How do LCD screens work?

How do LCD screens work?

Do you own one of those old computer desks where you can put a whole new shelf behind your flat computer monitor? Have you ever wondered why there even is so much, seemingly wasted space back there?

The answer is quite simple-computer monitors, and TV sets were much bulkier some 20 years ago. Those were the so-called CRT (cathode-ray tube) televisions. Aside from being big, some other differences are that the screen itself was curvy; they used up more energy, and also, got really warm if they were on for too long.

Today’s computer monitors and television sets use the LCD (liquid crystal display) technology- much as calculators, digital watches and smartphones do.

What is the first advantage that comes to mind when you compare LCD and CRT monitors? It’s its size. Today, you can put your TV on the wall without the need of an entire shelf to put it on. They are lighter and easier to carry if you are moving, spend less energy, and last but not least, they look far more elegant.

Now, have you ever thought about how they actually work? How do you see the picture that you see? And, If you know that a TV screen uses only three colours to form an image, how is it that we see all the different shades of all the different colours? The answer lies in understanding how polarized light and those liquid crystals work.

What is polarized light?

Light generally travels in all the directions. If that was not the case, your regular light bulb would shine only on things directly under it. This is where polarization comes into play. The polarized glass allows the light to come through only from one direction. If you have a vertical polarizer, only the vertical light can come through, while the horizontal light beams are blocked. The rule is the same for horizontal polarizers.

If you have a pair of sunglasses with polarized glass, you will notice that, if you tilt it, the image gets darker. This is because light beams in other angles cannot go through the polarized glass.

What are liquid crystals?

It is known that all matter in the world comes in three different states: solid, liquid, and gas. Crystals are inherently solid, so how can there be liquid crystals?

A scientist, F. Reinitzer, discovered this mixed state of matter, which later found its application in technology, mostly thanks to the ability to go from crystal to liquid in many stages. There are 2 phases that are important for understanding how LCD screens work and they are a nematic and smectic phase.

In the nematic phase, the liquid crystal molecules are twisted in a rather same direction, in the smectic phase, however, they form layers that go in the same direction but do not have any physical contact between them.

How does this apply to LCD screens?

LCD screens are divided into small rectangle – pixels, and each pixel is divided into subpixels – three rectangles all in different colours: one blue, one green, and one red. Each subpixel has a certain value (from 255 to 0), and when it is 0, that pixel is turned off.

For example, if green subpixel is turned off we get purple colour on the screen, however, if the green subpixel is at a value of, say, 100, we will also get purple but in a different shade.

But, how does a subpixel get turned on or off? This is what goes on behind each subpixel.

Each LCD screen has a backlight which is behind the entire monitor. The white light that it emits goes through the horizontal polarizer, and only horizontal light beams can go through it, the vertical ones are blocked.

The horizontal light beams then go to the liquid crystals. Stimulated by electricity, the liquid crystals’ molecules twist (nematic phase) and in that way, bend the horizontal light beams and make them into the vertical light beams.

The vertical light then goes through the vertical polarizer, and that is how a subpixel lights up.

If the liquid crystals are not stimulated by electricity, they will be in the smectic phase, and they will not bend the horizontal beams into the vertical ones. That way, since the next filter is the vertical polarizer, and the vertical beams were already blocked, no light will reach the subpixel, and it will remain turned off.

This happens for each subpixel in each of the pixels simultaneously, creating an image that we see on our LCD screen. An HD monitor has about 2 million pixels.

Conclusion

And now you know how an LCD screen works and what brings so much pleasure to your eyes.

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