Hands On - Ask the labs
Q&A
by Karlen McLean, ABOC, NCLC

POLARIZED PROOF

Q: Can you recommend the best way to verify that lenses are polarized?

A: Look through a polarized lens at a flat surface that has reflective glare, such as a building or pavement. At the 180 degree position the glare should disappear. Then, if you rotate the lens 90 degrees, the glare becomes visible. Some folks prefer to use a polariscope, a device that helps ascertain and exhibit the properties of polarized light.

— Susie Lesher,
vice president – sales, Brondstater Optical America, Inwood, W.Va.

GETTING CLOCKED

Q: Recently, I dropped our lens clock. Should it still read properly? How can I tell if it's off? If it is off, how do you recommend that I get it fixed?

A: First, check for visible damage such as a cracked crystal, a dented case, or damaged pin. Check for damage to the pin ends by moving the pins of the clock over a plastic lens and checking the lens for scratches.

Second, check the movement of the center pin and dial needle. Note the position of the needle and depress the center pin several times. The pin should move up and down smoothly, and the needle should turn freely and return to its original position.

Third, check the "0" calibration by holding the lens clock perpendicular to a perfectly smooth and flat surface, like a pane of glass. The needle should point to "0." If not, turn the center pin and retest until the clock zeros out.

Fourth, check the lens clock against a lens of a known base curve. Most lens clocks are calibrated to the refractive index of crown glass (n=1.523), so you'll need to use a conversion factor based on lens material.

Fifth, check for pin shaft wear by holding the lens clock perpendicular to the same smooth, flat surface and sliding the clock side-to-side. There should be less than 1/8 diopter movement in the needle. If there's more than this, there is excessive wear and the lens clock should be replaced.

If the lens clock does not check out properly, send it out for repair. Whether or not it can be repaired depends on specific damage, age of the clock, and the manufacturer.

— Thomas M. Sadler, president, Sadler Optical Tools and Findings, Attleboro, Mass.

Going Swimmingly

Q: We have a patient who wants Rx swim goggles. We would like to order the spherical equivalent of her power -4.00 -1.50 x 90. What formula should we use?

A: Typically 40 percent of the cylinder power would be used when fitting a spherical equivalent Rx. For those Rxs with a 90 degree axis, 40 percent of the near is done. So in this case, a -4.62 sphere would be prescribed. The closer to axis 180 you get, the more you would increase the amount. At 180 degrees, 60 percent of the cylinder power would be added.

— Dave Gohman,
customer service manager, X-Cel Optical Co., Sauk Rapids, Minn.

If you have a question you'd like to have answered in Ask the Labs, send it to Karlen McLean. E-mail: mcleank@lwwvisioncare.com. Fax: 215-643-1705. An archive of past Ask the Labs columns can be found on the Eyecare Business Website at www.eyecarebusiness.com.