How to find material index

[Blake]

It was mentioned in an earlier post that there is a formula to determine the refractive index of a lens based on its front/ocular curves. What is this formula? The only thing I know is the old "drop it on the table and see if it sounds like a poker chip" trick. The problem is, I'm not much of a poker player ;-) .

Blake

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[Dave Nelson]

The formula itself takes the thickness, index, and surface radii into account. The "easy" one is simply F1+F2=net power. For clarity, consider a plano front,or base curve. (plano is plano, regardless of the index.) Using a lens clock,which measures the diopric value of a single lens surface with an index of say,1.49 (plastic)You clock the front at 0.00 and the back at say -6.00. Now check the lens in the lensometer. If it reads -6.00 the index is 1.49, since the clock is calibrated to that index. If the power reads higher than -6.00,its high index. I am certain formulas exist to determine how high the index is, and I leave it to someone else to post it. (Darryl?)but the higher the index, the stronger the lens will read against the "clocked" power. In the previous example, a 1.56 might read as a -6.50 a 1.70 as a -7.00 ect. If checking a toric lens, power crosses can be used to determine the gauged power. Aspheric lenses, for obvious reasons, wont work with this method. (but dropping them to hear the "poker chip" sound does) All the best, D. Nelson

[Darryl Meister]

Hi Guys,

As requested, the formula for calculating the index of refraction is:

Index = CT * Front * (Power - Back) / (Power - Back - Front)

However, this formula is a bit cumbersome, and requires that the center thickness (CT in meters), front curve, back curve, and lens power be accurately determined. (Which is pretty difficult with the kind of devices used in dispensaries.) Moreover, since high-index lenses are often made in aspheric form, it is nearly impossible to measure the front curve accurately (as Dave pointed out). It is generally easier to determine the material type using other empirical methods, like listening to the sound the lens makes on a flat surface, looking for engraved manufacturer logos, estimating by edge thickness, hunting down previous job records, etcetera.

Best regards,
Darryl