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.

• Of the current display technologies, CRTs perform the best. CRTs are capable of producing very deep blacks, and since it is much easier to cut the black level in half than double the light output, very deep blacks will always mean high on/off contrast ratios.
• Digital technologies don't perform as well as CRTs in this regard, but among them DLP's have traditionally performed the best. The native contrast of DLP has steadily increased with each successive release of a new design. The most recent design, the DarkChip 4 chip, is just now appearing in high-end displays, and promises very high on/off contrast.
• Recently JVC has introduced new technology that dramatically improves the on/off contrast of their LCoS front projectors, which now measure close to what we used to see with CRTs. Other LCoS manufacturers (Sony, for example) rely upon a dynamic iris to achieve very high on/off contrast.
• The better plasma displays offer very good contrast. In fact, the best (the Pioneer Kuros) have blacks levels so low that they are difficult to measure. Even manufacturers who don't meet this standard can provide displays with very satisfying contrast.
• LCD generally provides the worst on/off contrast numbers, rarely much above 1000:1. This is because LCD technology relies upon a light source behind the panel, so it is difficult for LCDs to provide low black levels. However, a recent advance in LCD technology that provides this backlighting from hundreds, or even thousands, of individual LED light sources seems to offer much better contrast because the light sources can be locally dimmed. This allows the display to maintain high output in the bright areas of the image while the dark areas stay quite dark.

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.

• Realism: A high contrast display will impart to an image, especially dark images, an enormous amount of realism.
• Depth: Contrast (on/off and ANSI) is an important predictor of a display's ability to provide an almost 3-D effect.
• Good color: Colors seems much more subjectively pleasing when viewed on a display with high contrast.

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):

• Sharpness
• Color accuracy
• Freedom from artifacts
• Depth

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.