Wednesday, November 17, 2010
Why not achromatic eyeglasses?
I've been reading about the history of camera lenses, and it got me thinking about the yellow and purple fringes that I always see around bright white objects. Why do my glasses have such poor chromatic aberration? I will spend all of my adult life looking through them, so I'd like them to have a bit higher fidelity. Do they make achromatic eyeglasses? Would it be too expensive? Too heavy?
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An achromatic doublet would be significantly thicker and heavier than a single element of the same correction - you have to combine a positive and a matched negative lens, so you have to increase the curvature of both to increase the prescription, which makes the entire lens thicker.
ReplyDeleteSince it is generally only noticeable for people with higher prescriptions and color sensitivity, it would be of limited use. It is actually cheaper to select a material with less chromatic abberation (higher Abbe number) than to try to correct by using doublets
It would also be much more expensive, as the curvature at the junction of the elements would have to be precisely the same. Photographic lenses deal with this by designing the doublet once and making a lot of them at the same time, also photo lenses don't have to worry as much about weight.
It might also be more trouble-prone, as you now have a glued surface in the middle of the lens.
I still believe we must be at a point where we have suitable plastics with different refractive indexes and ABBE values and the engineering ability to create lightweight achromatic doublets suitable for spectacles. Also remember that spectacle lenses are mass produced and cut to fit a frame; it wouldn't need any more complicated fitting procedure than any other type of lens.
ReplyDeleteOf course I do expect such lenses to be thicker and heavier than is available and certainly a lot more expensive, but I'm surprised the option isn't out there already.
By stacking 2 layers you could probably halve the minimum thickness of each layer because the mechanical strength of the lens is likely to be the main limiting factor. The cost is likely to be high -- where have I heard that before? They make regular lenses CD-sized and discard 80-90% of the material, so the base-line cost is obviously not such a big deal. Even if welding 2 superior plastics together makes the lens 10x more expensive as it's shipped out of China, I'm sure the optometrists wouldn't notice the slight dent in their astronomical mark-ups.
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