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Fix and Fit

Knowing Lens Thickness Makes You Look Good

Eyecare Business December 1, 2013
fix and fit

Knowing Lens Thickness Makes you Look Good

Alex Yoho, ABOM

When a patient walks in with a new prescription in hand, one of his first questions is, “How thick will my lenses be?” This gives the dispenser a distinct advantage in controlling the frame selection to design eyewear that will be both practical and beautiful. If we want to determine the thickness of a lens, there are several ways to answer the patient’s question.

THINKING THICKNESS

If we boil down the main factors that control thickness, we can come up with a good approximation.

■ MATERIALS MATTER. First there is the thickness that ANSI requires. This is either the center thickness on a minus lens or the edge thickness of a plus lens. For common materials such as regular plastic or glass, this is about 2.0 mm. Tougher materials such as polycarbonate or Trivex can be made thinner, usually 1.0 to 1.5mm. Depending upon the material, we start with this thickness.

Number Crunching: THICKNESS CALCULATIONS

If we create a formula using the thickness-causing factors described in this article, we can approximate the thickest point on a lens:

Thickness = thinnest point + (1/2 Diameter)2 × (power) / 2000 × (index - 1)

So, with a CR-39 Rx of −4.00 in a 55mm frame:

Thinnest point = center thickness is 2.2mm

(27.5)2 × 4.00 / 2000 × (1.50 − 1)

3025/1000 = 3.025

2.2 center thickness + 3.025 = 5.225 edge thickness

■ CALCULATING CURVES. Second, the power of the lens creates thickness as a result of the curves that cause the power. In exact thickness calculations we would consider both surfaces, but for approximating the thickness we will think of the lenses as having a flat surface on one side and the Rx power as the second curve. Of course, the more power, the more curvature (or Sagitta depth), which means more thickness.

■ DETERMINING DIAMETER. Next is the diameter of our theoretical lens. On a minus lens, the edge gets thicker the farther away from the center we get. On a plus lens, we must add thickness to get the curve to stretch out to the edge at a given diameter.

■ INDICATING INDEX. Index of refraction is the last thing to consider. Each material has its own index of refraction, which is the material’s ability to bend light as a result of its density. So, a material with a higher ability to bend light will need less curvature to produce the same power. This means thinner lenses.

Keep in mind that eyesize can’t be the sole determinant of the diameter. We must include the decentration plus the eyesize as the total diameter.

With a bit of practice you’ll be calculating lens thickness and really impressing your patients. The best part is they will have a good understanding of why you are recommending the frames and lens materials for them and why your knowledge is a benefit to them. EB

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