Contrast ratio

The contrast ratio is a common in the consumer electronics measurement that is used to represent the maximum relative differences in brightness between black and white. It quantitatively describes the ability of a monitor or projector to produce a high-contrast image, and is the quotient of the maximum and the minimum representable luminance.

The larger this ratio is, the higher the contrast and the sharper and more vivid the picture appears. If the quotient is smaller act matt color, because the color saturation can be achieved by the addition of white portions only low values. The image looks faded and black areas of the image are often perceived as dark gray. The contrast ratio has a direct impact on the gamut ( the set 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 the perceived saturation by increasing the luminance ) or the Bezold - bridge effect ( color shift when changing the luminance ) affect the perception of color and also can change the appearance significantly.

Particularly important is a good contrast ratio for home theater applications has been proven which depend on the realistic reproduction of both pure black and pure white surfaces. However, it should be noted to the fact that the perception of the contrast very dependent on the ambient brightness ( relatively dark environment at home theater ). In a brighter environment, the black level of a monitor will look darker and thus increases the perceived contrast ( simultaneous contrast ). If the environment is too bright, you experience adverse effects and the contrast and color perception are effects like the Transient weakened adaptation. The perception of the black level is highly variable and dependent on both the ambient light and from the state of adaptation of the observer.

Typical contrast ratios for different screen and projector types:

• Projectors with passive matrix LCDs: 15:1
• Active-matrix LCDs: up to 100:1
• Mikrospiegelaktor: 175:1
• Poly -Si LCD: 300:1
• 35 mm slides: 500:1
• S -PVA TFT: up to 3000:1
• LCD TVs: up to 1500:1
• Plasma TV: up to 5,000,000:1 ( Stand: 12 /2010 | LG 50PX950 )

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

Dynamic contrast

In liquid crystal displays, a dynamic contrast is often given. This should not be confused with the static ( or native ) contrast and a comparison between the two contrast available is invalid.

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

Higher contrast values ​​are currently only accessible with a dynamic contrast of liquid crystal displays. Here, in addition the strength of the backlight is changed: In a generally dark image, the backlight is reduced to make the picture look even darker, with a generally bright picture when it is enhanced to additionally brighten the image. If this trick applied skillfully, it leads to the impression of a stronger films General contrast. In this case, however, the contrast is not increased within a single image, but the one between two successive images. The dynamic contrast is therefore only effective in moving images, static applications such as Image processing it is irrelevant. It must be even switched off for such applications to obtain a uniform working environment.

How little meaningful is the dynamic contrast can be illustrated by a simple consideration: Contrast is the ratio between the light intensity of the brightest and the darkest point. This ratio can be combined with dynamic contrast but can be arbitrarily increased by simply in dark scenes the backlight is being reduced. Will she even completely shut down, it gives an infinitely high value for the dynamic contrast. It is immediately clear that this is at the expense of image quality in dark scenes.

In some LED backlit displays, it is possible selectively to change the light in a specific display region and so as to enhance the contrast of an image frame (that is, also the static contrast). This technique is currently causing even with generally dark image content in bright image regions a perceptible form ( Halo ), since the resolution of the backlight is much smaller than the actual image resolution.

Semantic

In trading, the linguistically incorrect designation has " contrast ratio ", in contrast to the correct term " contrast ", which describes the brightness ratio between the largest and smallest representable brightness of an image or screen. A contrast ratio describes, taken literally, the ratio between two contrasts (eg, if one contrasts the two monitors compares ).

• Optics
• Moving
• Technical Optics