John, another great video! You can also use a lens clock and lensometer to tell the material. From an article I wrote for a state association news letter a number of years ago...
Determining Index of Refraction
How do we determine the index of refraction of an unknown lens material? Polycarbonate is easy to tell, we all use the “pling” test, verifying the distinct sound they make dropped on a table top (they sound like a poker chip).
But how do we tell CR39 from a mid-index, or 1.60 from 1.67? There’s a neat mathematical trick to get a close approximation. The formula is..
N = .53 X ( TP/CP ) + 1
N = Index Approximation
TP = True Power
CP = Clock Power
True power is what you read in your lensometer. Clock power is your power calculation using a standard lens clock. Let’s go over using a lens clock quickly. To get a “clock power” take your standard lens clock and read the front surface curvature. Next clock the back curve ( for this calculation use the flattest back curve if the lens has cylinder). For a minus lens, subtract the front curve from the back curve to get the clock power. ( For plus lenses subtract the back from the front for the clock power).
Some examples pulled from my stock lens inventory;
Lens #1 Clocks +2.50 front/-5.75 back. -5.75 - +2.50 = -3.25. Lens reads
-3.75 in the lensometer so;
.53 X ( 3.75/3.25 ) + 1 = 1.611 This lens is in fact a 1.60
Lens #2 Clocks +1.25 front/-7.75 back. -7.75 - +1.25 = -6.50. Lens reads
-8.25 in the lensometer so;
.53 X ( 8.25/6.50 ) + 1 = 1.672 This lens is in fact a 1.67
Keep in mind this is an approximation formula. Aspheric lenses can affect your answers because of changing curvature. But this can be a useful formula when trying to determine the index of an unknown lens.
I'll post a link out to this from our FB page.
Thanks!
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