(laser hums)
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- Hello, I would like to welcome you
to the Laramy-K Optician Works Training Center.
Where today, we are going to talk about three terms.
We're gonna talk about sphere, astigmatism, and axis.
Let's hit the bench.
Three terms, sphere, astigmatism, axis.
If you need more information on the
common refractive errors of the eye,
which is closely tied to this stuff,
well then, there's a video for that.
Go ahead and watch it.
If you need to know more about
how lenses correct those refractive errors,
well, there's a video for that, too.
Go and watch that.
Today we're just gonna nail these three terms.
One of the best ways to do that is
working with our three dimensional model.
It'll really help you out here.
If you ever are gonna understand lens design,
point of wear measurements, or contact lenses,
you must grasp these ideas in three dimensions.
You must, there is no way around it,
you'll never get it, the light bulb will never go on.
Let's dig into our eyeball here for just a second,
let's see if I can get this open, here we go.
All this stuff is inside the body
on the outside of the orbit, or the eyeball,
nothing to do with us.
This is the vitreous humor,
the gel-like substance inside the eye.
Inside the eye, nothing to us, not our problem.
Crystalline lens back there,
inside the eye, not our problem.
Iris and the pupil,
inside the eye, not our problem.
Then there's the cornea.
The cornea, some of it's inside the eye,
but a lot of it's on the outside.
And that's where we take over, right?
Everything from the front surface of the cornea
all the way to the sun and everything in between,
is what opticians need to know about.
Of course we cover that in all the other videos
and on the website.
I could use this cornea,
but of course it's really hard to see.
So I'm gonna switch to a couple of balls.
A spherical cornea is a cornea that is perfectly round.
And if I took it off, and you think about a cornea being,
you know you can see it, go look in the mirror.
If I sliced off the top half of a perfectly round ball,
well that would be a spherical cornea.
The radius of curvature from the center point out
would be the same in every direction.
Quite literally, if I take my
P.D. stick and I measure across,
about a hundred.
And if I measured it across in the other direction,
other meridian, I'd get about a hundred.
It's a sphere, perfect, round.
And, I'm always going on about how we,
you know the concepts that are presented
are always in very simple forms.
We're not trying to be a doctor,
we're not trying to be optical engineers.
But, especially if you get into contact,
you'll realize that when we're talking about the cornea,
spheres, astigmatism, toric,
it's actually really pretty darn close.
So, that's a spherical cornea.
What is astigmatism?
Astigmatism simply means that the human's cornea
is shaped more like this football.
Instead of being perfectly round like that,
instead it's shaped like this.
And it really is, pretty much, shaped like this.
(laughs) It's one of those few cases
where things work out pretty well for us.
The astigmatic cornea, a cornea with astigmatism,
a person with astigmatism simply has a misshapen cornea
which will have a steep curve and a shallow curve.
Those curves often follow the shape of the human eye,
it's called with the rule astigmatism.
But it is not always the case.
And with the rule means that it's gonna be somewhere around
the 90th meridian, not perfectly up and down, 180, 90.
And it's just gonna be around there somewhere.
In extreme cases, against the rule,
things would look like this.
Now of course my cornea doesn't move
once I develop as an adult, and things are kinda fixed.
If my cornea happens to be astigmatic,
and it's formed in the body at about that position, say,
it's gonna pretty much remain there.
Right, unless I have LASIK or some other kind of thing.
Does it move a little sometimes?
Yeah, but, right it's still there.
Could be there, could be anywhere,
you know 180 degrees, anywhere around.
Now, let's just say that this person developed
and their astigmatic cornea ended up about there.
Which would be, and I need to look at this,
which would be, let's say roughly about 85 degrees.
In order to correct their vision,
you'd do refraction, and you would
come up with a lens power,
or lens powers, to correct it.
If I take that lens, I need a way
to communicate with the person making the lens
and the person making the glasses,
how to properly position this lens, the curves on this lens,
so that they correspond to the curves on the cornea,
two curves, two curves, to correct vision.
That is axis.
Axis is a notation of position.
If it was, if I found that I needed my strongest
power to align with the steepest curve
on this particular astigmatic cornea,
and I needed that aligned at 85 degrees.
85 degrees, cut it and mount it in a frame,
it would align with the steepest curve there
if that was what was necessary
to correct this particular refractive error,
and I would have a person that
would be happy with their glasses.
So, that's it.
There we go.
We have got sphere, a perfectly round cornea.
We have got astigmatism, a cornea that is misshapen,
much like this football.
And we have axis, a notation of position
so that I can rotate lens around
a lens that has two different principle meridians,
two different powers,
so that it aligns correctly with the
curves that are on the cornea to correct vision.
Phew, I think I did it.
All right, thanks so much for watching,
I will see you again next week.
I do hope you took something away from that little lesson.
If you did, please hit that subscribe button down there,
hit the like button, leave me a comment.
And make absolutely sure
that every uncut lens in your optical world
comes from Laramy-K.
Worth his wild z (mumbles).
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