The Liquid Crystal Display LCD

Liquid Crystal Display (LCD) is a broad term that has been used to describe many display technologies and it is often a common belief that a Liquid Crystal Display LCD is the same as CRT (Cathode Ray Tube), LED (Light Emitting Diode) or Plasma displays. However, this is not the case. A Liquid Crystal Display LCD consists of two pieces of glass, with a liquid between two.

liquid crystal display 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 liquid crystal display to a venetian blind. Blinds can be fully closed, so that they block light completely, they can be fully opened allowing all of the light to pass through, or they can be angled allowing only partial light to shine through.

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. A liquid crystal display works similarly to this, with one significant enhancement: a liquid crystal display has the ability to block light in some areas and allow light to pass in other locations of the glass.

What a Liquid Crystal Display Is NOT

The liquid crystal display are not emissive displays. CRT’s, LED’s and plasma displays produce their own light and are referred to as emissive displays. It is because of this that they require more power than a liquid crystal display. This make liquid crystal displays more energy efficient.

However, a major drawback of the liquid crystal display, is that they cannot be seen at night. The solution to this problem is to install a backlight behind the LCD. Backlights do require more power than the liquid crystal display itself, but they can be turned ON only when necessary. Many products that are powered by batteries, like cell phones and watches, have the backlight dim or shut OFF after a certain amount of time.

How does a Liquid Crystal Display LCD work?

A picture is worth a thousand words. Take a look at the drawing to the left. The liquid crystal display is built the same as a sandwich. The top of the display contains a polarizer and a glass substrate. There is also a polarizer and a glass substrate on the bottom. Between the two layers is a twisted nematic liquid crystal fluid.

The basic structure of the liquid crystal display can be seen here. Light passes through the bottom polarizer and then the layer of glass. The light has two options.

  1. the fluid is rotated to allow the light to pass
  2. the fluid blocks the light from passing through the front of the glass.

What job does the Polarizer perform?

The polarizer is an optical filter that allows light waves to pass through in one direction and blocks the light waves in the other direction. It combines the light of mixed polarization into a well-defined focused beam. An example would be the covers for license plates which to make them difficult to read unless you are directly behind the car.

What job does the glass substrate perform?

The glass contains a substrate with an ITO (Indium Tin Oxide) coating. ITO is a transparent and colorless material, if applied in thin layers. It is used as a coating on the glass because it is transparent, conducts electricity and it acts as a barrier to hold in the twisted nematic liquid crystal fluid.

What job does the twisted nematic fluid perform?

The twisted nematic liquid crystal fluid is oval in shape. Below is a cross sectional drawing that shows the fluid’s shape. The molecules will rotate to block light or allow it to pass when an electric field is applied.

In conclusion…

When selecting the best liquid crystal display for your product it is good to have a basic understanding of LCDs, what they are and what they are not. It also helps to understand all the options available to you and how they affect the appearance and function of the display, this is covered in various other article journals. We have written several journals for engineers to be quickly informed so they can choose the best LCD for their particular product. This article provides a simplistic explanation of what an LCD is.

If you have questions about LCDs for your next project, contact us today!