B1 Intermediate US 29 Folder Collection
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- Hello just flew back from the lab in Iowa
and boy are my arms tired.
(rimshot)
(pop music)
Have you ever wondered how the lab knows
how well their AR coating will last?
Well the test used to determine that is
called the salt water boil test.
And we just happen to catch one being done
while we were at the lab and want to share it with you.
A few moments at the whiteboard before I show you
the actual test.
You may have noticed that I have a cold or I'm sick
there's a reason for that.
It was because I was traveling.
As the old saying goes it was almost like
I was asking for it.
I had my sleep cycle interrupted.
I was in airports and in planes.
I was in all kinds of public places.
I went out to eat, I went to the Y and worked out.
We were at the Iowa State Fair
50,000 other people were there.
I was on the factory floor shaking hands,
touching all the equipment that other people were using.
After about five days I was sick
(sneeze) as if I was asking for it.
What we're about to do with this test
in the AR coating is kind of the exact same thing.
We're gonna scratch the front of the lens,
we're gonna score it up, boil it in some abrasives.
We're gonna actually take adhesives
and try to yank that AR coating off
the front of the lens.
We're gonna thermal shock the lens,
plunge it into boiling water and then take it out
and put it into room temperature.
We are trying to make that AR coating sink.
(sneeze) Normally that would take about
one to two years.
You don't kid yourself, even the very best AR
after a year starts to degrade.
You might get two years out of it.
And we're gonna try to mimic that one
to two years of use down to about 15 minutes.
Remember that the AR stack itself is actually
working the manometer level.
That the one-billion of an inch level.
You'll see that we use a microscope
even to capture the the scoring
that we do and the lifting that might occur.
The stack itself it could be
three layers, five layers, seven layers
anything depends on the manufacturer
and what they're calling a layer.
Those are elements and the elements play
pretty well together.
They they have good bonds and they like to stick together
and play together.
So it's fairly rare that those separate
as individual layers.
The stack if you took all these layers and slammed
them down to the molecular level,
this has to stick to the front surface of the lens.
That front surface of the lens has
that thermal hard coat on it.
That is prepped in a special bath so that
these can hold on to the front of the lens.
That's what we're testing, how well the AR layer
is adhering to the thermal coat layer which in turn
is adhering to the front surface of the lens.
This is everything.
You're going to see in a moment a pattern
that we scratch or score into the front of the lens
with a razor blade.
If I took one of the squares
in the six by six grid and
blew it up this would be my lens.
And because my layers are nanometers
and this could be a couple of millimeters,
this wouldn't be terribly unrealistic.
The layer that sticks to the top portion
this would be that the front surface of my lens here
is exposed because we score it on four sides.
So it is far more likely to have an issue with
the chemical that we're using, the tape that we're lifting
with, all of the things to want to peel away
from that little tiny millimeter section that it has.
And by creating the grid system we're giving it so many more
opportunities to fail.
That's why we do that.
The first step is to score the lens.
We do this using a basic razor blade and
we cut this six by six grid of approximately
one millimeter squares,
Just as I showed you on the whiteboard
this is done to greatly increase exposed edges
of the AR stack or layers.
(pop music)
After scoring we move to the lift pull test.
This is done with a special tape designed
specifically for the test.
The tape is firmly pressed across the grid
and allowed to sit for 30 seconds.
After that the tape is a bit away and closely
examined for any AR layers that may have pulled off.
(pop music)
After the lift pull test we get to the actual
boil part of the salt water boil test.
The salt is added for two reasons.
One, it's as a acts as a mild abrasive.
And two, it keeps the temperature more consistent
throughout the beaker.
In reality this test is a thermal shock test.
The lens is plunged into boiling water
for about two full minutes.
After that it's quickly removed and again dropped into
a beaker of room temperature water for one minute.
(pop music)
Once this shock test has been completed
you remove the lens and we inspected using this microscope.
And the microscope allows us to take a really up close look
at each of the edges within the grid.
What we're looking for is any kind of failure
or lifting around each one of the squares.
Next we move on to the hydra layer test.
This is also a two part test.
The first part test uses the lift pull tape
but what we do is we mark the lens
using an ordinary AR pen.
And then we try to peel that ink away
and hopefully it's going to remain on the tape,
which it does.
(pop music)
The second part of the hydra layer test
is the water droplet.
And all of you who are familiar with contact lenses
will remember wetting angle.
Basically it's the same idea.
What you're looking for on that water droplet
is that it's a consistent beautiful
spherical shape and the angle between
where the water droplet ends and it meets the lens
is good and sharp it's an acute angle.
And as you tilt the lens and allow the water droplet
to run off, you don't want one residual water.
You don't want a trail that behind it.
Because we're trying to duplicate two years
worth of use and abuse
and kind of compact it into a 15 minute test,
what we do now is run through this series two more times.
Once we have the results of that series of three tests
we have these guides or rubrics that allow us to judge
the results of each of the tests
against both subjective and objective measures.
What you're always looking for is a lens to score
four or higher, which this particular lens does.
As always, thank you so much for watching.
Don't forget if you are watching this on YouTube
be sure to hit the Subscribe button
down there in the corner.
If you're watching us on Facebook be sure to share it
with as many people as you can.
Thanks so much.
I am headed out on vacation so next week you are
stuck with a rerun and I'll see you the week after that.
(pop music)
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Testing Your AR Coatings: How It's Done

29 Folder Collection
wei published on December 15, 2018
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