What Is A Slab-Off and How Do We Calculate It?


– Hello, and as always, welcome to the Laramy-K
Optician Works Training Center. Have you ever just have
one of those weeks? Well this one is heading that direction. Somebody had posted on social media, “Hey can you help me visualize slab-off?” I said, “Sure, I can do that.” Well about seven versions of this video that you’re about to see later, 40 some emails back
and forth with the lab, and several embarrassing moments, I think I finally caught it. Without further ado,
let’s hit the white board. (bouncy, energetic music) All right, a little bit of
a tough job ahead of me, but let me see if I
can’t help you visualize what the concept of slab-off is all about. Let’s begin with an ordinary prescription. Plus four OU, plus four on the right, and plus four on the left. An ordinary pair of single vision glasses. The optician that made
this pair of glasses did a really outstanding job. Monocular PDs, did an OC height. The lens OC for each of
these is perfectly placed in front of the wearer’s pupil. They’re looking through the
optical center of the lens, so there is no prismatic effect. The lens is there strictly to bend light, push it through the eye,
and have it hit the retina where it needs to be so
that person can see clearly. What are they seeing clearly? They’re seeing the object out there a little bit in the distance. When they look at the object,
it’s clear, it’s crisp, good visual acuity, and
they see one of these. Now, what happens when I do this? What happens when I
take my right plus four, and I make it a measly,
tiny little plus two, and I take my left lens, and
I take it from a plus four to a huge, whopping plus eight. What happens then? Nothing, absolutely nothing. Everything’s good, life is good. Why? Same reason, they’re
looking through the OC, a good pair of glasses made well, even though they have
that high prescription, there’s a big difference. looking through the optical center, there’s no prismatic effect. Looking out at the world, they see objects as they’re supposed to. No what happens when it’s bifocal time, and I have to give them
a nice, old fashioned straight top 28. Slab-off is also done on progressives, but we’re going to work
with a straight top 28. Now there’s a problem. This is where things get ugly. When they’re looking in the distance, they’ve got that nice crisp,
clean OC they’re looking for. They have to leave that
to reach this segment. Their eye must leave here and pass through the portion of the lens,
that actually creates prism, image shift, light shift to
reach the center of the segment, in order to read. When they enter this portion of lens, they get prism. Here’s my segment… Here’s my segment reading area. What happens to the image
viewed through a prism? It is shifted towards the
apex, because my plus eight is so much more powerful than my plus two, in my brain I’m gonna have double vision. This lens, this eye is
gonna see the object as shifted far more towards
the apex than this one is. How do I correct that? How do I get this back to here? Fusion slap-off. Because of the optics involved, I’m going to do the slab-off
on my my most minus, or least plus. What I’m going to do, is I’m
going to add a prism wedge on the back of this lens, to bring it to equal out this one. This prism wedge is slab-off. When I place that wedge on the
lower portion of this lens, and I mimic this prism amount here, then I can move the perceived
object back into one. How much I need to place
on the back of this to counteract this, is what
we’re gonna cover next. A few brief housekeeping items before we get into our
calculations for today. One, so much of the written
material that’s out there is quite old. You’re going to see a lot about glass, particularly about glass in slab-off. The lens blanks that you
need to do slab-off on glass have been discontinued. Glass slab-off is obsolete, so I am asking you to, let’s get over it, and move on. No more glass slab-off. Number two, reverse slab-off. You’re gonna see questions about that. I guarantee there’s gonna
be a question about it on the ABO. It’s really a shortcut technique. They try to mold the slab-off
amount onto the front surface, the finished front
surface of the lens blank, and then the lens gets back surfaced. To get the best results, you
wanna do a real slab-off, and just find a great lab like us, and we’ll do that for you, and you’ll have a lot better results. The example I’m about to do
is the simple plus eight, and a simple plus two. It’s a sphere, so we
don’t have to worry about calculating power in the meridian that we’re gonna be working with. When we’re working, slab-off calculations, the power is at 90 that we need. I know that seems a
little bit weird to me. It seems like it’s a
little bit off of the 90th, but that’s just he way my brain works. If the prescription given,
and it has a cylinder amount, is not at 90 or 180, because then you can just
do a flat transition, you will need to use the powers
in oblique meridian formula, or the 30/45/60 rule. Chances are pretty good that they may make you work through that, before you can work the calculation that we’re about to do on the APO. We have great videos
on that on the website, and on our YouTube channel as well. Okay, it’s now time to figure out how we come up with the
amount of that slab-off, or prism wedge that we’re going to put on the back of the lens. I think it would be a whole lot easier if we just called these
simple prism problems, because that’s really all they are. For some reason slab-off
just seems to make everybody, including myself go crazy. I’m not sure why that is. For instance this formula. My goodness, we’re all familiar with that. P is equal to hcm times D. That’s all were doing. I’ve got a right of plus two,
and a left of a plus eight. My drop, or my hcm is 10. Where does 10 come from? If I have my distance OC
the distance from that, to the top of my segment
is five millimeters. Generally lenses, straight top bifocals are surfaced with the OC five millimeters above the top to the segment. From the top of the segment
to the center of the segment is an additional five millimeters. So there’s your total of
10 millimeters of drop or movement down into the part of the lens that creates an error. That’s where our 10 comes from. Just the way it works out. Two times 10 is 20. 20 divided by 10 to
convert our centimeters to millimeters, gives
us two diopters of prism in our right. Our left. eight times 10,
80 divided by 10 to convert gives us eight diopters in our left. What prism direction do we have? We have base up, we have base up, because we’re looking
below the optical center, the 180 line, we’re looking
down towards that segment. Base up, base up prism cancels, so I take my eight, I subtract my two, and it end up with six diopters of base up in the left, total prismatic
error, or vertical imbalance in this pair of lenses. The two eyes working
together, the total result. If I need to overcome that, if I need to come up
with that six diopters to counteract that, and
bring those two objects back into focus, and fusion, I’m going to put six diopters
of base up on my right lens. It’ll look just like that. That will make it match my left, bring everything back to what I need, and that person can wear a pair of glasses and use a reading area or
bifocal without double vision. One last thing to mention about slab-off. I do need you to know that it
comes in both conventional, and freeform. That’s right. At Laramy-K, we put our
years of experience, and our freeform technology to work, and we make a digital,
or freeform slab-off. The advantages are a lens that
has a smoother transition, where that ledge begins, so it is more cosmetically appealing when you’re looking at it. It’s easier to wear, because
that blend is smoother, and it is actually thinner
and lighter as well. As always, thank you so much for watching if you are watching
this on YouTube, be sure to hit the subscribe button. If you’re watching us on Facebook, be sure to share it with
as many people as you can. If you enjoy these frequent videos, why not join OpticianWorks.com? As a full member, what
you’ll have access to a whole lot more great optician training. Thanks. (bouncy, energetic music)

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