Contrast ratio

The contrast ratio is a measure of a display system, defined as the ratio of the luminance of the brightest color (white) to that of the darkest color (black) that the system is capable of producing. A 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.

Contrast ratio ratings provided by different manufacturers of display devices are not necessarily comparable to each other due to differences in method of measurement, operation, and unstated variables. Manufacturers have traditionally favored measurement methods that isolate the device from the system, whereas other designers 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.

Methods of measurement

Many display devices favor the use of the full on/full off method of measurement, as it cancels out the effect of the room and results in an ideal ratio. Equal proportions of light reflect from the display to the room and back in both "black" and "white" measurements, as long as the room stays the same. This will inflate the light levels of both measurements proportionally, leaving the black to white luminance ratio unaffected.

Some manufacturers have gone as far as using different device parameters for the three tests, even further inflating the calculated contrast ratio. With DLP projectors, one method to do this is to enable the clear sector of the color filter wheel 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 an iris), 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, image quality may be over exposed.

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/m²). 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 with 5000:1 dynamic and 1000:1 static contrast ratio when the input signal contains a full range of brightnesses from 0 to 100% simultaneously. They will, however, be on-par when input signal ranges only from 0 to 20% brightness.

Contrast ratio in a real room

In marketing, literature for emissive (as opposed to reflective) displays are 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, making it harder to distinguish between different devices with very high contrast ratios. 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.

A clean print at a typical movie theater may have a contrast ratio of 500:1 Dynamic contrast ratio is usually measured at factory with two panels (one vs another) of the same model as each panel will have an inherent Dark and Light (Hot) spot. Static is usually measured with the same screen showing half screen full bright vs half screen full dark. This usually results in a lower ratio as brightness will creep into the dark area of the screen thus giving a higher luminance.

External links

• On static contrast ratio of the eye
• Practical Home Theater Guide