LCD screens in a nutshell (page 6: Understanding the brightness and the contrast ratio)

Understanding the brightness and the contrast ratio

The contrast ratio is measured by opposing the lighter and the darker pixel produced. In order to get a good contrast ratio result, white has to be as white as possible and black as black as possible.

So with a measuring tool, we have to measure the white level: let’s say that this measure is 250 cd/m². Then we measure the black and this time it is 0,5 cd/m².

The formula to establish the contrast ratio is:

Black / White = contrast ratio.

Applied to our case, the result is:

250 / 0.5 = 500 : 1 .

These results are representative of flat screens.

Now it is tempting for the manufacturer to extricate its screen by submitting a higher contrast ratio.

To succeed in this operation, there are two possibilities:

The manufacturer can improve the black colour, but this operation is very difficult. They need to improve the panel filter or work on the liquid crystal movements to facilitate the backlight blocking by placing them even more on a vertical position.
The other possibility is to rise up the white colour of the lightning by adding neon tubes or replacing them by brighter neon tubes.

Under those circumstances it is possible to reach a 500 cd/m² white.

If this “improved white” is used in our formula, the result will be:

500 / 0.5 = 1000 : 1

With such a contrast ratio, this screen should be perfect from a graphic designer point of view. But in fact this screen will only be annoyingly brighter than the other screens.

Actually, the recommended brightness should not exceed 110 cd/m². CRT screens brightness is 90 cd/m². All 250 cd/m² screens (and most of the LCD screen are) are too bright. This brightness is annoying, it is a strain on the eyes and you will almost certainly reduce it manually.
In a nutshell, contrast ratios are currently measured in a non-representative way. To be accurate, this measure should be based on the same white level for all screens, for example 110 cd/m². Let’s hope that the upcoming ISO norm incorporate this kind of requirement. Henceforth, don’t pay a lot of attention to this characteristic.

This norm ISO-13406-2 should really be reviewed soon! (two time)