All about Contrast
Unfortunately, manufacturer specifications for contrast are almost useless. They routinely cite figures under conditions that no one would ever watch (inaccurate color and contrast turned up to 100%, for example). The Imaging Science Foundation (ISF) is working on a specification for contrast that consumers can rely on. High-end audio provides a good exemplar for how this might work. Many years ago stereo manufacturers agreed to cite the power output of amplifiers according to a realistic and pre-defined standard (number of watts within a specified frequency range at a set amount of distortion). Until this RMS standard was widely adopted, power specifications were almost meaningless. Because the video world has yet to catch up with the audio world, consumers cannot rely on manufacturers to accurately report contrast figures. Until the video world adopts something similar to audio's RMS specifications, you should take officially published contrast numbers with a huge grain of salt.
Contrast is measured in 2 ways: On/Off and ANSI. Each measurement is important, but they measure different aspects of display performance.
On/Off contrast, as the name implies, expresses a ratio between the darkest dark and the whitest white that the display can produce. Because this number is always higher, for marketing reasons manufacturers generally cite this figure. Figures for ANSI contrast are rarely published at all.
The ability of a display to produce a high contrast ratio depends on two factors: the type of display technology and the manner in which that technology is implemented.
On/off contrast is an important specification because it measures the ability of the display to accurately reproduce very dark scenes. A display with poor on/off contrast will portray naturally dark scenes as though one is looking through a milky haze. Try watching Dark City on a display with poor on/off contrast. You won't like it. This specification is determined entirely by the display and is relatively unaffected by the room.
ANSI contrast, on the other hand, is a measurement of the ability of a display to reproduce black and white simultaneously, and is measured by comparing several averaged luminance readings from a 4x4 checkerboard test pattern. This figure is always much lower than on/off contrast. High-end DLPs provide the excellent ANSI contrast figures in the 500-800:1 range (front projectors are better than rear projectors). Oddly, CRTs, which perform so well with on/off contrast, offer relatively poor ANSI contrast figures of 75-150:1. LCoS displays offer numbers somewhere in between DLP and CRT. The champ for ANSI contrast though are plasma and LCD, which offer ANSI contrast figures that are much closer to the measured on/off contrast than what you find with the other display technologies. I measured one Pioneer Kuro with an ANSI contrast figure in excess of 3000:1!
The important point to remember about ANSI contrast is that this figure is profoundly affected by the room. If the room has a lot of reflective surfaces, then the light from the bright part of the image will bounce off the walls and ceiling and reflect back onto the screen, washing out the dark areas. This issue is worse for front projectors than with direct views and rear projectors, but it is still a factor even with these displays.
It is important that your display produces high contrast, both on/off and ANSI, for three reasons.
The Contrast Trap
Don't get caught in the contrast trap. This is when consumers chase a high on/off contrast ratio to the exclusion of all else. As important as contrast is, it is only one measurement of image quality. You should evaluate the image a display provides based on a variety of criteria, including (in addition to contrast):
Engineering good contrast is generally not cheap, so high contrast displays will usually offer great images in any case.
Dynamic vs. Native Contrast
Manufacturers have realized how much consumers pay attention to contrast specifications so they will perform almost any trick they can to inflate the numbers. One popular way is to offer some ability to manipulate the output of the display so that it is darker on dark scenes and brighter on bright scenes than it otherwise would be. Although this approach may offer somewhat better images on very dark scenes, it is of no use on everything else, and has zero effect on image depth and color. The native contrast—that is, the contrast it produces without the aid of a dynamic iris or some other tool—is the much more important specification.