How LCDs Work

If you’re an electronics hobbyist of any sort or even just the curios type, you may wonder how LCDs work. The LCD screen is ubiquitous. From our big screen TVs to our computers and “smart” devices like watches and phones to small displays we use in our projects, the LCD is now truly the go-to display for hobbyists and manufacturers alike. While newer technologies like organic LEDs (OLEDs) and quantum dots threaten to loosen some of the LCD’s grip on the electronics industry, it still reigns supreme as the king of displays as of this writing.

If you read this blog often, you know I sometimes like to start my posts off with a bit of quick history on the device or topic of discussion.

Toward that end, the story of the LCD began in the late 1960s when a few of RCA’s scientists invented the device. The goal: to make flat, wall-mountable TVs. Today, most of us have already disposed of our heavy, bulky, power-hungry CRT (cathode ray tube) based TVs, but back then CRTs were the only available option for screens. While those of us living in current times take our thin, cheap, lightweight LCD TVs for granted (along with our laptops, phones, and even displays on our appliances), the scientists and engineers involved in the creation of the original LCDs failed and could not create a viable wall-mounted LCD TV at that time. Because of this, RCA did not commercialize the LCD.

But good technology has a certain persistence about it, tending to eventually emerge victorious despite roadblocks and setbacks. This is due — at least in part, to the onslaught of time plus the relentless work of various scientists, engineers and even hobbyists. Eventually, the LCD did start to appear in various electronic devices, and it took off.

The rest is history; now they’re everywhere and I’ll abruptly conclude my throw-back with that.

But before we take a look at the inner workings of LCD screens, I want to let the reader know what they can expect from this article.

I’m not going to talk about how to wire or hook up specific LCDs or LCD breakout boards nor discuss the specs on said LCDs/boards. Also, this post doesn’t cover writing code regarding the use of an LCD. Wiring and especially writing code for a particular LCD is dependent on the LCD of choice and the microprocessor/board one chooses to use as the brains of their project.

Rather, I’m going to offer a solid treatment on the basics of how LCDs work and discuss the basic types of LCDs you may run into. This will be a primer for future, more detailed articles concerning the subject.

Now, let’s take a look at how LCDs work.

How LCDs Work – Light Polarization

LCD screens rely on the polarization of light to work, so it’s appropriate we start with that.

But what is light polarization? Good question – let’s take a quick minute to figure it out.

Light (or any electromagnetic wave) can be polarized.

To visualize this, imagine you have a rope in your hand and start to shake it. Waves will form and travel along the rope. They will be in one plane, assuming you don’t rotate your wrist or wiggle the rope in a different direction. The plane the waves along the rope travel in may be vertical or horizontal, as we can see in figure 1.

Figure 1: waves along a rope. In part (a) the waves are vertical and in part (b) they’re horizontal. The waves in the picture are transverse waves, just like light and all other electromagnetic waves.

In both cases, we can say that the waves along the rope have a linear polarization because the waves are in one plane.

Now, let’s say we put an obstacle in the path of the waves with a slit in it. If the slit is vertical, the horizontal waves will not pass through it. Similarly, if the slit is horizontal, the vertical waves will not pass. If we use two obstacles, one with a vertical slit and one with a horizontal slit neither will pass. Figure 2 offers some intuitive insight into how light can be polarized.

 

Figure 2: this figure offers an intuitive demonstration of how polarization works. In part (a) the vertically polarized waves pass and in part (b) the horizontally polarized waves do not pass because the orientation of the slit will not allow it. Note that both ropes have linearly polarized waves because either way, the waves reside in only one plane. One can polarize light because, like the rope, it travels in waves.

 

Figure 3: two pairs of polarized sunglasses at 90 degree angles. Notice no light passes through where they overlap. This is because both vertically and horizontally polarized light waves are being blocked.

Now that we have a basic intuitive grasp on light polarization, let’s talk about liquid crystals.

Liquid Crystals 101

The acronym LCD stands for liquid crystal display. As the name suggests, you can’t make an LCD without liquid crystals, but what exactly are liquid crystals?

You already know what a liquid is. In case you don’t know, a crystal is a solid whose atoms arrange themselves in a high degree of order, like table salt (which is a crystalline solid) or the cool looking crystal “rocks” one may find in a cave.

Figure 4: close up of table salt. Notice how the salt crystals look like cubes. This is because the sodium and chlorine atoms arrange themselves in an orderly fashion. Other crystals do the same, though the shapes may be different for different crystals.

Figure 5: rock crystals similar to those one may find in a cave. Again, the molecules in the crystals arrange themselves in an orderly fashion.

Liquid crystals are materials that flow like liquid but, like crystalline solids, pack at the molecular level with a high degree of order.

The molecules are mostly organic (though I’m not suggesting you crack open your display and eat them!) and appear in a lot of places, including the cells in your body. What makes them useful for LCD screens are their electrical and optical properties.

The liquid crystal molecules of interest are rod shaped and are electrically polarized. Some clarification on this is in order. Though liquid crystals can polarize light (as we’ll soon find out), I’m not talking about their ability to alter light waves traveling in a certain plane in this context. I’m talking about the molecules having more electrons in certain areas than others. This gives them a molecular dipole (sort of like an electric charge) and enables one to control their orientation with an electric field or a voltage.

Figure 6: a typical liquid crystal molecule. Notice the rod shape. The red area has a high electron density, the blue areas a lower electron density, making the molecule an electric dipole.

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How LCDs Work – the Practical Stuff

Now we know something about the polarization of light and liquid crystal molecules. It’s time to put it all together and talk about how the displays you’re familiar with work. Light polarization, chemistry, and electronics all work in tandem to give us the displays we use today.

Common Types of LCD Displays

Before we dissect an LCD screen and talk about its individual parts, it will be helpful to quickly discuss the basic types of LCD displays hobbyists are likely to use.

Refer to figure 7 for the following discussion.

Figure 7: the layers of a typical LCD screen. Some screens have an anti-glare layer on top of layer 1, but this is optional and not all screens have this layer.

Reflective LCD screens rely on ambient light passing through the layers and then reflecting back through the screen again to the viewer. In this type of LCD, the last layer (layer 6 in figure 7) is a mirror so the light that enters the screen reflects back. Since the light must pass through layers 1-5 of the screen twice, some light is “lost.”

Transmissive LCDs use a backlight rather than a mirror. The backlight corresponds to layer 6 in figure 7. In this case, the light only needs to pass through the screen layers one time. Transmissive LCD displays are great for viewing where there is no good external light source. The downside is that the backlight eats up a fair amount of power relative to the power consumption of the whole screen. Newer ones use an LED backlight which cuts down on power consumption though, saving battery life. Many smaller modern LCDs are transmissive.

Transflective LCD displays are a hybrid of both the above. They use a special polymer between layers 5 and 6 which can both reflect light and transmit light through it from the backlight depending on ambient lighting conditions. These often find a home in situations where ambient light is variable, like a car.

Anatomy of an LCD

Now that we know about the basic types of LCD screens, let’s talk a bit more about the individual layers of a common LCD. Refer to figure 7 again.

The first layer is a usually a polarizing light filter. In figure 7, this layer vertically polarizes light. The fifth layer is similar but polarizes light horizontally. When light waves oriented in all directions enter the first layer, only waves with a vertical orientation pass and enter the next layer.

Next, we have layer 2, the electrode layer. The electrodes are mostly clear, thanks to some clever materials science. This is because light must be able to pass through them so they don’t interfere with the display.

LCD manufacturers use indium tin oxide (ITO) for this layer. They deposit ITO on the surface in the shape(s) they wish the LCD to display.

Layer 3 is the liquid crystal itself. Before we go any further, here’s a little more detail on liquid crystals.

As we know, there are different varieties of liquid crystals.

Twisted nematic (or TN for short) is one type which refers to the alignment of liquid crystal molecules when no electric field is present. In this situation, this material forms a helical shape and is able to polarize light passing through by 90 degrees. When an electric field or voltage is present, the material untwists, and, because of this there is no polarization. In other words, when we apply a voltage to it, it does not polarize the light. This causes the light passing through to be blocked by the front vertical polarization filter’s (layer 1) interaction with the horizontal polarization filter (layer 5). Thus, the characters on the screen are now visible. By varying the strength of the electric field, the brightness varies. Figure 8 offers more detail on how this works.

Figure 8: another representation of an LCD in action. Here, the top plate and bottom plate refer to layers 2 and 4 from figure 7, respectively. The LC layer is the liquid crystal layer. Near the middle of the figure, we can see light entering the first polarizer from all orientations (thus the star shape) and exiting it with a vertical polarization.

Also, this section of the figure shows the LC molecules are aligned due to an electric field, so light does not continue to pass through the second polarizer. Rather, the segment of the ‘2’ from the top of the figure appears dark so the viewer can see it. When there is no electric field, the LC molecules are in the twisted arrangement. Because of this, they rotate the light once again allowing it to pass into the second polarizer. Then, the mirror reflects the light back to the viewer making the segment of the ‘2’ appear to be clear, thus almost invisible.

The twisted nematic design was okay for early LCD displays, but TN material has some issues. It isn’t very responsive and doesn’t have a clear threshold of operation. So, in some instances, an electric field forces the liquid crystals to align, but in others – say a different temperature – the same electric field may not.

Enter super-twisted nematic (STN) liquid crystal.

The original incarnation is a bit slow to respond but the polarization twist is greater than TN liquid crystal. Though this type is an improvement over TN, better versions have been released since.

These include FSTN- and FSTN+ where the F stands for filtered. These variants use an additional layer between STN liquid crystal and the rear polarizer. This gives the screen better contrast and sharpness.

Finally, note that color LCDs use red, green, and blue (RGB) color filters (not shown in figures 7 and 8) and/or pixels to generate color images. We’ll talk a bit more about this in the summary section.

More Recent Developments in LCD Technology

It’s time to talk about some newer LCD technology that you’re likely to see in your TVs, laptops, and phones. We won’t go into gory detail (at least this time) in how they work, but the concept is similar to the older LCD incarnations from above.

First up, we have IPS and TFT LCD displays.

TFT stands for thin film transistor. For more on transistors and how they work, see An Introduction to Transistors.

This type of display uses transparent FETs (a type of transistor) on glass for controlling the rows and columns of the screen. This allows much more current to move around and changes the electric field the screen uses to control the liquid crystal material quickly.

IPS is an acronym for in plane switching.

Here, the electrodes are on the sides of the pixel so the electric field that controls the liquid crystal orientation is parallel to the glass. This gives the viewing angle a significant boost, greatly increasing it. In turn, this makes the color consistency better. You may have tried to view an older LCD TV at a wide angle and noticed the color appeared to be altered and washed out. IPS screens help reduce this phenomenon.

Check Your Cholesterol

Another, newer LCD technology worth mentioning is cholesteric LCD or ChLCD for short.

The name refers to the cholesterol-like liquid crystal material.

These displays are reflective and sunlight-readable, whereas many transmissive screens tend to wash out in sunlight. ChLCDs break the mold we discussed earlier (that a lot of light is ”lost” in reflective LCDs) because they lack the polarizer layers from figure 7. Because of this, cholesteric LCDs reflect a higher amount of incoming light than standard screens.

The coolest thing about these LCD displays is that they are bistable, which is just a fancy way of saying the picture stays on after the power is removed. If you order a cholesteric LCD, don’t freak out if it arrives with the manufacturer’s logo and tagline still visible! This is perfectly normal. They also sport a high contrast and good viewing angle due to the lack of polarizers. One of their drawbacks, however,  is the relatively slow refresh rate, especially at colder temperatures.

Keep an eye out for ChLCD in electronic paper and other uses.

How LCDs Work: Summary & Recap

• LCD stands for liquid crystal display.
• Most LCDs rely on light polarization to work.
• The molecules in solid crystals arrange themselves in an orderly fashion. Table salt is a good example of a common crystalline solid.
• Liquid crystals have both properties of a liquid and of a crystalline solid. They are organic molecules which can polarize light. They also have an electric dipole which allows them to be controlled with an electric field.
• There are 3 common types of LCD displays. Reflective displays use ambient light. Transmissive screens have a back light. Transflective displays are a hybrid of both.
• A series of layers make up an LCD display. Figures 7 and 8 graphically depict the layers of a typical LCD.
• Voltage controls the orientation of the liquid crystal material in the LCD due to the liquid crystal molecule’s electric dipole. A dipole is just an abundance of electrons in one area of a molecule, giving it electrical properties. This, combined with polarizers, allows light to pass or not pass, which controls which (if any) characters show on the screen.
• A pixel is the smallest controllable element on a screen. They are shaped like little squares. Many of them grouped together and turned on in a certain pattern can form a meaningful picture.
• Color LCDs use color filters or color pixels in addition to the layers. A colored pixel is actually three pixels in one. The three “mini pixels” are red, green, and blue. By varying the intensity of each color “mini pixel” within a particular pixel, it can display any of all the possible colors. This can number in the tens of millions depending on the specs of the screen. Displays like this usually find their home in TVs, computers, and phones. Other smaller, simpler LCD screens are often monochrome (meaning one color, usually black or grey) or are able to display only a limited number of colors.
• Newer LCD technologies include thin film transistor (TFT), in plane switching (IPS) and cholesteric LCD (ChLCD).
• Other new technologies like OLEDs and quantum dots may eventually replace many LCD displays, but right now the LCD is still the most common and cheapest display in use.

Seeing How LCDs Work Clearly

So now we know the basics on how LCDs work.

Of course, it gets a bit more complicated when you drill deeper into the physics and chemistry of the devices, but this was just an introduction to LCDs.

After reading this, those who are using LCD screens in their projects should have a better understanding of how LCDs work.

Drop me a comment and tell about your projects involving LCDs. Are you using any of the newer technologies? What are you using the LCD display for?

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