By Tina Lahti and Amer MouradWe’ve all seen it—the job that comes back from the lab that seems much thicker than it should be. Sadly, these jobs often lead to finger pointing. We blame the laboratories but they delivered what you, the optician, ordered.
Using a simple job as a sample, let’s review the impact of center thickness, material index of refraction and decentration on lens edge thickness. We’ll look at center thickness followed by the material index of refraction and finally the effect of decentration on edge thickness. Our job, -8.00 sph in 1.50 plastic with 4 mm of decentration and a 2.0 center thickness, has an edge thickness of 9.4 mm.
Center thickness: Often, when a job is deemed too thick, a dispenser will start by measuring the center thickness and insist on a laboratory remake to grind the lens thinner. What is the impact of decreasing the center thickness from 2.0 to 1.5? A -8.00 sph in 1.50 plastic with 4 mm of decentration and a 1.5 mm center thickness has an edge thickness of 9.0 mm. This is a decrease of only 0.4 mm or 4 percent. For every 0.1 mm, the center thickness is reduced on a minus lens; the edge is reduced by roughly the same amount. Reducing the center thickness will generally not do much to improve the appearance of a high minus job.
Material index: Patients, especially those with higher powered lenses, are often offered higher index materials as an upgrade. What happens if we increase the index of refraction of our lens material by 10 points from 1.5 to 1.6? A -8.00 sph in 1.60 plastic with 4 mm of decentration and a 2.0 mm center thickness has an edge thickness of 8.1 mm, a reduction of 14 percent. Quite a bit more than a reduction in center thickness but still perhaps, not as much as the patient would like.
Decentration: What about decentration? Horizontal decentration is, quite simply, the difference between the patient’s mono PD and the mono frame PD. To determine the mono frame PD (aka Geometric Center or GD), take half of the “A” measurement and add half of the DBL. For example, if a frame has an A measurement of 52 mm and a DBL of 20 mm, the mono frame PD would be 26 + 10 = 36/36. If the patient’s mono PD is 32/32, then each lens will be decentered in 4 mm for each eye.
What happens if decentration is zero? With an Rx of -8.00 sph in 1.50 plastic with 0 mm of decentration and a 2.0 mm center thickness, the lens has an edge thickness of 6.3 mm, a reduction of 33 percent from the original job. Decentration affected the lens thickness of this job the most.
Choosing the frame: How would we build the ideal job for this patient? We start with choosing a frame with zero decentration, a lens with the highest index of refraction, and request that the lab grind a thinner center thickness. Ideally, we would offer our -8.00 patient a frame with no decentration, a 1.74 high index lens with a 1.5 mm center thickness. This ideal job would have an edge thickness of 4.5 mm, less than half of the thickness of the original job.
Combine options: In the real world, we can’t always create the ideal. Patients will often insist on larger frames, and the highest index materials are not available to all opticians. Our best option is to find a compromise that both reduces decentration and increases the material index. A -8.00 sph in 1.67 with 2 mm of decentration with a 2.0 mm center thickness would have an edge thickness of 6.1 mm, a 34 percent reduction from the original job.
- Digital processing, patients with high minus prescriptions can benefit from improved peripheral optics in free-form digital lenses. Some of these designs offer automatic lens thinning that can reduce thickness by a small amount, usually less than 5 percent.
- Base curve selection, creating a wrap for a high minus, or keeping them in the same base curve as their power increases, can have a detrimental impact on lens thickness. This same job, created on a 6-base lens, would be 15 percent thicker.
- Bevel placement, while this will not change the edge thickness, it can go a long way toward making the final pair of eyewear attractive. Front, center and 2/3 bevels are commonly requested by opticians to improve the cosmetics of each job.
Now you know, the best way to deal with thick lenses is to circumvent the issue during the frame and lens selection process. Work with your patients to make better frame and lens material choices. And help them understand that these decisions affect the attractiveness of the finished eyewear. With a little more thought and care before ordering the job, the lens calipers never need to come out, and you can avoid the sinking feeling in your gut when a job arrives with thick unsightly lenses.