Going Premium:
Providing your Patients with the Best in Aspheric Lenses
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In today’s tough marketplace, it is important to differentiate the professional optical dispensary from one in which just orders are taken. One way to do this is to offer premium products to the patient. In order for this to happen, one must be knowledgeable of the latest in lens technology. One such technology is aspheric lenses.
Aspheric Lenses
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Aspheric lenses -just like the name implies - are not spherical. In other words, instead of one curve on the front of the lens, and one or two curves on the back of the lens, the front curve of the lens is designed to have a gradual power change from the center of the lens to the periphery. This has multiple benefits to the patient. It minimizes or eliminates the blurry images created when the eye rotates away from the optical center of the lens, also known as marginal astigmatism. It also allows the lens to have a flatter profile. This creates a better looking lens that not only doesn’t compromise optical quality, but actually enhances the optical quality. The result is a lens that is flatter and thinner, has a wider undistorted field of view, fits better in most frames and reduces the magnification and minification of both the patient’s eye and their retinal image size. |
Creating the Aspheric Lens Design
Computer programs are needed to create the gradually changing, multi-curve surface of an aspheric lens. These computer programs use a polynomial, mathematical equation to determine the rate of flattening the lens surface will have. The reason each lens manufacturer has a different asphericity in their lens design is because each one uses different coefficients in their polynomial equation. Most aspheric lenses are either a true aspheric or semi-aspheric. In a true aspheric lens the change in curvature begins at the optical center. The curvature can begin steep or it can begin at a more gradual rate. In a semi-aspheric lens, the lens has a spherical center in widths varying from 10mm to 45mm. Although a semi-aspheric lens is not as cosmetically appealing as the aspheric lens, it is still thinner than a conventional spherical lens and is more forgiving if the fit is off.
Fitting Aspherics
Although accuracy is important when fitting any patient, aspheric lenses are not as forgiving as some other lens designs. Therefore, it is important that the ECP brush up on his or her fitting skills when designing a pair of glasses using aspheric lenses.
Rules to fitting aspheric lenses include:
Following the manufacturer’s recommendations - Because each manufacturer designs their own proprietary computer systems to create their lenses, each one has unique fitting guidelines. Know and utilize the guidelines for each aspheric lens that you use.
Use a pupilometer - Take monocular PDs.
Center the eye - The frame selected should have the eye in the center of the lens, both vertically and horizontally, as much as possible. However, the majority of the time you will find that the eye sits above center vertically so try to limit the asymmetry.
Measure the optical center - This is the point most ECPs either forget to do or perform improperly. First, dot the pupil just as one would do when measuring a progressive lens. Measure, and then drop the height 1mm for every 2 degrees of tilt, but do not exceed 5mm. If it exceeds 5mm, refit the patient with a better frame for him or her. The alternative method is to have the patient raise his or her chin until the glasses are perpendicular to the floor, have the patient look at the ECP and then dot the pupil while the patient remains in that position. Both of these methods are done to help reduce the amount of induced spherical or cylindrical power in the lens.
Adjust, adjust, adjust - Good frame adjustment is key, both at the fitting and the dispensing. Ensure that the frame fits properly, is comfortable and has the correct amount of pantoscopic tilt and face wrap. It does not matter how much time was taken in measuring for the lens if the frame does not fit properly. If the frame is crooked, too tight or too loose, the optical center will not follow the optical axis and the patient’s vision will be off. Proper adjustment is crucial both at the time of fitting and at the time of dispensing. Make sure that the weight is distributed properly along the fitting triangle of the bridge of the nose and the top of the ears. It is important that the frame front is level and is placed at good vertex distance from the eye. Adjustment to the temple and along the temple bend will keep the lenses stable in relation to the optical axis. Also, make certain that the pantoscopic tilt is proper for the OC placement.
Use Anti-reflective Coating
Most aspheric lenses are flatter than conventional lenses, so they create a highly reflective, mirror like surface. Also, aspherics are generally manufactured with high index materials, so increased internal reflections are also a consideration for the eye care professional. One way to help with this is to utilize a multi-layer AR coating. The use of a multi-layer AR coating can bring the amount of light transmission into the eye to 99.5%, thereby improving visual acuity. Also, a multi-layer AR coating reduces the amount of reflections that can occur from the flatter lens surface. Premium AR coatings should be scratch resistant, as well as repel oil, water and dust.
Comprised of a series of layers, the AR coating consists of metal oxides, adhered to the lens in thin films. Each film is designed to cancel out a specific light wavelength. The more layers of film on the coating, the more wavelengths that are blocked and the higher the light transmission of the lens. Working by the principle of destructive interference, multi-layer AR causes the light that reflects from the inner and outer surfaces of each film layer to become equal yet exactly opposite and thereby canceling each other out. In addition to the metal oxide layers, premium AR coats have:
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Oleophobic layers that make multilayer coatings more effective by reducing smudges.
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Hydrophobic layers that are applied as thin layers of silicone on top of the AR. It provides a smooth surface that decreases the wetting angle of the lens. The result is that liquids form small beads and can be easily removed from the lens.
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A secondary top coat that is a result of the seal that occurs when oleophobic and hydrophobic layers are used
together
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An Anti-static or Electro Magnetic Interference (EMI) layer that is placed on the lens to prevent static build up. By reducing static on the lens, airborne particles are a lot less likely to be attracted to the lens and adhere to it.
Conclusion
Patients go to a practice because they expect the best, and it is the job of the eye care professional to give it to them. It is imperative that the patient be offered the best in lens technology to fit his or her needs. Aspherics are an excellent tool for the ECP to use to give the patient the thin, lightweight and visually superior lens he or she desires.
With contributions from: Brian A. Thomas, P.h.D, ABOM
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