Contrast ratio is a ratio that indicates the range between the brightest point and the darkest point that a display can produce. For example, if the ratio is 500:1, it means that the brightest white is 500 times brighter than the blackest black. This measurement is often used in marketing language for both playback and recording devices, and like many “number wars” in the consumer electronics industry, it is very complicated, and consumers should approach it with care.
Ideally, contrast ratios would be tested in the exact same conditions, using the exact same procedures, with carefully calibrated equipment and neutral professionals administering the test. Unfortunately, this is not the case, which means that the measurements can be extremely variable and not always very reliable. Companies may test their contrast under various conditions, using various methods, and the claims they make may be difficult to reproduce.
The most popular method is the full on/full off, in which a display shows an all white and then an all black image, usually in perfect darkness. This provides the biggest number, because the contrast will be quite radical, but these conditions are rarely seen in the real world. Some companies use other methods such as a checkerboard of white and black squares, which provide more realistic conditions and therefore a more accurate contrast. The method used is not always disclosed by the company, however, which can make it difficult to judge the reliability of the stated measurement.
Theoretically, the better the contrast ratio, the better the quality of the display, but displays can also be affected by the conditions where they are used. A television, for example, will have greater contrast in darkness than it will in light conditions. The quality of the material being displayed can also have an impact, as a poor recording will look bad even on the best display. Furthermore, the human eye's ability to discern contrast and detail are limited, which means that two displays with different ratios can look very similar to the average consumer.
In addition to being important for displays, this ratio also has an impact on the quality of recording devices like cameras. If the ratio is high, the device will be able to reproduce high levels of contrast, creating cleaner, crisper, better-quality images. With a low ratio, quality will also be lower, and it will usually be impossible to clean up or improve the image because the necessary data will be missing since it was never captured.