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How LCD Screens Work

Liquid Crystal Display (LCD) screens have played a major role in the rise of personal digital technology. The first LCDs were used in calculators and digital watches in the 1970s, and the technology began to make its way into televisions and computer displays in the 1980s. During the early 2000s, the technology made its way into smartphones and PDAs. In 2007, the same year Apple introduced the first iPhone, LCDs surpassed CRT (cathode ray tube) displays in both sales and picture quality.



Every LCD consists of 6 basic parts. At the back of the display is a backlight (though in some devices like calculators and digital watches, it’s just a reflective surface). In most displays - including the one on your phone or tablet - the backlight consists of an array of light emitting diodes (LEDs). In front of the backlight is a polarized film (like you’d find in a pair of polarized sunglasses). This film only allows the light waves that are oriented vertically to pass through. Next is a layer of glass with electrode film on it. The middle layer contains the actual liquid crystals. These are tiny crystals that only allow light through in certain orientations, and change their shape based on whether current is applied. In front of the layer of crystals is another layer of glass with electrodes. Finally, a second polarizing filter sits at the front of the display.

Before we get into how all those pieces work together to make an image, we need to take a second and talk about polarized light. If you’ve ever worn a pair of polarized sunglasses, you’ve probably noticed that when you look at an LCD display - anything from your computer screen to the readout on a gas pump - you sometimes have a hard time seeing what’s on the screen, and that it can change depending on how you turn your head. That’s because of the way light works. Without getting overly technical, light is weird. Sometimes it acts like a particle (these particles are called photons), and sometimes it acts like a wave, moving up and down or side to side through space. Polarization works because of that wave-like movement. Polarized lenses like the ones you find in LCDs or some sunglasses or in the 3D glasses at a movie theater work by only letting through the light waves that are going in a particular direction. What’s more, if you take two polarized films or lenses and hold one still while rotating the other, you’ll notice that eventually no light comes through at all. That’s because one polarized film is blocking light waves that move vertically, and the other blocks the ones that move horizontally, which means that no light gets through at all.

Okay, so, now that we know how polarization works, we’re ready to take a look at how all the pieces of an LCD display work together to make images. When you turn on the display, the backlight begins to generate light. This light then hits the first polarized filter, which is arranged to only let waves that are moving vertically pass through. The vertical waves then pass through the glass with the electrodes, and strike the liquid crystals. Here’s where things get interesting: if you recall, we told you that the crystals change their shape depending on whether (and how much) electricity is applied. When a crystal is completely straight, it allows the vertical light waves to pass straight through, where they strike the polarizing filter at the front of the display. Since the front polarizing filter is arranged to only let through light that is moving horizontally, all the light from the backlight gets blocked, and the pixel is black. When a crystal is completely twisted, it reorients the light that passes through it, meaning that the light waves that strike the crystal vertically get rotated as they pass through, strike the front filter horizontally, and pass right through to your eyes.

So that’s how the light gets through, but that doesn’t answer the question of how images are generated. The crystals are arranged into rows pixels, and each pixel consists of three sub-pixels, each with a different colored filter (red, green, and blue). Each sub-pixel is controlled by a transistor that governs how much electricity is applied to the crystals inside, and therefore how much light passes through to your eyes. Controlling the flow of electricity through each sub-pixel causes the pixel to appear a particular color. Since your eye isn’t able to distinguish the pixels from one another without some sort of magnification, the pixels blend together to produce a seamless image.

If you’re wondering at this point whether this has anything to do with some of the terms that get thrown around in conversation about screen resolution, it absolutely does. The arrangement of the pixels into rows is where we get the terminology we use to talk about screen resolution. A high-definition screen is usually labeled as either 1080p or 780p. As you might expect, the “p” stands for “pixel,” and the number refers to how many rows there are. So, for example, a 1080p LCD television has 1,080 rows of pixels. Each row is 1,920 pixels wide, for a total of 2,073,600 pixels in your average high definition television. And if you’re wondering about the new 4K Ultra High Definition (UHD) TV’s that have hit the market recently, the terminology is basically the same. When you buy a TV or Blu-Ray player that says something like “4K UHD,” or “4K Ultra HD,” or if you’re streaming content from Netflix or Amazon with a similar label, then that means the image being is 3,840 pixels wide and 2,160 rows tall.

So there you have it, a basic primer on how LCDs work. At Phone Medics Plus, we don’t just want to be here for you when you need cell phone repair or computer repair. We also want to make sure you have the tools and knowledge you need to get the most out of your technological life.

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