|A free-form PAL like Seiko’s Superior is backed up by a patent.|
|Lens manufacturers, like Nikon, use a variety of methods to assess a lens.|
|PINPOINT ACCURACY The free-form process basically involves two items-a CNC direct-to-surface generator and point files. The generator uses a precision diamond tool that cuts curves with multiple cutters and a single point diamond cutter controlled by computer software called “point files.” Point files tell the generator how much and exactly where to remove the lens material in order to create the desired surface. This combination can produce virtually any complex multi-curve surface.|
With many PALs being produced using free-form technology, ECPs can follow these tips to determine which ones to use.
The key to finding a free-form PAL that suits your expectations, and the needs of your patients, is to evaluate the lens. Here are some suggestions.
BACKED BY A PATENT
As you begin to research, you will undoubtedly find your way to a lens manufacturer’s website or you’ll speak to a sales representative. As you probe for information, one measure of a lens’ merit you should explore is whether the claims made by the manufacturer are backed by patents. A patent usually means that the manufacturer has developed features and processes that have gone through significant analysis to ensure they are unique. This can additionally reassure you about the lens’ unique design elements.
Lens designers and manufacturers use a variety of methods to assess a lens. One of the most popular ones is the use of contour plots. They show the location and amount of unwanted astigmatic power changes on the surface of a progressive lens.
Designers and lens manufacturers use these plots to help them through the design and refinement stages of progressive lens development. An astigmatic contour plot illustrates the unwanted surface astigmatism of the lens by showing lines of equal cylinder power. The plots are developed by using a special surface power reading instrument that looks at a large number of points on the lens, interprets these powers, then sends the data to graphing software that produces the plot. Cylinder contours are displayed in dioptric increments, usually 0.50D of cylinder power, but any increment can be used.
One of the problems of using contour plots for assessing lens performance is that patients all have varying levels of tolerance to blur from unwanted cylinder: some are very sensitive to the least amount of blur while others find high amounts acceptable. It is safe to guess that the average patient will experience blur with more than 2.00D of unwanted cylinder. This human variation cannot be accounted for by a contour plot.
Another tool for the ECP is the eyepath power profile. This plot illustrates the increase in plus power along the center line of the lens. Some PALs may have an increase in power that builds slowly and steadily while others may start their power increase gradually and then rises rapidly towards the near zone. The eyepath power profile also illustrates the corridor length of the lens and the power at the specified reading zone of a lens. While this information is useful, the ECP must remember that the eye’s pupil size in average daylight is about 3mm, so the eye experiences a range of powers along the corridor, not just a single one.
The only true predictor of lens success is to test the design on progressive lens wearers. This will discover if a design works. Wearer trials take the lenses and test them with the very people who will be wearing them. The results should help you decide if the lens is a keeper or not.
Randall L. Smith is the Opticianry Program Director at Baker College in Allen Park, MI.
WHERE TO FIND IT:
Nikon Optical USA, Inc.
877-767-8033 • nikonlenswear.us
Seiko Optical Products of America, Inc.
800-235-5367 • seikoeyewear.com