Contrast ratio

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The contrast ratio is a metric of a display system, defined as the ratio of the luminosity of the brightest and the darkest color the system is capable of producing. High contrast ratio is a desired aspect of any display, but with the various methods of measurement for a system or a part thereof, remarkably different values can be measured of the same subject.

Manufacturers of display devices have traditionally favoured those methods of measurement that isolate the device from the system, whereas designers of practical display systems have more often taken the effect of the room into account. An ideal room would absorb all the light reflecting from a projection screen or emitted by a CRT, and the only light seen in the room would come from the display device. With such a room, the contrast ratio of the image would be the same as the device. Real rooms reflect some of the light back to the displayed image, lowering the contrast ratio in the seen image.

1 Contrast ratio as the property of a device
2 Methods of measurement
3 Dynamic contrast ratio
4 Contrast ratio in a real room
5 See also
6 Notes
- 6.1 Old notes

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Contrast ratio as the property of a device

Examples are 800:1, 700:1, and 500:1 from higher to lower capability. Infinite contrast ratios can be achieved by devices capable of emitting no light at all as their darkest color. Contrast ratio is most commonly considered in connection with transmissive displays<ref name=termuse>www.webopedia.com. On use of the term in relation to different display technologies</ref>, such as LCD, in which all pixels share the same light emitter, and manipulate the brightness of transmitted light individually. Technological challenges make it hard to design a mechanism to shut off 100% of transmitted light in these displays. Additionally, any optics in front of the matrix of light modulators that can potentially mix the light from different pixels, such as the lens of a DLP/LCD projector, will also degradate the contrast ratio.

Emissive display technologies - where all pixels emit light individually, such as OLED, plasma, FED and SED - are capable of achieving a very good contrast ratio. This is also true with the case of CRT Template:Ref which have a theoretically infinite contrast ratio and practically achieve such a high contrast ratio Template:Ref that this terminology usually does not refer to them.

Poor contrast ratio manifests itself in the lack of true black, and in noticeably desaturated colors (the darker is the supposed color - the stronger is the desaturation).

A common marketing myth is that too much contrast is a negative attribute however this myth has been propagated by salesmen to sell cheap LCD displays.

A notable recent development in the LCD technology is the so called "dynamic contrast". When there is a need to display a dark image, the display would underpower the backlight lamp (or decrease the aperture of the projector's lens using a shutter), but will proportionately amplify the transmission through the LCD panel. This gives the benefit of realizing the potential static contrast ratio of the LCD panel in dark scenes, when the image is watched in a dark room. The drawback is that if a dark scene does contain small areas of superbright light, they may be sacrificed and blown out. This may not sound too bad though, as the static contrast ratio of a human eye is somewhere between 100:1 - 1000:1 Template:Ref so the details in those highlights might not be resolvable anyway.

The trick for the display is to determine how much of the highlights may be unnoticeably blown out in a given image under the given ambient lighting conditions.

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Methods of measurement

Many manufacturers of display devices favor the use of the full on/full off method of measurement, as it will effectively cancel the effect of the room completely, giving as high ratios as possible. Same proportion of light will reflect from the display to the room and back in both measurements, as long as the room stays the same. This will inflate the light levels of both the "black" and the "white" measurements in the same proportion, unaffecting the black/white luminance ratio.

Some manufacturers have gone as far as using a different device parameters for the two tests, even further inflating the calculated contrast ratio. With DLP projectors, one method to do this is to enable the white sector for the "on" part and disable it for the "off" partTemplate:Ref. This practice is rather dubious, as it will be impossible to reproduce such contrast ratios with any useful image content.

Another measure is the ANSI contrast, in which the measurement is done with a check-board patterned test image where the light intensities are measured simultaneously. This is a more realistic measure of device capability, but includes the potential of including the effects of the room into the measurement, if the test is not performed in a room that is close to ideal.

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Dynamic contrast ratio

It is also common to market only the dynamic contrast ratio capability of a display (when it is supported), which should not be directly compared to the static contrast ratio. A plasma display with a static 5000:1 contrast ratio will show superior contrast to an LCD display with 5000:1 dynamic and 1000:1 static contrast ratio when the input signal contains full range of brightnesses from 0 to 100% simultaneously. However they will be on par when input signal ranges only from 0 to 20% brightness.

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Contrast ratio in a real room

Note that the contrast ratio promoted in marketing literature for emissive (as opposed to reflective) displays is always measured under the optimum condition of a room in total darkness. In typical viewing situations the contrast ratio is significantly lower due to the reflection of light from the surface of the display <ref name=realenv>www.poynton.com. On practical contrast ratios in real environments</ref>. How much the room light reduces the contrast ratio depends on the luminance of the display, as well as the amount of light reflecting off the display.

It should be noted that even a presence of relatively dim light in a room will render devices with infinite contrast ratios virtually indistinguishable from those with poor ratios Template:Ref.

A clean print at a typical movie theater may have a contrast ratio of 500:1Template:Ref.

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