Contrast ratio

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The contrast ratio is a measurement value commonly used in entertainment electronics, which is used to represent the maximum relative brightness differences between black and white . It describes quantitatively the ability of a screen or projector to generate a high-contrast image and is the quotient of the maximum and minimum displayable luminance .

The larger this quotient, the higher the contrast and the sharper and livelier the image. If the quotient is smaller, colors appear more matt, as the color saturation can only reach low values ​​due to the addition of white components. The image appears faded and black areas of the image are often perceived as dark gray. The contrast ratio has a direct effect on the gamut (the amount of all displayable colors) of a display device. Other effects, such as the Helmholtz-Kohlrausch effect (perceived brightness increases with increasing saturation despite constant luminance), the Hunt effect (increase in perceived saturation by increasing the luminance) or the Bezold Bridge effect (color shift when the luminance changes) also influence color perception and can significantly change the appearance.

A good contrast ratio has proven to be particularly important in home theater applications, where the realistic reproduction of both deep black and pure white surfaces is important. However, it should be noted that the perception of the contrast depends very much on the ambient brightness (relatively dark surroundings in home cinema). In a lighter environment, the black level of a monitor appears darker and thus increases the perceived contrast ( simultaneous contrast ). If the environment is too bright, the opposite effects occur and the perception of contrast and color is weakened by effects such as transient adaptation . The perception of the black level is very variable and depends on both the ambient brightness and the adaptation status of the viewer.

Typical contrast ratios for different screen and projector types:

In January 2007, Texas Instruments presented a prototype of a rear projection television at the electronics fair CES in Las Vegas, which uses LEDs as a light source and has a contrast ratio of 100,000: 1. Sony presented two OLED screens with an alleged contrast ratio of even 1,000,000: 1.

Dynamic contrast

In the case of liquid crystal screens, a dynamic contrast is often specified, also called ACR advanced contrast ratio . This must not be confused with the static (or native) contrast and a comparison between the two types of contrast is not permitted.

The static contrast in liquid crystal screens is the ratio between the light intensity of the brightest and the darkest point of an image and is produced by the different light transmittance of the liquid crystals. At the moment (2009) a maximum contrast of around 1000: 1 is achieved with TN and IPS technology, and up to 2500: 1 with AMVA technology.

In the case of liquid crystal screens, higher contrast values ​​can currently only be achieved with dynamic contrast. The strength of the background lighting is also changed: In the case of a generally dark picture, the background lighting is reduced in order to make the picture appear even darker; in the case of a generally bright picture, it is intensified to brighten the picture. If this trick is used skilfully, it leads to the impression of a stronger general contrast in films. In doing so, however, the contrast within a single image is by no means increased, but that between two successive images. The dynamic contrast is only effective with moving images; it is irrelevant for static applications such as image processing. For such applications it even has to be switched off in order to maintain a uniform working environment.

How little meaningful the dynamic contrast is can be illustrated by a simple consideration: The contrast is the ratio between the light intensity of the brightest and the darkest point. This ratio can be increased as required with dynamic contrast by simply reducing the background lighting further and further in dark scenes. If it is even switched off completely, the result is an infinitely high value for the dynamic contrast. It is immediately clear that this is at the expense of image quality in dark scenes.

With some LED-backlit screens it is possible to change the lighting systematically in a certain screen region and thus to increase the contrast of a single image (ie also the static contrast). This technology currently still causes a perceptible glow (halo) in bright image regions with generally dark image content, since the resolution of the background lighting is much lower than the actual image resolution.

Individual evidence

  1. Panasonic TX-32PD30D, self-measured: white point 280 ± 14 cd / m², black point ≤0.01 cd / m²