Frame EDs, Friend or Foe?
Frame Measurements have become increasingly important with the addition of digital lens designs. As a lab, we have lost control over the safety margin on lens cutout. The lens designer gives us all of the parameters to process the lens, and we are at their mercy.
Our biggest issue has been the Frame ED (Effective Diameter) measurement. Unfortunately, many have been misinformed about how to measure this.
When we get a job that hasn’t cut out for a customer, what we often see is the LD, or Longest Diameter, placed incorrectly in the ED box. One of the top results in a quick Google search even incorrectly explains how to measure it.
How to Correctly Measure ED
To avoid mistakes (and the resulting delays) in measuring frame ED, we are going to show you how to properly measure the ED. To start, we must first box the frame.
Our frame A is 51mm as listed, across the top. You get this by measuring the distance between two vertical box lines. The B measurement is 35mm, which you get by measuring the distance between the two horizontal lines. Now, we must find the geometric center of the lens. To do this, we make an X from the corners in the newly drawn box.
Now that we have drawn the X, the Geometric Center is right in the middle. We can now figure out what the ED measurement is. The ED measurement is twice the distance of the farthest point from the Geometric Center.
The green line now indicates the ED. When I measure this line, I come up with 27mm. So the ED is 2 times 27, or 54mm. This ensures the best cutout, and best thicknesses.
Another way to get the ED, is to run the frame through your tracer before ordering the uncut lenses, and storing it for retrieval when you get the uncut lenses in, saving you a step later.
Order by Lens Shape Number
We have also developed a way to more accurately estimate the frame ED, using shape numbers. If you pick a shape number from our list (see below), we will calculate the ED, based on the A and B measurements, and the selected frame shape.
I hope this helps provide you better thicknesses, and better cut-outs, and a better quality product for your patient.