LCD Monitors

Now shipping with all the but the cheapest complete PCs are LCD monitors. Advances in display manufacturing and associated cost reductions with economies of scale have brought LCD monitors into the mainstream, shipping with budget systems that start at just £400. LCD monitors come in all shapes and sizes, have differing resolutions and inputs. The purpose of this TekSpek is to provide a basic understanding of how LCDs work, delineate their desirable characteristics, and to offer basic buying advice.

The technology

Underlying technology
LCDs work in a relatively easy method to understand. Firstly, behind each LCD screen, where screens are defined by the number of pixels on a given panel, a light source is shone through from the behind to panel, through two panels of glass sandwiching the LCD, to your eyes. Each pixel on the LCD display has an array of liquid crystals (from whence the name arrives) whose molecules can be charged with a variable amount of voltage from electrodes over each subpixel (where each pixel is divided into red, blue and green filters), resulting in varying levels of light, from the source, passing through. Oversimplifying it somewhat, it's the combination of subpixel voltage-switching and light source that make up the images you see.

LCD technology encompasses digital watches right up to ultra-high resolution displays. For the latter, each pixel can display one colour to another, activated by the variable voltage-switching from the pixels' transistors, detailed above. Faster switching between opposing colours (usually black-to-white), quoted in milliseconds, is usually a reasonable indicator of the panel's effectiveness in displaying fast-moving images. The lower the ms time, the less time needed to redraw an image and, consequently, the lower the chance of ghosting or smearing when displaying fast-moving images. Manufacturers often have different methods of determining pixel response time, so it requires a little time investment on your part, to correctly determine whether the quoted figures between competing manufacturers is a consistent measure.

Types of LCDs
LCDs can be defined by the technology used to control the amount of refracted light (and by inference, colour) passing through from the source to your eyes. Broadly speaking, three technologies exist, and each has its own benefits and disadvantages. Twisted Nematic (TN) LCDs literally twist the liquid crystals in the display. The greater the twisting, controlled by the pixels' transistors, the less light passes through the LCD. Colours can then displayed in relation to just how much light is being passed through each pixel. This approach offers fast colour switching (response time) but usually at the cost of poor colour rendition for dark colours, especially blacks.

In-plane switching (IPS) uses two transistors per pixel, where the voltage is applied at each end of the horizontally-placed liquid crystal. The benefit is a better viewing angle, resulting from the alignment of the liquid crystal molecules. Vertical Alignment (VA) take the viewing angle benefits of IPS displays but only use a single transistor to do so. With no voltage applied by the pixel's transistor to the naturally vertically-aligned liquid crystal, deep blacks are displayed, from the complete blocking off of light. Voltage application turns the liquid crystal from vertical to horizontal, matching the underlying layer and producing a white image. Colour consistency, therefore, is one of the hallmarks of VA displays.

The use of millions of transistors to create an electric field around liquid crystals can lead to the unwanted side-effect of dead/stuck-on (sub)pixels, arising from transistors not working correctly and thus not applying the correct voltage (or at all) to the LCs. LCD manufacturers generally claim that a non-perfect screen, that is, one with a small number of erroneously behaving pixels is a by-product of keeping LCD costs down. Recently, though, advances in manufacturing have allowed certain manufacturers to offer a zero bright-dot policy, where the display will be replaced if it ships with dead pixels.

Size and brightness/contrast considerations
LCDs are usually offered in a number of sizes, measured from corner to corner, with the display's native resolution (number of vertical and horizontal pixels) and pixel pitch largely determining the optimum size. Pixel pitch, the space taken up by a single pixel, defined in miilimetres, usually ranges from 0.2mm-0.3mm. A lower pixel pitch adds in more pixels for a given size of display, thereby offering a sharper image. We've listed a number of LCD sizes and the native resolutions you're likely to find with each.