Q. More about Computers

In your "Ask the Labs" section of March 2000 Eyecare Business, you answered the question about computer lenses. Your answer was incomplete. No discussion of available computer lenses should omit the AO Technica. This was the first lens specifically designed for computer specific tasks and is still available. It also differs from many other computer lenses in that it does have a quite usable distance portion.

-David Schurgin, O.D., FAAO Stoneham, Mass.

A.
We didn't include the Technica because the reader had asked about variable focus lenses. Technica does not fall into that precise category because it is a true progressive and provides a distance window, unlike variable focus lenses. It is an excellent lens for computer users however.

Q. A Paradox
 I see plano wrap-around sunglasses advertised with the claim they have polycarbonate "decentered" lenses. How do you decenter the optical center in a plano lens when a plano lens has no optical center?

A.
The following comes from a technical paper written in 1982 by the late Donald B. Whitney of American Optical: 
We find the answer (to decentering a plano lens) in an examination of the famous Prentice's Rule, still one of the first things taught in ophthalmic optics. This famous rule, however, is not valid for low powered prescriptions less than 1.00D. Consider a plano lens in glass with a convex curve of +6.00 diopters and a center thickness of 2.2. If we perform the calculations, we find the rear curve must be steeper than -6.00 diopters if the lens is to have zero power. (His calculations indicated a rear curve of -6.052). 
If a lens has the same curve on the front and rear surfaces, it will have plus power due to lens thickness. Performing all the calculations listed in Whitney's paper reveals that:

• A plano lens does not have the same curvature on each surface
• A plano lens does not have surfaces with concentric radii
• A plano lens has a steeper concave than its convex curve
• A plano lens does not conform to Prentice's Rule
• A plano lens has an optical center

Q. Gold is where you find it

A.
As a rule, blocking that much infrared light is not normally a concern for sunglasses. You may have to address this issue the way NASA does. In space, blocking infrared in an astronaut's face shield is critical, more for preventing heat build-up in the space suit than for protecting eyes. If IR were not completely blocked, temperature in space suits would rise to dangerous levels. NASA utilizes a pure gold coating applied to the face shields, which blocks virtually all infrared.

-Dan Torgersen, OLA technical director

Editor's note: We checked with a leading ophthalmic lens coater to see if it is possible to apply a pure gold coating to ophthalmic lenses and at what cost. The response: We've never had a request for pure gold coating on ophthalmic lenses. It's certainly possible. It's costly because the process will literally coat the chamber walls as well as the lenses with gold. The wholesale cost of the process would likely be in the neighborhood of $500.

-Edwin Ellefsen, Opticoat, Franklin Park, Ill. EB

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