What is the index of an unknown lens?

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Experienced practitioners may be able to identify some of the higher-index materials, for example the unique sound a polycarbonate lens makes when it is tapped. But as professionals you need to demonstrate skill, not guesswork (Table 1).

Imagine the scenario where a patient says that he is wearing a thinner lens, he remembers paying extra for it.

Your new patient is wearing a 1.7 index lens, but you had assumed that it was 1.6. Your patient would be happy that your lenses were cheaper (half the price in the above data) but may decide that the lens was not as thin as the previous one, and insists that you remake them. Conversely if you assumed that the lenses were 1.8 and dispensed that, the patient may think that you have over-charged him, and not appreciate the decrease in thickness. Had you calculated the index and dispensed the same, the patient would have been fairly charged, and received the thickness of lens that he was expecting.