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Everyone loves a fulsome flat screen. And why not?

You can now bag yourself 22 inches of half-decent LCD panelry for under £150.

But how much do you really know about the technology that goes into these widescreen wonders? The next time you buy a monitor, will you have a clue what you're buying? Or will you be winging it based on little more than size, price and maybe the number of input options?

Fear not - we're going to give you all the information you need to sort the good panels from the bad.

Monitor makers may have a nasty habit of fluffing up spec sheets with all manner of distracting disinformation. But we'll help you keep them honest with the most powerful weapon of all – knowledge.

Not all LCD monitors are the same

If you come away with just one insight from this LCD technology expose, for the love of liquid crystal make it the appreciation that not all LCD monitors are the same.

In fact, there are three major types of LCD panel technology - TN, IPS and VA. Each has its own very distinct characteristics in metrics such as pixel response, contrast and viewing angles. No single panel type is superior across the board.

However, before we trot out the various pros and cons, it's worth understanding the basic principles that underpin any LCD display, from a simple monochrome panel in an ancient pocket calculator to the latest high screens with millions of eye-popping colours. LCD, of course, stands for liquid crystal display. Each picture element, or pixel, in an LCD panel is made up of just that, a cell containing liquid crystals.

On a desktop LCD monitor, there are thousands – if not millions – of these pixels, arranged in a grid pattern. Behind this grid is a light source, literally the backlight. When an electric current is passed through the crystals courtesy of a pair of transparent electrodes, they realign themselves. Precise techniques vary between panel types, but the idea is that there are two basic positions or states the crystals can assume. In one, light is allowed to pass through, in the other it's blocked out.

OK, it's actually a little more complicated than that. The crystals don't block light, they polarise it in a certain plane, and in combination with static polarisation filters, light either passes through the LCD panel or is blocked. What's more, the crystals can assume intermediate positions, allowing some but not all light to pass through. But you get the idea.

Now, on a full colour LCD panel, every pixel is actually made up of three individual elements, each with a colour filter – red, green and blue. However, these elements are so tiny, the light emitted by them is amalgamated at normal viewing distances. Hence, by varying the intensity of light that passes through these elements, a full palate of hues is generated and Bob's your mother's technicolour brother.

Those principles all LCD panels share, but there are lots of detail differences. For instance: some panel types block light more efficiently, allowing deeper blacks and greater contrast.

In others, the liquid crystals change state more rapidly, delivering better pixel response and hence sharper moving images. Still others offer finer control of the positions that the crystals assume, giving superior colour accuracy. But which are best in what areas?

TN – Twisted nematic

TN or twisted nematic panels are easily the most common in today's desktop monitors. For the most part, that's because they are dirt cheap to manufacture. Unsurprisingly, TN panels aren't exactly the sexiest lookers.

A dingy, washed-out image with mediocre contrast and very restricted viewing angles is the hallmark of a TN display. Much of this is because the default position for the crystals in a TN panel allows light to pass through. Only when a voltage is applied do the crystals 'twist' into a position that blocks light.

Fine control of the crystal states is another TN shortcoming, leading to poor colour accuracy. However, one of the upsides is rapid response. The crystals in TN panels change position more quickly than any other panel type, making them great for fast-paced frag fests and action movies. In terms of quoted specifications, TN panels therefore lead the pack for pixel response, achieving very low single digit milliseconds times.

Elsewhere, things aren't so impressive. Static contrast ratios for TN screens are usually well below 1,000:1 while viewing angles are typically below 170 degrees in both horizontal and vertical planes. However, all of these specifications are subject to abuse by manufacturers.

IPS – In-plane switching

At the opposite end of the scale for everything from price and image accuracy to frequency of use is IPS or in-plane switching technology.

Where other panel types have just one controller transistor per picture cell, IPS screens have a pair and therefore offer the best control and colour accuracy. But, this added complexity not only leads to increased cost. It also blocks more light and can lead to slightly less saturated colours.

Initially, this might sound like a somewhat disastrous disadvantage when compared to the competitor panel types, but it's not: the finer crystal control of IPS technology delivers the widest and most consistent viewing angles. The reasons for this are a little complex, but involve the way crystals rotate about their axis rather than the way that they tilt up or twist at one end in a TN panel.

Anyhow, the upshot is that crystals in an IPS panel present a more consistent face to the viewer at varying angles and therefore allow a more constant quantity of light to pass through each of the red, green and blue picture elements. That prevents the colours from going wonky when viewed from an angle.

However, the definite area in which IPS panels are recognised to fall short is concerning pixel response. The greater range of movement makes the crystals in IPS panels a little more reluctant to shift. The longer that takes, the blurrier a panel looks when rendering moving images.

That said, the latest IPS panels are capable of grey-to-grey response as low as 6ms, so you'd hardly call them slow. As for viewing angles and contrast, look for figures of at least 170 degrees and 1,000:1, respectively.

VA – Vertical alignment

The final panel type is VA or vertical alignment. There are actually two types of VA technology: PVA (patterned vertical alignment) and MVA (multi-domain vertical alignment). PVA is much more common, but both share the same basic structure and give similar image quality.

In most regards, VA panels are a half-way house between IPS and TN technology. For price, pixel response and colour accuracy, they fall neatly between the two. However, VA panels have a few foibles that set them apart from other panels. For starters, the default position of the crystals blocks light from passing through. Consequently, VA panels have the deepest, inkiest blacks available and the highest contrast ratios.

They also boast the highest levels of colour saturation (though not colour accuracy) and viewing angles that are nearly on a par with the best IPS screens. All these factors together make VA the most vibrant, eye-catching panel technology of all.

The richness and depth of a higher-end VA monitor gives TN and IPS technology a thoroughly good spanking. Static contrast ratios of 1,000:1 or more are common as are viewing angles in excess of 175 degrees.

But, there is one catch. VA panels are not the speediest. In an attempt to push response times below 10ms, many VA monitors use a technique known as pixel overdrive. While it's a pretty effective measure for speeding up raw pixel response, it introduces a new problem: input lag.

Making your choice

That's how the different panel types stack up. Funnily enough though, many monitor makers are doing their best to avoid mentioning panel types at all, particularly when the panel in question is TN. But there is tell-tale sign you can use for panel spotting: price.

Pretty much all 22-inch widescreen monitors and certainly anything below £250 will be TN-based. Just about any 23 or 24-inch screen cheaper than £300 will almost definitely be powered by a TN panel. Likewise, there are a number of even larger 26 and 27-inch TN monitors priced around £250 to £350.

PVA is the next most popular panel type. It's most commonly seen in 24-inch monitors priced in the £350 to £500 range. A handful of MVA screens also fall into this range.

A number of manufacturers also knock out 26 and 27-inch PVA panels; again, these will be significantly pricier than their TN-based equivalents. Finally, Samsung makes a monstrous 30-inch PVA panel. It's arguably the most spectacular monitor currently on the market.

As for IPS technology, it's becoming increasingly rare. With a few exceptions, IPS is restricted to seriously expensive panels aimed at graphics professionals who are willing to fork out for the very best colour accuracy.

Tricksy tech

As important as panel type may seem, it's not the only factor that defines image quality. Recently, several new electronic image enhancement technologies have emerged, not all of which have been unambiguously successful.

The most infamous of these is dynamic contrast. The theory behind it is simple enough. Motion video and 3D games contain scenes that vary in terms of light intensity from brightly lit daytime vistas to dingy dungeons and nightscapes. So why not vary the backlight and therefore enable deeper darker blacks to be rendered without compromising on vibrancy and brightness for brighter pictures?

That's exactly what dynamic contrast attempts to do by analysing image data as it reaches the monitor and adjusting the backlight to suit. In theory, this all sounds rather impressive, but unfortunately in practice, the result always turns out to be pretty clunky.

Typically, the backlight is noticeably slow to respond, and when it eventually does it tends to crush the fine details in darker colours. We've yet to see a dynamic contrast implementation that actually benefits image quality.

Then there's digital colour enhancement. Here, the monitor's colour balance is digitally processed with a view to either compensating for the shortcomings of the panel itself or just jazzing up the visuals. As with dynamic contrast, we've yet to see a colour enhancement technology that really delivers.

Final screening

By now you'll have a good idea why some monitors look so much better than others and how the different panel technologies compare. More importantly, armed with this information, manufacturers can no longer bamboozle you with superficially spectacular specifications.

If you see a TN monitor with a sky high contrast ratio, for instance, you'll know there's some dodgy dynamic contrast technology at work.

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First published in PC Format, Issue 223

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