Every eye care professional knows the term high index. Even the newest member of the field is told that high index is better, high index is lighter, high index is thinner.
However, for many it’s a vague term. The average ECP knows the term and the benefits, but do not know the why and whether the above statements are entirely true.
What is Index?
Index of Refraction, or refractive index, is a measurement of the rate of speed that light passes through a substance. In this case, the substance is the lens. Conversely, the index of refraction could be thought of in terms of how much the substance resists the passage of light through it. The index of refraction helps manufacturers calculate the refractive power of the lens and also the dispersive power of prisms within certain materials. To determine the index of refraction, the ECP takes the speed of light in air, divided by the speed of light in the lens material.
N = speed of light in air
speed of light in material
Since it is known that the speed of light in air (C) is 186,000 miles/sec this is constant for the ECP. So, if a lab is trying to determine the index of a lens polymer, and they know that the light within the medium is traveling at a speed of 118,000 miles per second it can be determined that the index of refraction is 1.57.
Why is Index Important?
Index of refraction is a guide that tells us how much the lens refracts, or bends, light. Refraction, when used in the optical system, occurs when light rays travel in the air at one angle and then passes into the lens and is bent at another angle. This phenomenon is best explained by Snell’s Law.
Snell’s law shows the relationship between the angles of incidence and the angles of refraction that occur when a ray of light passes from air into the lens. First, the angles must be determined. To do this, the ECP must envision an imaginary line called the normal. The normal is a line that is always drawn perpendicular or at 90 degrees to the surface of the lens at the point of incidence, or where the ray enters the lens. The angle of incidence is the angle formed by the ray of light as it approaches the lens surface and the normal. The angle of refraction is the angle that is formed by the normal and the degree to which the entering ray of light is bent.
The angle of the incoming ray is equal to the angle of reflection. The Angle of Refraction
is determined by the ratio of the sines of the Angle of Incidence to the Angle of Refraction and the ratio of the dielectric constants for the upper and lower layers.
Let’s imagine that an ECP is designing a lens and is using crown glass with an index of 1.523. Since the lens is in air we know that the incident medium, or medium one, is 1.00 because air has an index (N) of 1.00. The angle of incidence is 30 degrees. By using the formula, the lens designer knows that the angle of refraction of the lens is 19 degrees. If the lens designer picks a new lens material with an index of 1.67, the angle of refraction of the lens becomes 17 degrees. The narrower the angle, the more the light is bent by the material, therefore the less material that is needed to bend the light to the desired power. The result is usually a thinner and sometimes lighter lens.
Why do high index lenses have a greater problem with reflections?
Internal reflections occur within the boundaries of a lens when a ray of light within a transparent material of higher index approaches another medium of lower index at more than the critical angle. When light enters a lens, since the lens is a different refractive index than air, the light beam is partially refracted, partially reflected and/or partially absorbed. The rays that are refracted within the lens, then move to the lens boundary. Most will pass through the lens and exit immediately, but some will be reflected internally before they leave the lens. These that are reflected internally are bound by the rule of critical angle. The formula for critical angle is used to determine how much internal reflection a material will have when the index of the material is known.
The critical angle formula for a lens is:
Sin i = 1/ n
Using the above formula, the ECP knows that the critical angle of a lens with an index of 1.523 is 41 degrees. This means that all light rays traveling within a lens at an angle to the lens boundary at an angle greater than 41 degrees will be reflected internally. If the index of the material is 1.67, the critical angle is 37 degrees. Since the critical angle of the 1.67 lens material is lower, that means more rays of light will be reflected within the lens material. Therefore, the ECP now knows that the higher the index of refraction the higher the rate of reflections within a lens.
This translates to less light exiting the lens and reaching the patient’s eye. Naturally, this would impact visual acuity. This explains why some lens materials are only available with anti-reflective coatings. Without the coatings, the patient’s visual acuity would suffer from the lens selection. With the coatings, the lens will transmit nearly all of the incident light rays and visual acuity will actually improve.
Why do some materials have more rainbows than others?
As explained earlier, refractive index describes how much light is refracted or bent from its original pathway. Since white light is composed of multiple colors, each color within the spectrum is bent at a different angle than the other during refraction. Therefore, each component of light has its own refractive index. Blue light has a higher refractive index than red light and is therefore bent more than red when it passes through a lens. Since high index slows light within a material down, the higher the index of refraction, the more dispersion will occur. The result is chromatic aberration or dispersion. This dispersion results in a chromatic aberration that can, at times, be observed by the patient. However, most frequently the complaint is that off axis viewing is more blurred than central viewing due to the color images overlapping one another.
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The degree to which a lens disperses light is commonly referred to as Abbe value or constrigence. Lenses with a high Abbe value have less chromatic aberration than those with a lower Abbe value. Generally, higher index materials have lower Abbe values than the conventional lens materials of CR-39 or crown glass. Fortunately there are new higher index lens materials that are exceptions to this rule.
How can an eye care professional minimize problems with high index materials?
Fit
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Assist the patient in selecting frames that fit close to the eye, fit well along the bridge, and are able to be adjusted properly with a minimum amount of vertex distance and sufficient lens tilt. The less the vertex distance needed, the better the peripheral vision and the fewer problems with chromatic aberration.
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Keep the effective diameter (ED) within 2mm of the frames A dimension.
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Select frames that allow the eye to be centered within the lens so that the ocular position coincides with the amount of tilt in the frame. Take monocular pupillary distances and optical center heights and decenter vertically when necessary. Keep decentration to a minimum. For every 2mm of decentration you are effectively increasing the lens thickness by one diopter of strength. If you decenter a minus 2.00 diopter lens 6mm, it will have the thickness of a minus 5.00 diopter lens.
Lens Selection
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Use the newer, higher quality high index lens materials when required, but do not expect miracles. Good opticianry techniques, like those described above, will provide better results than poor opticianry with a high index lens material.
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Use either an aspheric lens design or select a lens with the best form base curve for the patient’s prescription. This minimizes some of the aberrations that compound the problems produced by chromatic aberrations.
Use Anti-Reflective Coating
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Reflections and chromatic aberrations are reduced
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Less eye fatigue due to greater light transmission
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Better cosmetic appearance
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Enhanced visual acuity
Conclusion
It is no surprise that high index lenses are part of the wave of the future in the optical industry. High index lenses can help to provide the best solution for some patients with higher prescriptions and an eye for fashion. With the proper understanding of the technical aspects of high index lens materials an eye care provider can give the best possible vision solution to their patients.
