Contrast ratio

The contrast ratio is a measure of a display system, defined as the ratio of the luminosity of the brightest color to the luminosity of the darkest color that 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 its part, remarkably different measured values can sometimes produce similar results.

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 seen in the image.

Moving from a system that displays a static motionless image to a system that displays a dynamic, changing picture slightly complicates the definition of the contrast ratio, because of the need to take into account the extra temporal dimension to the measuring process. Thus the ratio of the luminosity of the brightest and the darkest color the system is capable of producing simultaneously at any instant of time is called static contrast ratio, while the ratio of the luminosity of the brightest and the darkest color the system is capable of producing over time is called dynamic contrast ratio.

Contrast ratio as the property of a device
Examples of reported contrast ratios 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 (so generally the higher the first number, the better). Contrast ratio is most commonly considered in connection with displays that share the same light emitter for all pixels and employ an array of modulators for individual pixels, which manipulate their transmissivity or reflectivity. Technological challenges make it hard to design a mechanism to block 100% of input light in these displays. Additionally, any optics in front of the array of light modulators that can potentially mix the light from different pixels, such as the lens of a DLP/LCD/LCoS projector, will also degrade the contrast ratio. The higher the contrast ratio the better the longshot will look.

Emissive display technologies (where all pixels emit light individually, such as OLED, FED and SED) are capable of achieving a very high contrast ratio. This is also true with CRTs, which have a theoretically infinite contrast ratio and practically achieve such a high contrast ratio that this terminology usually does not refer to them. Plasma displays are subject to contrast ratio because the black areas on a plasma display are not truly black, but emit light due to the need to maintain gas discharge (priming pulse).

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

With display systems that support 8-bit color channels, contrast ratio is the ratio of the screen's whitest white (output level 255) to its blackest black (output level 0).

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. Equal 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 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" part 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 checker board patterned test image where the luminosity values are measured simultaneously. This is a more realistic measure of system 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.

It is useful to note that the full on/full off method effectively measures the dynamic contrast ratio of a display, while the ANSI contrast measures the static contrast ratio.

Dynamic contrast ratio
A notable recent development in the LCD technology is the so called "dynamic contrast" (DC). 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.

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.

Brightness, as it is most often used in marketing literature, refers to the emitted luminous intensity on screen measured in candela per square metre (cd/m^2). The higher the number, the brighter the screen.

It is also common to market only the dynamic contrast ratio capability of a display (when it is better than its static contrast ratio), 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.

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. 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.

Even the presence of relatively dim light in a room, or even as low as a single candle, will render devices with infinite contrast ratios virtually indistinguishable from those with poor ratios.

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