Contrast ratio and high definition TV – why black-and-white is so important to colour TV

One of the most important figures in the specifications for TVs and other displays is the ‘contrast ratio’. This figure gives you an idea of the range of brightness of which a display is capable, from the most eye-blasting whites to the inkiest blacks.

That’s the theory. In practice, most TV panels produce about the same maximum brightness. Where they vary is with their black levels.

It’s odd, isn’t it? You would think that the easiest thing for any display to do would be to produce black: just don’t switch on those bits of the screen!

But there are, in fact, technical reasons why producing black is not all that easy. LCD TVs work by blocking parts of a backlight with LCD pixels. But the LCD cannot completely stop all light. It’s just a fact of nature (so far, anyway). For plasma TVs it is, well, complicated. For the moment, going to complete black is not possible although they do a better job of it than LCDs and are getting quite close.

So if you are sitting in a darkened lounge room watching a dark scene in a movie, you may see a slight glow in the screen. This will tend to wash out some of the subtle variations in the levels of darkness, so it can be harder to see what’s going on.

GadgetGuy tip

One thing that you will rarely notice with the 2008 Beijing Olympic Games is any deficiencies in the contrast ratio of your TV. All the events will be brightly lit and in such circumstances the black levels of a display become quite unimportant.

The only way you can really tell how well a TV is going to perform in producing blacks is to take a DVD with you which you know has night scenes, and ask the store to set up the TV with a DVD player in a room with the lights dimmed. If the blacks in this scene have a kind of pearly glow, you may find the TV dissatisfying if you like to watch it with the lights down low.