How LCD Screens Work

General Features

Brightness

Contrast

Viewing Angle

Response Time

Connections

Product Lifespan

Dead Pixels

LCD Monitor vs LCD TV

Summary of LCD TV Buying Tips

Ever since the invention of the television in the late 1920s, size has always been an issue. Even today, in a world obsessed by miniaturisation, traditional TVs are massive beasts, brooding bulkily in the corner of our living rooms.

Thankfully, the rapid development of colour LCD (Liquid Crystal Display) technology over the past few years has finally brought super-slim flat-screen TVs to life as a practical alternative to traditional models.

This guide aims to steer you through the pros and cons of this new technology and provide practical advice to make sure you get the best value for money when buying an LCD TV.

How LCD Screens Work

 LCD TVs use the same technology that is used in LCD computer monitors. At the rear of the display is a fluorescent white backlight that gives LCD screens their characteristic bright, smooth illumination. Above this is a grid of hundreds of thousands of tiny LCD pixels.

Each pixel is made up of three LCD chambers, coloured red, green and blue, that open and close like shutters. The amount of electricity fed to each chamber determines how much light is let through--and therefore the apparent colour of the pixel to the human-eye. The speed at which these pixels change--known as their response time--is particularly important as it determines how swiftly the image on the screen can change. Slow response times will make fast moving objects appear fuzzy and blurred.

General Features

LCD TVs have many advantages over their conventional cousins. Not only are they smaller and lighter than conventional televisions but they are also much more economical to run too.

Their small size and slim profile means that they are perfect where space is limited, as well as where style is important. Most models feature a stand for use on flat surfaces but they can also be mounted on (or recessed into) the wall or even hung from the ceiling.

Brightness

The brightness of LCD TVs varies considerably and is an important factor to examine when considering a purchase. Most LCD screens tend to be considerably dimmer than plasma or conventional CRT screens and don't perform as well when used in bright lighting conditions.

A screen's brightness is often expressed in Cd/mē. Cd is an abbreviation for candela, a unit of measurement of the intensity of light. An ordinary wax candle generates one candela while a LCD TV may generate 400 Cd/mē or more--the higher the number, the better the screen.

Contrast

This is a measure of how many times brighter the white tones produced by an image are than the black tones.

The more contrast a LCD TV can produce, the better, as it means the image will be crisper, cleaner and sharper. When looking at the specifications for a display you'll see this expressed as a ratio. A typical ratio might be 400:1, although these figures are increasing all the time, with figures upwards of 800:1 are not uncommon.

Many manufacturers feature their own technologies to improve their screens' contrast capabilities, so be sure to look out for these when deciding which model to buy.

Viewing Angle

Depending on how you intend to use your LCD TV, viewing angle may well be the most important consideration to make when thinking about your purchase. Unlike a conventional TV set, visible equally well from pretty much any angle, the way an LCD screen looks may vary tremendously depending on where you are looking at it from, potentially rendering the image unviewable from your favourite armchair.

When making your purchase it's therefore very important to make sure the TV you are buying will be visible from every position you'd want to view it from. Look at the specifications to see how wide the viewing angle on any particular model is and think about the shape of your room and where the LCD TV will sit. Most LCD TVs offer a horizontal viewing angle somewhere between 150 and 170 degrees and a vertical angle of around 130 degrees.

Response Time

An LCD TV's response time refers to the amount of time it takes for the LCD pixels to switch totally from black to white and back again. It is measured in thousandths of a second (milliseconds or ms) and is an important factor in determining how well a LCD TV will handle swiftly changing images.

If the response time is not fast enough (especially on larger screens), scenes that feature lots of fast movement (eg: car chases and other action scenes) will appear muddied and blurry. The smaller the response time, therefore, the better.

On an average LCD TV, response times will tend to be around 20-30ms. These figures are falling all the time though and some LCD TVs are already reaching figures of 15-17ms, matching the performance of a conventional TV screen.

Connections

As with any TV you'll want to make sure you can connect your video recorder, DVD player, game console and other equipment to your LCD TV.

Look for at least one SCART socket (preferably two and ideally with RGB input for superior picture quality), s-video and component inputs as well as a standard arial socket. It's also worth looking out for a PC monitor connection if you'll want to connect your computer to the TV at any point.

Product Lifespan

An LCD TV's backlight, as with any light bulb, has a limited lifespan. Depending on the manufacturer, this is likely to be between 20,000-50,000 hours of continuous operation. Don't panic though--assuming four hours of viewing per day this equates to somewhere between 13-24 years of operation, so you shouldn't find yourself having to find a replacement too quickly.

Dead Pixels

While most LCD TV screens will function perfectly throughout their lifetime it is worth mentioning the issue of dead pixels.

With in excess of three-quarters of a million pixels on an average LCD panel, each controlled by three individual transistors (electronic switches), it is perhaps unrealistic to expect every single screen to be 100% defect free. A dead pixel occurs when the switches that turn an individual pixel on and off fail. To the human eye this appears as a tiny black spot on the picture.

Different manufacturers have different levels of fault tolerance for their products and varying policies with regard to dead pixels. Some will replace the screen when just one pixel fails while others only consider replacing it if two, three or more have failed.

LCD Monitor vs LCD TV

Although LCD TVs have much in common visually with LCD computer monitors and many even feature a socket with which you can connect them to your PC, there are actually some important differences between the two devices.

Monitors are built to a much lower specification, primarily for close-up viewing from straight on, and are not designed with the wide viewing-angle required by a general-purpose TV. They also tend to be slower to react to moving images and do not perform well with fast moving video.

Some manufacturers have been marketing TVs based around LCD screens designed originally as monitors, simply by adding a tuner and making some simple styling changes. Check for signs that the model you are looking at was designed for TV use by following our buying tips (see below).

Also beware smaller LCD TVs that come with a separate tuner box as this may be another sign that they were originally designed for PC use. Similarly, avoid LCD monitors "with built in TV-tuner" unless you're planning to watch TV sat at your computer and genuinely want to use the screen primarily as a monitor.

Summary of LCD TV Buying Tips

Brightness: the higher the better. Look for 300Cd/mē and upwards

Contrast: Tte higher the ratio the better. 400:1 is average, 800:1 is better

Viewing angle: the higher the better. Look for at least 150-170 degrees horizontal and around 130 degrees vertical, unless you have specific limited requirements (i.e., you know you will only be watching the TV from one specific angle, such as a favourite chair)

Response time: the lower the better, preferably <20ms

Connections: look for at least 1 SCART socket (preferably 2, ideally with RGB input), s-video and component inputs. Also computer connection if required

Avoid: TVs based around computer monitors