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

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Thanks.

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