Understanding LCD Contrast Ratio

Colin Tang

You’re thinking about buying the latest LCD HDTV and wondering what’s the latest offering on contrast ratio all about.Manufacturers are heavily advertising new LCD contrast ratio figures ranging from hundreds to thousands to one but you can’t make any sense of the numbers. How are these numbers calculated and how can they affect picture quality?

Contrast describes the difference or dissimilarities between elements and encompasses may aspects of our lives. It’s in the yin vs yang, white vs black, Mozart vs Metalica, 0 vs 1 etc. Contrast appreciation is not only an aesthetic concept, but a primal instinct honed through centuries of struggle for survival. Our caveman ancestors have long depended on our visual contrast sensitivity to spot predator and prey and come out top of the food chain.

Brightside DR37-P (right) with 200,000:1 ratio!!! 

There are basically two types of LCD contrast ratios, but the most common figure stated by manufacturers is the dynamic or sequential contrast ratio. It is measured by dividing the luminance (brightness) of peak white at one time by the luminance of reference black at another. Two points to take home here; higher values are better. And for the same amount of peak brightness, higher LCD contrast ratios generally indicate better black levels. But here’s the rub; there is no standard way to measure dynamic ratios and so contrast ratios are rarely comparable between manufacturers.

Simultaneous, or static contrast ratio measures the ratio between peak white and reference black reproduced in a single scene using a ANSI checkerboard pattern. This should give a better indication of the display’s performance. For many reasons (room gain, display flare, design etc), the static ANSI contrast ratio normally has a lower value than dynamic ones and therefore won’t be a favourite spec with manufacturers. 

ANSI checkerboard pattern

The contrast ratios above deal primarily with white and black luminance only, but what about the other shades of colour in between them?  Image contrast perception is also affected by the ratio of colour luminance between black and white. In the example below, the two pictures have the same white and black luminance (same contrast ratio) but the left picture looks more contrasty because of different `gamma’ curves.

In summary, higher LCD contrast ratio, whether ANSI or dynamic, can have the following benefits: better blacks, higher brightness, wider greyscale and colour range, better shadow detail definition and increased perceived sharpness. Although this ultimately depends on your TV design, you should do the following to maximize your contrast ratio on your LCD whenever possible.

1) Brightness control should be set at the correct black level level. Set too high and you will reduce the contrast ratio and wash out colours at the darker end.
2) Contrast control (white level) should be maximized to comfortable levels without losing detail in highlights.
3) Ambient light should be reduced as far as possible because contrast ratios are lowered in its presence.
4) LCD contrast ratio drops considerably after a certain viewing angle, so make sure you stay in the sweet spot.
5) If you have a gamma control, experiment with it and find the picture with the highest `contrast’. This setting will change depending on your ambient light conditions.