|CENTERING A LENS Some lensometers have a “1/2 of B” measurement indicator adjacent to the lens stop. The lever that moves the stage up and down has a tab that points to a scale in millimeters. When a lens is centered, the scale tells you how many millimeters it is from the center of the lens stop aperture to the stage. If you add the supplemental plastic stage, this indicator will read incorrectly.|
Each issue, Tech Tips will explore some interesting aspect of optical technology. This month we look at how to adapt a lensometer in order to read base curve eyewear.
You can craft a device for wrap sunwear to add to your lensometer’s stage.
From humble beginnings in 1919 when Sam Foster started the Foster Grant company and started selling this new product on the boardwalk in Atlantic City, sunwear has been a fashion accessory many Americans appreciate and enjoy. In fact, Foster Grant sunwear became a household name in the ’60s when the company ran a nationwide ad campaign with the slogan, “Who’s that behind those Foster Grants?” and used top celebrities of the day in their ads.
Today’s sunwear has evolved a good deal from those styles and have morphed into some pretty impressive offerings. One of the most popular styles is the wraparound model or “wrap” style for short. This design incorporates the use of a high base curve (usually 8.00D or higher) lenses with an aggressive frame styling that has a steep face form tilt (wrap angle) to it.
The use of steep base curve lenses causes a dilemma when trying to read the power of sunwear prescription lenses using a lensometer. The problem is that the frame is so steeply curved that the bottom of both eyewires won’t rest on the lensometer’s stage. Fig. 1 (right) illustrates this problem.
Notice how the left eyewire in Fig. 1 rests properly on the lensometer’s stage but the right one hangs off the back of it. The lensometer’s stage was developed to accommodate low base curve eyewear that is found with most clear ophthalmic eyewear styles. The purpose of the lensometer’s stage is to enable both right and left eyewires to rest on it so that their respective lenses are in identical vertical alignment in order that optical center placements and vertical prism can be properly read in the instrument. If the lenses cannot be identically aligned this way, they cannot be correctly read in the instrument. What’s needed is some way to get both lenses onto the lensometer’s stage just as if the frame had much less wrap.
The solution to this problem comes from industry veteran Kathryn Gross-Edelman at Pech Optical Corp. Edelman’s lab team developed a supplemental plastic stage you can make and add to your lensometer’s stage. Fig. 2 (right) illustrates this device. Using a piece of hard acrylic sheet material sometimes called Plexiglass (which you can purchase from a local hardware store), the lab cut a “U” shaped design the width of the lensometer’s stage and about twice its depth. The supplemental stage can be held in place with double-sided tape, edger blocking pads, or some other inventive method. You will want to take this stage off from time to time so use a securing method that allows you to do that easily but which keeps the stage in place securely while being used for reading wrap sunwear.
With your supplemental stage securely in place, position your wrap sunwear on it. As you clamp one lens into place, notice that the other lens is resting comfortably on the extended section of the supplemental stage. The more important issue is that the frame is resting properly aligned on the supplemental stage. With this alignment, you can read your wrap sunwear the same way you would read any other prescription eyewear.
This solution is ingenious although it takes a little work to craft it. Having a supplemental stage takes the alignment problem out of reading wrap sunwear. The small amount of time and money it takes to make this device is well worth the reward.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.