A detailed knowledge of the technological background of LCD’s is not strictly necessary in order to be able to use them. An LCD Display basically consists of two very thin glass plates between which there is a liquid crystal layer some 10;.1m thick. This layer consists of a crystalline molecular structure. What is essential is that the molecular structure changes under the influence of an electrical field. Depending on the direction in which the molecules are organized, the liquid crystal layer becomes either transparent or reflective.
How LCD technology works
The inside surface of the two glass plates is coated with a transparent, conductive layer and this forms the electrodes. A voltage applied to them creates an electrical field which causes the molecules in the liquid crystal layer to change direction.
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The plane affected (or segment of a digital display) then alters in transparency. If we see an LCD cross section we would easily see the SiO2 layers. These insulate the electrodes from the effects of the liquid crystal and the two polarizers (polarization filter discs).
The alignment of the crystalline structure is such that transparency will not change until a voltage is applied. When an (alternating) current is applied between the two electrodes, the crystal molecules will be arranged horizontally.
As can be seen, the lower half has no drive current and so the liquid crystals are in a vertical configuration. In an unenergized state in a reflective LCD, a vertical and a horizontal polarizer are laminated onto the liquid crystal cell at right angles (or 900 rotated) to each other. Vertically polarized light entering the front of the cell (A) follows the rotation of the crystal alignment as it passes through the cell again rotating 90 degrees, the polarized light passes through the horizontal polarizer to the reflector (E).
The light is then returned through the cell again rotating 90 degrees, and passes out of the LCD through the vertical polarizer. In an energized state, however, across one or more of the character segments the crystal molecules in the segment align themselves with the electrical field. Rotation does not therefore occur in the energized segment.
The vertically polarized light from the energized segments cannot pass through the horizontal polarizer, but is rather absorbed by it. The segments therefore appear as dark images against a light background.
The opposite happens with parallel polarisation filters, the powered segments are transparent and appear as bright images on a dark background.
Things are different when a semi- transparent mirror is used as a reflector. It results in ’transflective' displays which can be illuminated from in front as well as from behind. When current consumption is of minor concern, in mains power equipment for example, the light source behind the display can be constantly on.
If the surrounding brightness is greater than the light intensity effected by the built-in lighting, the display operates in a reflective manner. lf the external brightness is less, 'transillumination’ or transmission occurs. There are also displays which operate exclusively on a built-in light source, that is to say, producing transmission without a reflector.
These are called transmissive displays. Recent developments seem to favor reflective and transflective displays, whereas the transmissive type tends to be pushed into the background. The former types nearly always display dark characters on a bright background, whereas the transmissive type features transparent characters on an opaque (dark) background.
courtesy: Elektor electronics