Liquid Crystal Display (LCD) technology
Liquid Crystal Display
With the advent of digital imaging systems it has become increasingly common to view medical images on electronic soft-copy display devices rather than film. This has many advantages, not least the cost-saving involved in not printing film and managing its storage.
LCD Displays Screen
For many years the dominant technology in electronic displays has been the cathode-ray tube (CRT), as it has been in television viewing in the home. Recent years, however, have seen the development of a number of alternative technologies for such small displays, including plasma, liquid crystal and projected displays. This report evaluates a display that uses liquid crystal display technology.
Liquid crystals exist in a state that displays some of the characteristics of a solid (i.e. molecules in the crystal are fixed in orientation within the material) and some of the characteristics of a liquid (i.e. the molecules are free to move around while maintaining that fixed orientation). A substance can exist as a liquid crystal over a relatively narrow temperature range; any higher than this range and it becomes a normal liquid and any lower and it becomes a normal
solid. Liquid crystals can exist in several different “phases” displaying different characteristics, and it is a particular instance of one of these phases that is used in liquid crystal displays – the “nematic” phase.
In the nematic state, the liquid crystals are naturally aligned. They can be forced to change their alignment by application of an electrical voltage across the crystal. The degree by which they change alignment can be controlled predictably by the amount of voltage applied. As the alignment changes, the polarisation of any light passing through the crystal is affected in a similarly predictable fashion. It is this property that makes liquid crystals suitable for use in display devices.
Liquid crystals are used in displays by sandwiching them between two layers of glass. On either side of this sandwich are placed two polarising layers. These two layers of polarising material have planes of polarisation that are perpendicular to each other. A backlight is placed on one side of the sandwich and the light from this passes through the first layer of polarising material, causing it to be polarised. The plane of vibration of the light is modified by the liquid crystal according to the alignment of the crystal, which in turn is controlled by the voltage across it. This voltage is applied by electrodes on one side of the liquid crystal cell, hence this particular construction is referred to as ‘In-Plane Switching’.
Finally, the transmitted light reaches the front polarising layer, and the amount of light allowed through this layer is dependent on the plane of vibration of the light in relation to this second polarising layer. By adjusting the orientation of the molecules, the plane of vibration of the light can be modified and thus the amount of light passing through can be controlled.