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  • - 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)

- Hello, and as always,

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B1 中級 美國腔

什麼是 "斷板",如何計算? (What Is A Slab-Off and How Do We Calculate It?)

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    wei 發佈於 2021 年 01 月 14 日
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