There are literally hundreds of lens styles on the market today and dozens of lens materials that are thinner and lighter. We have complex computer programs that can control thickness by using complicated lens geometry and algorithms to make the lenses as thin as possible. We’re also fortunate to have frames that are still relatively small and oval in shape. Combining all these important factors allows our laboratory to make lenses thinner than ever. However, all of this can be completely negated if the proper lens shape is not given to the lab when ordering. Unfortunately, it’s not uncommon for me to take several phone calls a week from accounts who don’t understand why the lens shape is needed. Our Customer Service Department attempts to take the order only to face resistance when asked for the shape. When I ultimately take the call I’m always asked “Why do I need the lens shape? I’ve already given you the frame measurements.” These types of calls can be from accounts who have been in the industry for a short amount of time but they can also be from Eye Care Professionals who have been in the industry for 20 years or more. It’s the calls from the latter that surprise me. So whether you’re in the industry for several months or many years, it’s time to review why we need the lens shape. With today’s technically advanced lens styles, lens materials and complex computer systems, it can’t be business as usual. It’s imperative to give us the correct lens shape so you get the lenses as thin as possible the first time without having to re-do the job due to thickness. Though lens thickness can be affected by several different factors, for the purposes of this article, we’ll be focusing on the lens shape.
When we do have an issue with lenses being too thick, in almost every case it’s a plus Rx with the incorrect shape given. You may ask: “how can the wrong shape be given? I’ve given you the frame measurements!” Therein lies the problem. What measurements did you give us and how did you measure the frame? If you gave us the “A” measurement and the “B” measurement so far, so good. However, what was that 3rd measurement you gave us? Did you take your millimeter ruler and measure the longest diagonal? If you did, than you simply gave us the longest diagonal and NOT the effective diameter which is so critical for controlling thickness (see accompanying article on page 4 regarding effective diameter). In addition, if you added or subtracted a millimeter or 2 to “compensate,” or “fool” our computer system, then trying to get the correct thickness will be even more complicated.
When we receive the “A”, “B” and longest diagonal from you, all our computer system can do (and this would be true of any lab management software) is create a square or rectangular looking shape which can look much different than the actual shape. To illustrate this point, let’s take a look at a real live example of a job that we fabricated at the lab and we had to redo because the measurements that were given didn’t come close to the actual shape. To exacerbate the situation, the wrong “B” measurement was given as well.
The Rx called for: Varilux Comfort
Transitions Polycarbonate
OD: +5.00
OS: +5.25
Add: +3.25
Measurements given: “A”=50, “B”=37, longest diagonal=54.
Is the frame shape necessary on a job like this? It certainly is and here’s why. For those of you not familiar with the surfacing procedure or how lens thickness calculations are determined, you may think that the lens calculations will be based on the OD Rx of +5.00 and OS Rx of +5.25. This is not true. Since this is a Progressive lens, the lens thickness calculations will be based on the total power of the lenses or +8.25 in the OD and +8.50 in the OS. With an Rx like this it’s critical to have an accurate lens shape. Even being off a millimeter can effect thickness!
The shape above was calculated using the above measurements. Obviously, it’s very “rectangular” looking and experience tells me this probably isn’t the actual shape. Using this shape as the basis for calculations, our computer system determined that the center thicknesses would be 8.0 for the OD and 8.3 for the OS.
When the account faxed the actual frame shape to the lab (above), we were able to trace and digitize it. When this shape was used as the basis for the thickness calculations, the center thickness for the OD is 5.8 and for the OS 6.0; 2mm difference!
Now that we’ve illustrated the difference faxing us the actual shape makes verses giving us the “A”, B” and longest diagonal, the question becomes when is it necessary to fax us the shape? What guidelines should you use? Here’s what we suggest:
- On any plus Rx over +2.00. Remember, when working with Progressives, use the total power as the basis.
- Check with our lab to see if we have the shape already stored. We have thousands of shapes stored but obviously we don’t have every frame shape stored. If you’re sending in your orders via Remo, go to the frame icon below the word “Frame” in the lower left hand side of the frame and select “Lenses Only” and search for the shape by manufacturer. If we have the shape stored, you don’t have to do anything. We’ll “attach” the shape to the job and will then be able to make the lenses as thin as possible. You can also send the shape in Remo by interfacing a tracer with Remo. (For more information on this, contact your Tri-Supreme Account Manager, or Mark Cohen, Training and Marketing Manager).
- If you’re sending us the frame for us to edge, you need not do anything. We will trace the frame or use the stored shape in our computer system.
Your patients want and expect the thinnest lenses possible today. With a little “teamwork” between you and our lab, we can certainly accomplish this goal.
The Longest Diagonal Versus the Effective Diameter
As discussed in our accompanying article, there is much confusion in our industry regarding the longest diagonal and the effective diameter. They are not one in the same! The longest diagonal is simply that; measuring the longest diagonal of the frame. To the best of my knowledge it has no value for calculating lens thickness. However, the effective diameter is probably the most critical measurement you can give us. The Effective Diameter or ED is defined as: the smallest theoretical blank that can be used to achieve accurate centration and cut out on a given shape. This value is equal to 2 times the longest radius of the shape. The ED is very difficult to measure off a frame. It should be looked up in the manufacturer’s specs or Frame Facts. The longest diagonal is not a satisfactory equivalent to the Effective Diameter. Even today, I’m not sure how the longest diagonal worked it’s way into our everyday optical nomenclature and more importantly how it incorrectly got substituted for Effective Diameter. Maybe my memory fails me but if anyone knows how the longest diagonal got substituted for the effective diameter or what value it has, please let me know and I’ll be happy to print it in the next newsletter.