hands on
ASK THE LABS
by Karlen McLean, ABOC, NCLC

AR STEPS

Q: What are the steps that are generally involved in the AR coating process?

A: First, lenses are inspected for defects and then hand cleaned. Next, lenses are cleaned in an ultrasonic lens washer and rinsed and dried through a process based on the lenses' index and material.

A foundation hardcoat is applied, and then the lenses repeat the ultrasonic cleaning process and rinsing and drying steps. They are then oven cured to remove moisture. Now the lenses are ready to have AR applied in a negative vacuum atmosphere environment. After the coating is applied to the convex surface, the lenses are flipped and the coating process is repeated to the concave surface.

The last step is the application of a hydrophobic coating, similar to the AR process via a negative vacuum atmosphere environment. From start to finish processing takes about four hours. Also, manufacturers may add proprietary methods to the process.

— Keith M. Cross, vice president, and Edward Labarre,
AR coating manager,
Northeast Lens Corporation,
Newton Upper Falls, Mass.

EXTRA STRENGTH

Q: What's the different between tensile strength and flexural strength in a lens?

A: Tensile relates to a pulling action. Flexural relates to bending. Tensile and flexural lens strength are tested using two different methods.

On impact, glass, an inflexible material, receives tension on the outer edge when pressed or hit in the center. Ninety percent of the time, the fracture origin of the break (tensile strength) is on the outer edge.

On impact, plastic material gives and breaks in the center. Backside coatings help with scratching, but can deplete the strength of a plastic, polycarbonate, or Trivex lens. The results are measured in inches—the FDA minimum is 50 inches. We use 85 inches of strength as a safety margin, as that allows typical variations in the backside coating and will likely still meet FDA requirements.

Different coatings have different qualities, and coatings change over time. An older bottle of coating that's opened a lot can thicken and will weaken the lens. A new UV lamp in the curing oven can create brittleness.

Process control is important in order to maintain a safe lens.

— John Young, president, COLTS Laboratories, Clearwater, Fla.

WATERWISHES

Q: How can I change over my edger system that recycles water from a bucket to a system the uses only clean water? The maze of hoses is a little confusing—from one-in, one-out to two-in, two-out, with solenoids connected to each set of hoses.

A: It should be easy to change your current system to one water line. The solenoid will be on the water line going into the edger using fresh water; one water line on one source. We operate three edgers on one freshwater line. Our edgers don't use pumps. We have one hose for water into the edgers and one drain out of the edgers.

— Barbara Wooten, production manager,
Dietz Laboratories, Inc., Fort Worth, Texas

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.