August 3, 2013
Whether you’ve been wearing progressive lenses for years or just getting your first pair there are a couple of additional measurements that need to be made before you buy them. You obviously have to have your prescription figured out but it’s only part of what goes into getting you a new pair of progressive lenses. And for those of you wondering what a progressive lens is, they are the invisible, no line lenses used for distance and near vision. Some patients seem to like calling them bifocals or trifocals. Please! We never tell a patient that but that’s like a lot of things. We just internalize something that way I suppose to make it easier to understand.
So, your pupil distance….there’s the old fashioned way to measure this with a ruler and then there’s specialized equipment. I’m guessing the most common way that I know of is using a pupillometer. A simple looking device that kinda looks like a funky pair of binoculars. Generally this is a very accurate measurement so not much to worry about or go wrong with this method. A ruler on the other hand in an inexperienced hand or poorly trained hand, could surely result in a potential problem. I could go into the problems but let’s just leave that for if you have a problem, then hopefully your optician will check that again.
Most pupillary distances are 60ish…a little more for some, a little less for others. Sometimes the near pupillary distance is also taken. When your eye looks at a near object your eyes turn in some and result in a little shorter pupillary distance. Usually labs will automatically calculate this for the progressive lens. I said usually….an office might have some very specialized equipment that does some cool measurements for all of this or your optician will take another pupillary measurement for near.
On to the segment height. Let’s take a real bifocal for example. There’s a line in that lens. We’ll call that a segment which separates the top power of the lens from the bottom part. In a progressive there isn’t a line but there is an area where the reading power of the lens design has to be known. To get that measurement, an optician will ask you to look at their eyes and they will probably use a marker to put a dot on each lens that corresponds to where your pupil is. But what if you’re not the same height as the other person? You should have your eyes at the same level as the other person’s eyes who is doing the measurement. If you are standing higher than the other person they will possibly mark/measure your pupil too low and if you are shorter or below then your “seg” height (where your pupil is) might be too high. All this can have an effect on how much of the variable reading power your brain will have to deal with in the progressive lens optics and maybe even cause you to have problems adapting to the lenses.
I think even a mildly new optician will do the right measurements but if a patient does have problems getting used to how their new progressives work, these measurements should be remeasured and confirmed. There are many other variables that need to be accounted for in fitting someone with progressive lenses but I’ll save that for another post. One more thing, I hear this a lot. The patient who has trouble with the prescription might think the doctor got it wrong. We’re not perfect but I’m going to tell you, it’s probably not the doctors fault! It has something to do with the lenses!