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The prescription is the lens formula that describes lens power and in which meridian the power is located. Being able to visualize lenses is the starting point for picking the right choices that turn lens formulas into comfortable, attractive and wellperforming eyewear. Topnotch opticians see the final eyewear design in every lens formula. The formula is made up of the sphere power and any cylinder, axis and any presented prism.
First, the lens axis describes the two major meridian locations of the prescription. Lens powers are separated into two principal meridians, the axis and 90° from the axis. Figure 1 illustrates the standards used to describe the prescription axis. Zero is always to the left of each of the patient’s eyes, 90 above, 180 on the right. Axes are written only from 1 to 180 degrees because lens power rotates around a center and whatever happens below center happens above. Therefore, the axes 181-360 degrees are not used. Also, since zero and 180 mean the same angle, only 180 is used.
Here are the rules to determine power. There is only sphere power at the axis, .90 degrees from the axis the power is the sum of the sphere and cylinder values. In the Rx (Figure 2): OD +1.00-0.50 x 005. For +1.00 -0.50 x 005, at the axis, only sphere power or, +1.00D and 90 degrees at 95, the sum of sphere and cylinder, +1.00D and -0.50D is +0.50D. The power in the two meridians is +1.00D@005 and +0.50@095 for the right lens and +0.75@10 and Plano@100 for the left.
Follow the examples in the table, cover the answers and test yourself.
Axis and Eyewear Selection
Top-notch opticians see the final eyewear design in every lens formula. This helps to choose whether to recommend thin and light lenses or another benefit like photochromics depending on frame or budget concern. For example, in a - 3.00-1.00 x 180 Rx, the power at 180 is -3 diopters. Let’s say that due to the decentration and frame size, the lens diameter required is 62mm. Here are the thickness comparatives one should know. At this diameter, the edge thickness in plastic is 5.1mm (2.0 ct), 4.1mm for poly (1.5ct) and 3.6mm (1.3ct) in 1.67 ultra high index and 3.4mm (1.3ct) in 1.74 index.
Next measure the edge thickness of the frame and teach the patient how much the lens will show or not. Discuss the opportunity for a lens that shows less. In this way patients understand what their final glasses look like and the extra costs for thin and light lenses are justified. Also, when possible use frames that hide edge thickness if the patient is sensitive to it.
In the illustrations of frame eyewire thickness, one can see how matching frame width and lens material can ensure little or no lens thickness visibility.
Power at Any Meridian So far we’ve learned the power at each of the principal meridians, the axis and 90 degrees from the axis. What happens in between?
A good example is the Rx -2.00-2.00 x 45 i.e., when the axis is not 90 or 180. What is the power that creates the edge thickness at the furthest edge of the frame along the 180? This is the thickness that people see.
1. Determine the difference in degrees that the 180 is away from the axis and use the table to estimate the power at 180. 2. Multiply the cylinder power by the percent. 3. Add this to the sphere power to solve for the power at 180.
In this case, the axis is 45 degrees from the 180 or half the cylinder would occur at the 180. 50% of a - 2.00 cylinder is -1.00 diopters. Added to the sphere power the result is - 2.00 and -1.00 or -3.00 along the 180. It’s also logical; see the illustration of the two power meridians. A -3.00 is halfway in power and halfway in distance between the two principal meridians.
Use this to estimate the power in any meridian regardless of prescription and axis.
Prescription Precision Precision in prescriptions starts with the refraction. Understand that at the end of the refraction, the doctor asks the patient to choose 1 or 2 but when the doctor has done the right analysis the patient has little discrimination left to be sure about the right choice. As a result, they can leave the eye exam thinking that they made the wrong choice. This affects their confidence in the prescription and their confidence in spending many hundreds of dollars on a pair of glasses. You can change that.
First, the doctor should reassure the patient that they’ve made the best choices. Also tell patients that the end of the eyeglass exam tests there may be no difference in the way they see with either choice. In this way, they’re reassured about the prescription and spending for a variety of optical benefits.
Next steps? Ensure that everything from here on requires the most precise replication of the prescription in the best lens design and material. Add AR to all lenses since that ensures the best realization of the vision goals of any office. Photochromics and polarized enhance the prescription, meet every patient want and add safety and comfort. Demand the best from yourself and from those that supply the prescription. Use the latest technologies and fashions, learn their attributes and describe the brand, vision and comfort benefits with ease.
{Sponsored by Essilor of America and Luxottica Group} www.varilux.com • www.crizalpro.com • www.luxottica.com
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