hey everyone I finished
building my
first LCD test cell so check it out
I built this with two microscope slides
that I coded with I tio and then filled
with a pneumatic liquid crystal that
someone donated to my purpose so as you
can see the quality isn't exactly great
but it is modulating light for
comparison here's a commercial test cell
that I've filled with the same liquid
crystals and as you can see it works a
whole lot better and I'll be using this
test cell the commercial one to help me
improve my process so let me tell you
what I learned so you may have seen this
trick using polarisers where if you have
the polarisers aligned you get light to
pass through them but when they are
crossed you don't get any light to
transmit through them however there's an
interesting trick if we put another
polarizer between the two set at about
45 degrees light passes through again
which is an interesting sort of thing
that may not seem quite so intuitive and
in a similar vein if we put a piece of
plastic between the crossed polarizers
we get a really interesting effect there
and what's happening is the piece of
plastic is scrambling the polarization
of light and there's also some
wavelengths dependent things going on
which is where the colors come from so
early on some researchers at RCA thought
you might be able to build a display
with this technique using liquid
crystals and a liquid crystal is a
complex sounding name for something
that's actually fairly straightforward
in fact a lot of the proteins in your
body qualify as liquid crystals really
all they are is a crystalline substance
that can flow around like a liquid so at
the microscopic level a lot of the
molecular properties are crystalline but
it really does look like a liquid so
here's a vial of some right here so
liquid crystals have a characteristic
sort of murky liquid appearance to them
and the murkiness comes from the
crystalline structure that's distributed
throughout the liquid a liquid crystal
substance will turn clear if you heat it
high enough
the temperature depends on the substance
properties and it will also turn to a
full crystal solid if you cool it enough
so this liquid crystal region is
actually a kind of a new phase between
liquid and solid so it's long been known
that if you put a crystal between
polarisers you can use the crystals
properties to reorient the polarization
of light and very smart researcher at
Reed RCA was thinking you could build a
display with this if you could
electrically modulate these crystals
these liquid crystals and position it
between the crossed polarisers so then
you could basically have a system that
was either cross polarisers without any
sort of scrambling between them or we
could put if we could turn this
scrambler on and off electrically then
we would have a display that we could
switch on and off you know at our
choosing electrically so let's say we
had some liquid crystal that we could
turn on and off with electricity and
when it was on it would scramble light
polarization and it was off it wouldn't
do anything to it we could build a
display with a stack up like this this
is sort of a cross-section view and if
we didn't have any power applied what
would happen is the incoming light would
come in and be polarized by the top
polarizer and then pass through here
nothing would happen because this is
just glass and our crystal is off and
then it would get stopped at the second
polarizer because our polarisers would
be crossed like this and then if the
crystal were on we apply power and the
crystal turns on we'll talk about the
methods later then the light would come
in and get polarized and then as soon as
it hits the liquid crystal layer it
would become random again and then
continue on through the second polarizer
and this time it would make it through
because some of that scrambled light is
now of the correct polarization and then
hit the reflector and then basically
have the same thing happen on the way
back and that's exactly what's happening
with this stack up here so as you can
see the plastic is scrambling the light
and we're kind of it's kind of okay I
mean it's we do have contrast between
the black of the cross polarisers and
the sort of multicolored see-through of
the scrambled setting but this really
isn't so great I mean
isn't an awesome-looking contrast ratio
for a display and there's ways that we
can make this function quite a lot
better one step up would just be to have
the liquid crystals simulate a 45-degree
polarizer this way we don't have these
weird color effects at least the display
is now just black and gray instead of
black and multicolored but we can do
something even better as it turns out
there's two actually there's a number of
different kinds of liquid crystal but
the two that are relevant today are
called chiral and pneumatic and chiral
liquid crystals basically have a
corkscrew structure like this and the
pneumatic liquid crystals have like a
needle-like structure like this and when
I say is structure this is like the
shape of the molecule so to make the
display as efficient as possible what
would really be nice is if we could
convert the polarized light in this
liquid crystal layer to the other
polarization so basically if we had
something that would twist the light
from the polarization generated by the
first polarizer to the orientation of
the second polarizer that way we would
basically have half the light lost here
because if it's random coming in and
this is only selecting for one
polarization then we get half out then
all that gets twisted into the other
orientation and passes straight through
here and then in the same path comes out
so our display could be almost 50%
efficient in the on state which is
pretty good so early on the LCD
researchers really wanted to find a way
to get this 90 degree twist to happen in
the liquid crystal layer and they
figured out by mixing these two
different kinds of liquid crystals
together they could produce a cross
between these two that had just the
right amount of corkscrewing going on so
that it would complete a 90 degree twist
in this liquid crystal layer so the
chiral molecules have a pitch to them
the distance between parts of the
corkscrew and if we mix these together
something interesting happens the
molecules interchange structure a bit
and we can actually custom tailor the
pitch of the chiral molecules by adding
more
maduk molecules so if we know what the
distance is and we do because we're
building this display we can custom
tailor our molecules such that they will
complete 90 degrees of twist in the
space that we give them and this is
typically on the order of five to ten
microns so what we could do I mean since
we're building the display we can tailor
both we can we can change the thickness
here and we can also change the
molecular composition such that we get
that 90 degree twist and this is what it
looks like so the cell is half filled
and so up here in the blue region the
molecules are completing that 90 degree
twist for us and here in the dark region
you can't see through because that's
crossed polarisers I only got this cell
half filled with liquid crystals so this
is air on this side and liquid crystal
up here the reason that this is blue and
not totally transparent is because I
have a mismatch the pitch of my liquid
crystal probably doesn't match this test
cell exactly and so we're getting some
wavelength dependent effects here also I
want to give a big thanks for the person
to the person that donated these things
to me he gave me two different kinds of
liquid crystal to experiment with in a
number of these test cells so thank you
thank you so now that we have the basic
idea there's a couple of interesting
technical challenges that come up one is
how do we keep these two pieces of glass
separated by five microns across the
entire size of the display I mean at the
time they didn't have 50 inch television
screens but they you know did want to
make watches or alarm clocks that were
this big and they want those pieces of
glass to be separated by five micron
what you do is you actually just throw a
bunch of glass beads down into this area
and then squash the two pieces of glass
together and let the glass beads hold
them apart at the right distance sounds
primitive but that's actually what we do
today even so check it out I loaded up
my microscope with a commercial LCD and
if you focus down into the layer you
know focus past the dust that's laying
on the surface and focus down into the
display you can actually see a whole
bunch of
needs in there and just for comparison
here's the tip of a needle so I didn't
do careful measurement but I would be
pretty sure that those beads are on the
order of five micron next we need to
figure out a way to make clear
electrodes so what what happens when we
actually put power on this system as we
generate an electric field between the
two glass plates and the electric field
is what causes these pneumatic
needle-like crystals to you know
obtain a certain orientation so normally
we have this 90 degree twist going from
the chiral molecules but we can
overpower that by putting an electric
field on them and pull them into this
straight configuration so that the
chemical composition doesn't change we
have the same blend of chiral and
pneumatic molecules but when we put and
apply an electric field this behavior is
sort of discouraged in favor of this
because the electric field is putting
force onto our system so the way that we
make a display is we actually put clear
electrodes in the pattern of the thing
that we want to show on the display and
then when we set up when we apply a
voltage there's an electric field only
between those patterns so for a
7-segment display you know you've got
your your elements kind of like this
let's say and the actual electrodes on
the glass are in that very shape so that
we only have electric field wear
underneath those elements so the
development of clear electrodes was also
kind of a new thing that had to happen
for LCDs to come about I don't think
they had a lot of commercial use before
LCDs were possible and today this is
almost always done with indium tin oxide
and so you take your piece of glass that
I showed in a previous video and we can
sputter righty-o onto it to make a clear
conductive layer there's one other
interesting problem so I talked about
this distance being custom tailored for
our molecule or vice versa so that
there's exactly enough space in here for
our molecules to make a 90 degree twist
but who's to say that the molecules will
be lined up with our polarisers I mean
it's a 90 degree twist sure
maybe it's you know from zero to ninety
or maybe it's from 180 to 270 or what so
there's a very primitive way of fixing
this problem all we do is we take our
finished pieces of glass and rub them
with a cloth in the direction that we
want the molecules to take sounds really
ridiculous but this is also current
technology today what we do after making
our glass with the patterned I teo
electrodes on them as we coat it with a
very thin polymer typically just a
couple hundred nanometers of polyamide
and then you you just take the piece of
glass and you just rub it with a cloth I
mean at the factories they don't pay
people to rub it by hand we have
machines with rollers to do it but the
very small micro scratches that you put
into the plastic with the cloth will
cause the molecules to stick in a
preferable direction so of course we rub
this top one in the direction of this
polarizer and we rub the bottom plate in
the direction of this polarizer and that
will sort of help the molecules get that
90 degree twist in our preferred
direction so that it works with our
polarizer system in my case I didn't
have an easy way of making a 200 micron
or sorry 200 nanometer polyamide layer
on my glass so all I did was use a some
sandpaper very very fine sandpaper to
put some scratches into it and I don't
think this worked very well so I'm going
to have to come up with a plastic
coating technique this basic structure
is called twisted nematic since we're
combining elements of chiral molecules
and pneumatic molecules and you might
have also heard about other kinds of
displays called super twisted nematic or
other variations a super twisted nematic
is a system that uses a 270 degree
rotation here instead of a 90 degree
rotation and the reason for that is to
get better grayscale control in matrix
displays so for doing one or two
elements like a seven segment display
this system works out just fine but when
you want to do a high-density computer
display run into all kinds of additional
problems and then you might need
transistors to drive each pixel in the
list of technological improvements goes
on and on if you think about how many
billion LCD
have been built and how many many many
billions of dollars have gone into their
research it's not too surprising that
we've developed these quite quite
extensively okay well I've got my
photoresist system finally under control
and so I'll be making some patterned i.t
oh and hopefully some better quality
LCDs in addition to some other cool
stuff okay
s
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