LCD Technology: How LCD Displays Work

The Technology Behind How LCD Displays Work

This is the crux of how LCD displays work. LCD – or Liquid Crystal Display is the technology that uses the special properties of liquid crystals to create products including televisions, monitors, clocks, and control panels. It’s often used as a broad umbrella category to cover other image technologies including cathode ray tube (CRT), plasma and light emitting diode (LED) which can be used in similar applications. However, those technologies operate differently, and for this article we’ll focus only on the function of LCDs. This includes a look at the basics of how LCDs work, specifically monochrome LCDs, as well as a more in-depth review of the two modes of LCD function: positive mode and negative mode.

How LCD Displays Work: A High-Tech Blind

An LCD works as a valve, either allowing light to pass through, or blocking it. Unlike some other image technologies, it does not generate the light itself. To help explain this, let’s compare the LCD to a set of blinds… either Venetian blinds or verticle blinds.

As you know, you can adjust the blinds to alter the amount of light desired. When fully closed, the blinds block light completely; when open, all light passes through; and when angled, partial light comes in. An LCD works similarly to this. With one significant enhancement an LCD has the ability to block light in some areas and allow light to pass in other locations of the glass.

One significant advantage of LCDs over other technologies such as LEDs and OLEDs (organic light emitting diodes) is their power-efficiency. Because LCDs are not an emissive display technology – they control the light passing through them rather than producing the light themselves – they require less power to operate. Ultimately, this results in lower over all cost.

LCDs can be designed in two ways, multiplexed or static. In a multiplexed approach, components of the display – segments or dots — are ON between 20% and 80% of the time, in an alternating system. In a static display, all segments are ON 100% of the time. The result of a static display is a darker, more defined look. However, the trade-off is cost. A static display is more expensive to run since resources are committed to each segment 100% of the time.

Positive Mode – Letting the Light Shine

Positive mode is the more commonly used of the two ways in which LCD displays work. In positive mode, dark letters, numbers, and symbols appear on a light background. The diagram and photo below show an example of a positive mode static display

In this photo below you can see that the background is yellow/green where light is shining through and that black letters show where the light is blocked.

Positive mode LCDs are popular because they don’t require a backlight to function, and therefore use less power. Typical applications of positive mode LCDs include wrist watches, hand-held calculators and battery-operated temperature gauges.

Negative Mode LCDs – The Blackout

Negative mode LCDs work in reverse of positive mode LCDs. In negative mode the characters are brighter than the background. Take a look at the diagrams and photos below.

As you can see, this LCD features a dark blue background with white letters. The letters are created using a backlight.

Many users prefer to read from a negative mode LCD because the information stands out better from the background than in positive mode. However, though they are attractive and usable, negative mode LCDs have two drawbacks to consider:

  • The backlight must be on all the time to read the display. This increases power demand.
  • Hot spots are common in some negative mode displays. A hot spot is where there is a bright area close to the LED backlight and dark area, or cold spot, further away from the backlight.

Products that typically use negative mode LCDs include cell phones, GPS, credit card readers, and appliances such as microwaves and coffee makers.

How LCD Displays Work: Bringing it all Together

LCD technology works in several modes to be both cost and design-effective in a range of applications. By either creating letters by blocking the light emission, or using a backlight to let bright letters shine through a dark display, LCD technology is implemented in both everyday consumer products and high tech applications. For more information about how LCD displays work, please visit us the “journals” link on our site.