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All you need to know about LCDs

The industry of displays is one that has rapidly grown and developed. Liquid crystal displays (LCDs), particularly monochrome LCD graphic modules have persevered and remains a popular choice in a variety of applications, due to their reliability and cost-effectiveness. In your day-to-day life, you most likely come across several of them. From the display on your home thermostat to the one on your oven or microwave. LCD displays are everywhere.


In short, an LCD is a flat panel display that operates by using liquid crystals to create imagery on a screen. If you are involved with optoelectronics and display, then understanding LCDs, how they work and how they compare to other display technologies is vital. Maintaining and choosing new displays becomes simpler with this knowledge.

Monochrome LCD means that the display consists of a single colour LCD panel to display graphical content, thus unlike colour TFTs, which use RGB pixels, monochrome LCDs use a single colour pixel.


What are liquid crystals?

Firstly, you need to know what is meant by liquid crystals. Liquid crystals, as the name implies, are closer to a liquid state than a solid one. Their characteristics are between that of a conventional liquid and that of a solid crystal. Meaning that it does ‘flow’ as a substance, but the individual molecules carry a crystal-like solid orientation.

Liquid crystals are very sensitive to temperature. They are formed through a vast amount of heat, and it would only take a little more heat to turn liquid crystals into a real liquid. This is why LCDs can play up in extreme weather conditions.


How do LCDs work?

Within an LCD, liquid crystals are embedded in the screen. These are what work to produce an image of sequence on the display. Though it is important to note that liquid crystals do not produce light themselves. In illuminated LCDs, the light is emitted from a backlight which illuminates the liquid crystals. They operate by controlling the transmission of light through the liquid crystals.

So, to form imagery on the display, voltage is applied to specific pixels in the liquid crystal layer. This causes certain liquid crystals to align in a specific direction, altering the quantity of light that is transmitted through the display. By varying the voltage, the amount of light passed through can be controlled, allowing different shades of the monochrome colour to be created.

Polarized light denotes light waves with vibrations occurring in a single plane. In LCDs, this is accomplished by implementing polarized layers, with tiny nematic (twisted) liquid crystals positioned between the filters. Each pixel has polarizing filters on both the front and back. Polarizing filters dictate what light passes through the LCD screen and enable a better contrast ratio. Without these filters, the visual results of the display will have poor quality. Monochrome LCDs tend to have higher contrast ratios and faster response times than colour displays, making them ideal for applications requiring simple text and graphics.

When the liquid crystals are off, electricity is not being passed through to the crystals from the transistors. This causes the crystals to appear brighter because of the 90-degree twisting of the nematic liquid crystal. This enables light to travel through both polarizing filters on the pixel. In this way, the pixel appears illuminated because of the light passing through.

Comparatively, when the liquid crystal is switched on, electricity is being passed through the nematic liquid crystals, causing them to straighten out from their twisted state. The effect of this is that the polarizing filter in front of the liquid crystal blocks out the light. This leaves the pixel dark, in its switched-off state.


Backlights

As mentioned, liquid crystals do not emit light. A backlight is used for illumination in LCDs. LCDs can operate without a backlight, for example, pocket calculators where users need natural light to visibly perceive the information displayed on the screen. This makes them a more-energy efficient display technology as without a backlight they consume much less power than other display types. But in some more modern colour LCDs, a backlight will be installed behind the LCD panel to improve readability and aesthetics. The most common type of backlight is LED (light emitting diodes) based. LED semiconductors release photons and emit light after an electric current flows through them.


Lifetime

As with many other display types such as OLED and TFT, an LCDs lifetime will depend on serval factors, mostly relating to the conditions the device is stored in and used in and frequency of use. One benefit of LCDs over these other displays, is their extended lifespan, which have less frequent need for replacement.

But an LCD device is only as strong as its weakest component. The lifetime and durability of the backlight is usually the most influential component in determining an LCDs lifetime. When the backlight reaches the end of its lifetime, it will dim, making the whole LCD appear to diminish.

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