Nov
2004

L&T 101


An off-axis lens can cause headaches, eyestrain and visual disorientation before patients realize their eyes will not accommodate for the errors in their new eyewear. Off-axis error is often a result of operator and/or equipment mistake occurring during blocking, the initial stage of finishing, which most lab techs agree accounts for the lion share of finishing-related spoilage. “Finishing is dependent on accurate blocking and layout,” says Rob Cunningham, CEO of Cunningham Lens, a wholesale lab in Marlboro, Mass. “More than half of finishing-related spoilage, especially a lens being off-axis, happens at layout and blocking.”

Finishing blocking has three steps:  (1) lens inspection and verification in the lensometer; (2) lens layout—decentering the lens within the parameters of the frame; and finally (3) blocking the lens—affixing the lens to the finishing block. The lens and block assembly is then placed into the edger’s clamp or chuck, so the lens can be cut to the correct size and shape. The success of that edged lens is often dependent on finishing blocking. “If something is wrong with the blocking, you won’t know the lens is spoiled until after it’s edged,” says Fred Gross, optician at Bridge Opticians in Fort Lee, N.J.

Automated Blocking
Today’s blockers—which can be purchased as separate units or as part of an edger—automate what used to be manual tasks. In pre-automated blocking, after lens verification in the lensometer, technicians then marked the optical center of the lens, placed the lens in a “marking device” and using frame measurements and decentration formula, manually aligned the lens for distance decentration, axis and proper seg-height. The device stamped a mechanical marking on the lens, which served as a guide for where the lab technician affixes the block to the lens.

This process is automated by the latest generation of finishing systems. The blocker interfaces with the tracer; computer software calculates the optical center and aligns the lens according to the tracing parameters. Some blockers feature a lensometer in the blocker. With newer systems, the lab technician verifies lens power, then marks or “spots” the optical center in the lensometer before placing the lens on the blocker so it can read the mark. Following computer prompts, the lab tech inputs PD and other data into the blocker keypad. Lens alignment is automated; the unit moves the lens into proper position, further eliminating operator-error risks, particularly parallax error.

The blocker’s computer screen shows a cut out of the final lens shape from the tracing data, indicating whether or not the edged lens will be on axis. With lens lay out complete, instead of affixing the block by hand, the automated systems feature a small “arm,” which holds the block and positions it in the correct location on the front surface of the lens. A leap pad, which has a double-sided adhesive, affixes the block to the lens.

“You still need the optical knowledge with the new systems,” says Jay Binkowitz of American Eye Care Center in Queens, N.Y. “But once the lenses are marked, or in the case of progressives where you find the manufacturer’s laser markings and you enhance them—then the new automated technologies finds those markings, decenters the lens then blocks the lens. There is less handling of the lens which is always a chance for error.”

Tracing Interface
The interface between the layout/blocker and the frame tracer is key to the effectiveness of the new blocking technology. “A frame trace has more accurate information than using a ruler and calculating the decentration,” says Nicolas Snell, lab optician for Northern Illinois Optical in Rockford, Ill. “The computer memory also stores that tracing data, so when we’re just replacing the lenses for a patient, we can block and layout the lenses as they come in and edge them, so when the patient returns it’s just a matter of inserting the lenses into their frames.”

The popularity of three-piece frames makes the tracer-blocker interface particularly valuable because technicians can manipulate the shape of a lens while the computer determines its new decentration. “When you change the shape of a lens for rimless jobs, you are changing the A and B measurements of the lens, usually the B measurement, of what is essentially the frame shape,” says Binkowitz. “This moves the optical center and in the case of multifocals, the fitting heights. As you alter the original tracing on the grid, the computer calculates the new optical center.”

Polarized Miscues
Technicians emphasize that in spite of the advantages of the new blocking systems, three popular product categories—polarized lenses, smaller eye-sizes and hydrophobic, anti-reflective coatings—can create potential spoilage for uninformed lab technicians.

Some blocking units can have difficulty reading the optical centers of the darker polarized lenses. Robin Genden, vice president of Bayview Optometrics in Mashpee, Mass., has found that that the CD-R marking pens in different colors avoids problems with polarized lenses. “There’s no problem verifying polarized lenses in the lensometer, but in the blocker, there are some brands where the markings can’t be read, so we use a white or red CD-R marking pen.”

Block Selection
With smaller eye sizes, the pitfall is not in lens layout, but in block selection. In general, most blocks come in full size and half-eye sizes, the latter used for half-eye readers and children’s eye sizes. Current adult frames feature small eye-sizes, often requiring the half-eye block. “With the eye sizes being so small, we’ve been using a half-eye block for a lot of the newer frame styles, even though they are not half-eyes,” says Genden. “With the smaller frame styles, the full size block causes slippage.”

Many of the newer systems will prompt users to select the appropriate block, although there are some borderline eye sizes and a call to the edger manufacturer and/or frame company may be necessary. “There are some styles where the half-eye is better, no matter what the computer prompt says, but knowing which ones comes with experience,” Genden notes. Less experienced operators, when in doubt of which block to use, should contact their blocking unit’s manufacturer.

Leap Pad Crisis
One of the main causes of slippage, which usually results in off-axis lenses as well other problems ranging from lens warpage to cracking, is leap pad compatibility with the lens. “With the new blockers, the more than likely cause of off-axis lenses, is slippage, which is often due to the leap pad,” says Binkowitz. “Some leap pads don’t work as well with some edgers. Sometimes, the leap pad manufacturer will change factories, and suddenly you notice that one of 10 lenses is being cut off axis.”

Leap pad problems are also being reported with the latest generations of hydrophobic anti-reflective coatings—they are not consistently sticking to the new AR. Explains one leap pad supplier, “We’ve been getting a lot of complaints and returns of leap pads not working with the latest hydrophobics. Apparently, the new hydrophobics were developed without anyone thinking about leap pad adhesion. Leap pad manufacturers are working now with AR companies on the problem.”

Some lab techs use an “old optician’s trick” of applying hair-spray on a lens to make it “stickier,” although there is some concern about AR crazing. Other techs have gone back to using “surface-saver” tape (also called lens protector tape, or, because of its translucent color, blue chip tape) over the lens, to ensure leap pad adhesion. “I use the protective tape as a matter of protocol and have had no problems with slippage because of a leap pad,” says Gross. “I like the extra protection, especially with AR, even though edger manufactures claim you no longer need it. I make sure to trim the tape within the parameters of the eye size. If the cutting wheels of the edger cut the tape, it usually doesn’t cause a problem with edging, but it can gum up the cutting wheel for the next job.”

Blocking Basics
1 Verify lens powers in lensometer before beginning the finishing blocking procedure.
2 Use non-acetone markers when “spotting” the lens. Some polarized lenses may require a brighter color marking in order to be read by the blocking/layout unit.
3 Select the proper block. Some smaller frame styles may require a half-eye sized block even though the frame is “full-size.”
4 If lens protective tape is used, trim so it is within the parameters of the desired lens size in order to avoid undue cutting of tape by the edger’s cutting wheel.
5 Make sure your leap pad will adhere to the AR coating of the lens before edging. Follow the leap pad recommendations of your equipment manufacturer.

 

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