The backlight assembly
There are two types of backlight formats: the guided type for screen of 20 in. or less and the direct type for larger screen sizes. Both use cold cathode fluorescent tubes (CCFT) for their low energy consumption and low cost. The guided type is slim and compact but suffers from complicated structure and low light efficiency. By contrast, the direct type is thick in structure but simpler in structure with high efficiency.
The assembly of the guided type consists of one CCFT on either side of the screen with a light guide, and a reflector behind the light together with one or more microprisms and one or more diffusers in front of it (Figure 11.22). The light guide is based on a methacrylate material and it is used to guide the light through the layers. The reflector is located in the back of panel and it is used to improve the reflection of light. It is made of a material called poly- ethylene (PET). The diffuser layer has two functions, to diffuse and to collimate the light to make it parallel and uniform. Using the same material PET as the reflector, the diffuser improves brightness by 20%. The light from the diffuser is collimated by the next microprism layer which improves bright- ness by 40%.
The direct light assembly has more than two lamps as shown in Figure 11.23. The same layers are used as for the guided light type per- forming the same functions. Direct light diffuser are used to diffuse the light and to avoid seeing the backlight. A transparent indium tin oxide (ITO) sheet connected to ground is used to filter out the noise produced by the lamps.
When servicing a backlight assembly, care must be taken to ensure that the layers are replaced in the correct order as well as orientation other- wise a permanent pattern may appear on the screen. Other precautions that must be observed when servicing the LCD panel include keeping the surfaces clean and avoiding scratching their surfaces, keeping the LCD dry as water could cause electrical shorts and corrosion, avoiding swift temperature changes as dew and ice could cause non-conformance and malfunction, avoid electrostatic discharges by ensuring proper body earth before handing the LCD and do not operate the LCD for a long time with the same pattern as this would cause image persistence which may result in permanent damage.
