
|
*Utilizes the green wavelength instead of yellow |
While there are many advantages to high index lenses, including better ultraviolet protection than with standard plastic and glass lenses, there are some drawbacks as well.
High index lenses tend to have increased chromatic aberration. The flatter profile and poorer light transmission can cause increased internal surface and backside reflections. Where CR-39 lenses reflect about 7% of light, high index lenses can reflect substantially more. Distortion can be caused by the low Abbe values and off-axis viewing through a high index lens. The farther the gaze gets from the optical center, the more distortion is created. Base curve selection can also cause issues for myopes. This is especially true with stock high index lenses. Manufacturers tend to make their stock lenses on flatter base curves than corrective curve theory dictates to make them appear thinner. These flatter base curves can lead to distortion for the patient.
Recommending high index lenses
When recommending high index lenses:
-
always recommend anti-reflective treatment. This will cut down on the distracting reflections caused by the material and increase light transmission up to 99.5%.
-
suggest the smallest frames possible to decrease the amount of lens material required. This will maximize the benefits of the lighter, thinner properties of the high index lenses.
-
choose a frame where there will be minimal decentration. This will keep the eyes centered in the frame and reduce the center and edge thickness.
Finding the refractive index
The refractive index of a lens material refers to how much the material refracts or bends light as it enters the lens from air. The index is determined by dividing the speed of light in a vacuum by the speed of light going through the material. For instance, the speed of light in a vacuum divided by the speed of light in water gives the index of refraction of water.
n = index of refraction
speed of light in air = 186,000 miles per second (mps)
speed of light in a material = varies
n (water) = speed of light in air (186,000 mps) / speed of light in water
n (water) = 186,000/139,849
n (water) = 1.33
Finding the edge/center thickness
Now that you know the index of refraction of a material, you can calculate the center or edge thickness of the lenses. The formula is:
t = ((d/2) 2 X D) / 2000(n-1)
where t is thickness
d is diameter in mm
D is power
n is index
Using the formula for a -4.00 diopter lens and a 65 mm lens blank of 1.60 index:
t = (65/2) 2 x 4.00)/2000 (n-1)
t = 32.52 x4/(2000 (1.60-1)
t = 1056.25 x 4/2000 x .60
t = 4225/1200
t = 3.52 mm plus center thickness = edge thickness
High index lenses are considered a specialty lens because their properties are more complex and customized than standard plastic and glass lenses. They are an excellent choice for higher prescriptions in order to make cosmetically appealing eyewear. With anti-reflective and other lens treatments, such as roll and polish, high prescription lenses can be as light, thin and attractive as their low power counterparts. Now that you can determine index of refraction and lens thickness, you can provide to your patients the finest the eyewear industry has to offer.
With contributions from: Brian A. Thomas, P.h.D, ABOM
|
