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LCD Monitor

Bill tears down an LCD monitor to show how it works. He describes how liquid crystals are used, the structure of the glass panes, and the thin film transistor (TFTs) that allow for active matrix addressing.

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Transcript This monitor uses liquid crystals to display images. How this thing works amazes me! Let me show you.

Let's start at the back of the screen. If you look here you'll see a row of LED lights at the bottom called the "backlight." These are the only lights in the monitor! Next I'll put in what's called the "optical system" which make the light even across the back of the screen.

The first sheet makes a nice even white background for the light. The next piece is called a "light-guide plate." You see its covered with dots. When light enters from the bottom edge it propagates down the plate by total internal reflection, unless it hits one of the dots. They make some of the light rays emerge out the front. Then engineers place a diffuser film; it helps eliminate the dot pattern from the light-guide plate. Then comes a "prism film."

They use this because light from the backlight emerges not only perpendicular to the back surface, but also at oblique angles. This sheet will increases the perpendicular part a bit: You can notice that here where I have the sheet is much brighter than where it isn't.

So at this point once we put the last diffuser film on, we have a very evenly lit surface, all from the single row of LED lights at the bottom.

The "backlight" are always on when the monitors on, but what controls what we see is this piece of glass: It functions as a shutter. At the back and front of this glass sheet are two polarizers. They stick tightly to the piece of glass, but let me illustrate it with two sheets that I have.

If I lay this sheet on the optical system you can see that it passes light. And if I put this piece on top it also allows light to pass if I put this one on top of it. But if I rotate it exactly ninety degrees to the bottom sheet you'll see the light disappears. The bottom sheet creates polarized light which will only pass through another polarizer set to the right angle. Now, of course, in this LCD monitor the front polarizer doesn't rotate - other than the off switch the monitor has no moving parts! Instead what we do is place these two polarizers 90 degrees to each other - this configuration that allows no light through - and then, if we want light to pass, we "twist" the light within the glass pane to match the front polarizer. How? This simple looking piece of glass does all the "magic." Let me put it back on and you can see that the image reappears. I just love that! It's actually a sandwich of glass.

Engineers fill the space between the panes with tiny glass beads to keep them separated and with organic molecules known as liquid crystals. These crystals have interesting properties in that they do not allow light to pass uniformly along both axes. Grooves are formed on the surface of both pieces of glass at 90 degrees to one another. The molecules in-between line up in a beautiful helix.

When light from the backlight passes through the first polarizer and enters the sandwich it's rotated by the liquid crystals so as to allow it to pass through the second polarizer and emerge out the front of the screen. This is known as the normally white mode. Applying an electric field across the sandwich causes the crystals to line up lengthwise.

Now the light that passes through the first polarizer is not rotated by the crystals and can no longer pass through the front of the screen. We call this the normally black mode.

Now that we can control the light through the glass, who do we get color? Let's look in detail at the piece of glass.

By controlling the voltage between these transparent electrodes we can control the intensity of the light that passes through. Now, there's much more to the glass plate.

Let's examine this section where my sleeve meets the gold background. If we zoom in you can see its made of pixels. If I turn off the image and backlight the glass sandwich you can you see the screen contains red, green, and blue sections. These are sub-pixels: The three together make a single pixel.

In the sandwich these are simply colored tiles that overlay the front transparent electrodes. They follows the RGB color model: We adjust the "electrode-shutter" behind the sub-pixels so that they make up a particular color. For example, to get the color of the blue in my shirt we set the red sub-pixel to 12% of maximum intensity, green to 21% and blue to about 50%.

And now for the last critical piece in the glass sandwich: On the back pane engineers paint tiny devices called thin film transistors. That's why these monitors are often labeled TFT.

Each sub-pixel has transistor which controls it. This transistor you see right here functions as a switch that allows the screen to be updated row by row.

By applying a voltage to a specific row while keeping the other rows grounded we allow each sub-pixel in that row to receive video data coming from the top of the screen. Only one row can receive information at a time, but the speed with which this happens for each row is so fast that your brain blends it into a fluid image.

What an amazing device. And also the technology that allowed computing to go mobile: Image laptops, cell phones and tablets without lightweight screen. I'm Bill Hammack the engineer guy.