by Daniela Danilova
Before there was on-demand streaming, there were infomercials. Although nowadays a practically extinct form of advertising, these inserts used to be a staple in every child’s TV time, and the source of all of our most-wanted toys. Whether it was the Pillow Pet or the Fushigi Magic Gravity Ball, it was a must to call over your parents and aggressively poke the screen over the version you wanted most. Instead of voicing words of approval, their response was probably something along the lines of, “Quit it! You’ll break the screen!” Indeed, as your pointer made contact with the monitor, you noticed odd waves emanating from underneath, yet the image returned to normal almost instantly afterwards. This is the magic of the liquid crystal display, more commonly known as the LCD.
An LCD is a type of electronic display which cleverly uses polarized light, electricity, and something known as liquid crystal. Liquid crystal isn’t a single substance but rather a specific state of matter somewhere between a solid and a liquid. It consists of small molecules that are able to slip and slide along each other as if they are a liquid, but also to orient themselves in a regular grid-like pattern, mimicking a solid object. Although individual crystals may change location, the overall structure remains.
Liquid crystals were first described by Austrian chemist Friedrich Reinitzer and German physicist Otto Lehmann. They noticed that a sample of cholesteryl benzoate, a derivative of cholesterol, appeared to have two melting points, turning into a cloudy liquid at one temperature but a clear one if heated further. They documented the existence of this “orderly liquid” that seemed, at first, to not have any real purpose, but the existence of liquid crystals in this gray zone became the key to their ultimate success. Moreover, liquid crystals can exist in several different structures (Figure 1), and the state they end up assuming can be controlled via an electric current (this will become important soon!).
Figure 1: The varying structures of liquid crystals (Image Source)
Before we understand how zapping liquid crystals leads to some of our favorite inventions, we need to talk about the anatomy of light. A beam of light is made up of many different light waves, all traveling in various orientations. When you pass the light through a vertical polarizing filter, it only lets through the vertically-oriented waves, blocking out all else (Figure 2). If you then pass this filtered vertical light through a sheet of liquid crystals, the crystals will change the orientation of the light to match their own. What happens if you have multiple sheets layered atop one another? If they are laid out in a spiral pattern, you can gradually twist the vertical light to a completely horizontal position. This way, if there was another polarizing filter present, this time oriented horizontally, all of this light would pass through to the other side.
Figure 2: A beam of light passing through a vertical polarizing filter, letting through only vertically-oriented waves. (Image Source)
Now, cue the electricity! Remember mentioning how an electric current can change the shape of a collection of liquid crystals? Running a current through the setup above, you would notice that the liquid crystal spiral is now completely untwisted, meaning the vertical light that passed through them also emerges untwisted. This means none of it can pass through the horizontal filter at the end, which appears dark from the other side (Figure 3). Whether you realize it or not, we have just walked through the basic principles of an LCD screen. Full light and full darkness are just the two extremes. If you have a transistor, a tiny controller of electric current, assigned to each pixel, you will be able to precisely control the amount of untwisting in the liquid crystals, allowing the pixels to take on the full range of brightness. Add in red-, blue-, or green-colored filters to each, and boom – you have yourself a colored screen.
Figure 3: The amount of light that makes it through a pixel on a screen depends on the orientation of the liquid crystals within (Image Source)
Today, LCDs can be readily found in everything from calculators and digital watches to TVs and even mood rings. They’re the reason we’re able to enjoy screens the thickness of a small stack of papers rather than a textbook, revolutionizing the way we interact with and incorporate technology into our daily work and leisure. Next time you find yourself binge-watching a favorite show, you can thank these tiny swirling universes for bringing life to the beloved characters and riveting story lines that make them so memorable.
