Eyeglass Material II, Abbe Numbers

Okay, I’m a physicist, but I kinda slept through the optics part, so I’m okay on the concept of dispersion and having index of refraction being a function of wavelength, but I’m a little weak on just what that Abbe number means in absolute terms. But I do know bigger is bettter (as far as keeping dispersion low). Here’s what I found on materials.

Traditionally, eyeglasses were made of glass (duh), specifically crown glass with an Abbe number of about 60.¹ So that’s more-or-less my target for materials. The worst material for dispersion is the best if you happen to work in a machine shop. Polycarbonate is light, strong, and approved for saftey-glasses. It also has an Abbe number of about 29-31. Hi-index (I think the funny spelling is a trademark, but I’m not sure) is listed at 43 and CR-39, another plastic, is listed at 59-60. CR-39 is also the softest of the three and most scratch prone.

After looking around on the web, Wikipedia has the best short article on materials pointing out what I already found: "Standard polycarbonate with an Abbe value of 30 is one of the worst
materials optically, if chromatic aberration intolerance is of concern."

Sigh. I’ll probably end up with more than one pair, one for night vision with CR-39. Oh yes, and of course, you’re (well, my) ability to focus changes at night being in that over-40 crowd, so the ones for day vision may not work for night. We’ll see how this works out.

¹ The FAA flier on sunglasses says 59. askanoptician.com says 60. Close enough.