HOW THEY WORK:“CRT and LCD diplays”

As seen in our previous presentation, both the CRT and LCD are used for displaying different forms of data or information.  The applicability of one over the other is either a matter of cost, convenience or both. 

As seen in our previous presentation, both the CRT and LCD are used for displaying different forms of data or information.  The applicability of one over the other is either a matter of cost, convenience or both. 

A liquid crystal display (LCD) is a thin, flat panel screen used for electronically displaying information such as text, images, and moving pictures.

This ranges from Desktop computer, Laptops, PDAs (Personal Digital Assistants), Digital Watches, TFT monitors for computers, televisions, instrument panels, and a wide range of devices ranging from aircraft control panels, to daily life devices such as home stereos, gaming devices, clocks, watches, calculators, and cell phones, etc.

Common on the list are the lightweight devices, coupled with portability, and its ability to be produced in much larger screen sizes that are impractical for the manufacture of large CRT displays.

Its low electrical power consumption makes it viable to be used on battery and solar powered devices.

It is an electronically-modulated optical device made up of any number of pixels filled with liquid crystals and arrayed in front of a light source  (backlight) or reflector to produce images in colour or monochrome. 

By 2008, the manufacture of LCD TV screens greatly surpassed that of CRT units.

If you happen to disassemble an LCD display (highly not advised), you will find that, it has several layers of what may appear as thin transparent, tinted and mirror like layers with varying darkness intensity.  Several are made of the following:-

•    Polarizing filter film with a vertical axis to polarize light as it
enters.
•    Glass substrate with ITO (Indium tin oxide) electrodes. The shapes of these electrodes will determine the shapes that will appear when the LCD is turned ON. Vertical ridges stretched on the surface are smooth.

•    Twisted nematic liquid crystal.
•    Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter.
•    Polarizing filter film with a horizontal axis to block/pass light.

•    Reflective surface to send light back to viewer. (In a backlit LCD, this layer is replaced with a light source.)
It is important to note that, from the simplest to the largest LCD displays, they all have the above substances in order to work.

ITO is mainly used to make transparent conductive coatings for liquid crystal displays, flat panel displays, plasma displays, electronic ink applications, organic light-emitting diodes, solar cells, antistatic coatings and EMI shielding.

In organic light-emitting diodes, ITO is used as the anode (sometimes positive). 

In more sophisticated LCDs, some light florescent bulbs are added on the edge of the screen, these send light to the reflector that in turn reflects the light outwards.

When a current is applied to the twisted crystals, they untwist so that light’s passage through the polarised layer is blocked. Most computer displays are lit with built-in fluorescent tubes within the LCD.

A white diffusion panel (reflector) behind the LCD redirects and scatters the light evenly to ensure a uniform display.

This phenomenon is used in the Common plane based LCDs as used for simple displays that need to show the same information over and over again e.g. Watches and microwave timers.

Active-matrix LCDs depend on thin film transistors (TFT). Basically, TFTs are tiny switching transistors and capacitors. They are arranged in a matrix on a glass substrate.

To address a particular pixel, the proper row is switched on, and then a charge is sent down the correct column.

Since all of the other rows that the column intersects are turned off, only the capacitor at the designated pixel receives a charge.

The capacitor is able to hold the charge until the next refresh cycle. And if we carefully control the amount of voltage supplied to a crystal, we can make it untwist only enough to allow some light through.

By doing this in very exact, very small increments, LCDs can create a grey scale. Most displays today offer 256 levels of brightness per pixel. 

As for coloured displays, an LCD that can show colours must have three sub-pixels with red, green and blue colour filters to create each colour pixel.

Through the careful control and
variation of the voltage applied, the intensity of each sub-pixel can range over 256 shades.

Combining the sub-pixels produces a possible palette of 16.8 million colours (256 shades of red x 256 shades of green x 256 shades of blue), as shown below.

These colour displays take an enormous number of transistors. For example, a typical laptop computer supports resolutions up to 1,024x768.

If we multiply 1,024 columns by 768 rows by 3 sub-pixels, we get 2,359,296 transistors engraved onto the glass!


eddie@afrowebs.com

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