So my favorite animation movie is, Shrek. And, you know, who doesn't like Shrek?
And I especially like this dialog from the movie, where Shrek says that
you know, Ogres and onions they have a lot in common.
And what he says is that they have
layers, and layers, and layers in their personality.
So both of them are very complicated
creatures and they have you know, multiple layers to
their character and their personality. And what I want to emphasize
in this video is that the liquid crystal display, or the
LCD, which are the most prevalent display we see, in our, day to day
electronics in our smart phones, in our tablets, in our televisions.
They are very much like that as well. So shown here is you know, a cross-section
picture which shows the multiple layers in a liquid crystal display.
And to describe it a little more, so there are, you know, as you
can just see over here there are layers and layers and layers of things.
And each of them is you know,
doing something or achieving a particular function that is enabling this display.
So the very first layer you know, starting from below,
starting from the bottom and all the way to the surface.
So this is the bottom and this is the surface of my LCD.
So, I see, like, you know, seven or eight layers in in between this
two bottom and top surface.
And the very first layer at the bottom is the back light, so this is
something which is you know, giving the brightness to the LCD.
So this could be you know many of these cathode fluorescent tubes.
Or many times in our tablets these are these are LEDs which are placed either
at the very side of our display.
And and then there is a difuser in here which diffuses this light coming from the
sides and diffuses it evenly and distributes and
projects that in the, in the up-ward direction.
So there, this could be either by this
back-light, could either be powered by you know cathode
ray or florescent tubes or it could be powered by LEDs these LEDs
could be very much at the side, so it could be a side or edge LEDs.
So, this light from the back light is then first passed through
of the polarizer and note that there are two polarizers over there.
There is one polarizer towards the bottom, let's call it as polarizer let
me write it down in the proper colors.
This is polarizer one, and then this is polarizer two.
So, the way first polarizer, it polarizes
the light in you know, one particular polarization.
So, let's say it has a polarization looking like this.
So, all the light will have a polarization like this.
Then this light is passed through it has to pass through
this this thin film transistors backplane or TFT backplane.
And this essentially this is the set of transistors which
apply voltage to our liquid crystal layer and they control you
know each of the individual pixel of our liquid crystal display.
So then we have this TFT backplane
in our light has to pass through it and you know the
area which is
over here covered by transistors so that the light will be blocked by these
transistors which are you know partially occupying or blocking the light.
and there is
important metric over there called the fill factor
which determines you know how much of this area
in this backplane is blocked by these thin film transistors.
And that degrades,
you know, the overall efficiency of light or
the percentage of light that we can pass through.
So the next layer on top of these TFT backplane is the liquid crystal
and I'll just talk about it a little more, very soon.
But these liquid crystal essentially when you apply
voltage on them, they essentially can manipulate the polarization
of the light.
So they can either bend the polarization of the light coming
in, or they can you know, if you, whether it's on
or off, it could either bend the polarization of the light,
or it could do nothing to the polarization of the light.
And then on the top we have essentially these color filters, which are, determine,
determine whether the pixel is red, green or blue.
So remember all the back light, which is coming from below is all white.
And the only way you define colors is by passing that light through
these color filters so they allow just either the red, a green or
blue light and they block the others. And that way you determine what is the
color of the pixels.
So remember this is a very inefficient way.
Because we are generating white light and then
we are you know rejecting all the other
all the other you know, components and just
letting pass through the red, green and blue component.
And on the very top we have polarizer layer again.
And the unique thing about this polarizer is it has an
opposite polarizer on. It has a 90 degree
as compared to the polarizer 1 it has polarization which is orthogonal to polarizer 1
So these two polarizers in conjunction with our liquid
crystal display determine whether the light passes through or not. So now to understand how
LCD actually works what I am showing over here is a zoomed in view which shows one pixel of my each individual pixel of my
LCD display and as you can see it has two polarizers. This is polarizer 1 and this is polarizer 2
and then you have liquid crystal in between them so this would be where your liquid crystal is and these two polarizers are arranged orthogonal to each other. So as you can
see over here this has a polarization like this, and
this has a polarization which is orthoganol to this.
So this side of polarization is like this.
So what happens in this liquid crystal, liquid crystal is that, they,
these are, these are organic, molecule which have these helical structure.
So if you don't apply any voltage on your liquid crystal.
So essentially you have this liquid
crystal, and you just essentially have this open circuit over here.
These, liquid crystal by default, they have you know,
this helical, they are arranged in this helical manner.
And what they do is they, they rotate the polarization of the light coming in.
So, our light, as you can see, we passed it through this polarizer one on the top.
So, it has a polarization
which looked like this.
And what is a good crystal molecule, if you don't apply any voltage on them what
they do is, essentially, they rotate the light,
such that it now has orthogonal polarization.
So, if you do not apply any voltage on this
liquid crystal, they just rotate this polarization of the light.
And since these two polarizers have
orthogonal polarization, the light passes through and
essentially I, I get, I can say this pixel allows the light to pass through.
on other hand, what you if you want to turn off this
pixel or you don't allow any light through it, what you do
is you apply a potential across this liquid crystal so what
you do is you apply a potential across this this liquid crystal.
And these liquid crystals are, they have
these molecules of these liquid crystal have polarization.
Some have more of inherent polarization in them.
So they all line up in the direction of the electric field.
And now you can see that now when the light is
coming in, so it comes in and it passes through this polarizer.
So all of the light has the polarization of this polarizer.
And then, when it passes through the liquid crystal,
the liquid crystal does not bend the light anymore.
So when it comes through this second
polarizer this has orthogonal polarization, so
no light can pass through, and the pixel appears as if it's it's black.
Or it essentially does not allow the light to pass through.
So this liquid crystal, it seems like you
know, a very nice guy, it can, when you don't apply any
voltage to it, it by default does all the work for you.
It turns the polarization of the light around,
and the light passes through and my pixel is on.
When I apply a voltage
on it
you know it minds itself, it does not bother
or does not touch with the polarization of the light.
And I get no light through my pixel and my pixel is off.
But you know, there's a catch to it. There's, although it does this thing very
nicely and you know, it can do this thing infinite number
of times one of the limitations is it's not very efficient.
So when it you know, when you apply a voltage and you don't
want any light to leak through this you know, some light which leaks through
this liquid crystal, because it's not 100% efficient, you know, it still
will bend some of the light, or some of the light will still leave through these two polarizers.
Think of these as, you know, you have
two shades, and they are placed perpendicular, and
you have two shades on the window, but
there will be some light that still leaks through.
So, it does not have 100% efficiency of you know, turning the light off.
At the same time, you know when you turn when you don't apply a voltage the it.
Turns the polarization
of the light so that it can pass through.
But again it does not have 100% efficiency for this process as well.
So there's always some intensity lost when
you pass the light through this liquid crystal.
And keep in that mind that you'll be having two of
these electrodes through which the light has to pass through as well.
So what's shown here is is one of these pixels which
has the color filters as well.
So this is a individual pixel, and then when you are generating each of
these RGB colors, what you do is you have these three color filters over here.
So this would be red.
this would be a green, and this would be a blue color filter.
And, again you can see that over here you have
these, you have this, in this case it has these, Cathode ray,
back light, but it could be very much, LED based, back light.
Or you want to have this edge-LED based, back light which is
commonly used in your, smart phones and your tablets.
And then these, essentially these back-lights they generate this light and
it passes thorough my first polarizer. So this is my polarizer one.
think of it like a shade on your window, so all
the light passes through this shade, so it has that particular polarization.
And now, and, I run through this, glass, which is my substrate, and
then I have this liquid crystal over here. And I apply
the voltage on these, liquid crystals using,
two of, two of my transparent electrodes,
so shown here is one of these, electrodes, so this would be electrode one.
And electrode one, and it's, these electrodes are made up of these
conducting transparent oxides, so it's
called transparent conducting electrode, and the material
which is most commonly used to make these is Indium Tin Oxide, or ITO.
So i have two of these electrodes this is the first electrode.
And this is the second electrode.
And they are essentially used to apply, I use these electrode to apply a
voltage on my liquid crystal, or don't apply any voltage.
So what they do essentially is again when you either if you
apply voltage or you don't apply voltage it will either bend the polarization
of the light and it will let it through.
Or it will not bend the polarization of the light.
And the second polarizer is
orthogonal
to my first Polarizer.
So I can, I'll either get light or not get light through this liquid crystal.
And then what I do is I pass it through a color filter.
So this color filter, what it does is
essentially it, the light which is coming in.
It only lets the red component of this pass through.
And similarly this other color filter, it lets only the
green component pass through.
And this other color filter it lets only the blue component pass through.
So these, and that's how I get these individual RGB colors.
And then I can combine these three or, you know.
Combine them in which ever way I want
these primaries, to give me the required color.
So each of these RGB, as you know, are controlled individually, and then I can
you know, control the amount of light or even the intensity of each of these RGB
individually, and then I can combine these three
primaries to give me any color I want.
So, one thing I want to point out is, you know, how inefficient this, process is.
So, all these layers, what they're doing is each of these layers, think of
it, you know, that each of these layer, if you come with a intensity
I, you only get a fraction of that intensity after passing through this.
Each of these individual layers.
So this x is always between zero and one for each of these layers.
So, think of how many layers you have
over here, so you have this polarizer, one, then
you have this glass substrate, so this, you
know, this is layer one, this is layer two.
Then you have this another electrode,
so this would be layer three.
Then you have the liquid crystal, this would be layer four.
For, then you have this alignment layer that is layer five.
Then you have this another electrode, which is the second electrode
to apply voltage on your liquid crystal, that's your layer six.
Then you have this color filter, that's another layer.
Then you will top it off with, you know, another another glass substrate.
So that, each of these layers, you know, if it has, if it
has efficiency of x to pass the light through, I get, you know,
compounded off, these different, efficiency of these
different layers, and that essentially limits my overall brightness
of my LCD. So it makes the whole, Process, very
inefficient.
At the same time, I have, I have these color filters and, each
of these guys are only letting one third of the light through them.
So I, you know, I just, I pay, I spend so much time and,
you know, getting or controlling this light and getting it all the way to here.
And then when I am passing through this color filter, I
am rejecting essentially 2/3rds of my light and only
letting my blue, or in the case green or red light pass through.
So all these inefficiencies combine up and
they the overall efficiency of this whole. If you, you know if you pass 100
photons of light from over here.
You only get like five photons out from here.
So it's a very inefficient
process, and that's something to keep in mind, and it
comes from having all these multiple layers to your LCD.