Understanding Contrast

Unfortunately, manufacturer specifications for contrast are nearly useless. They will 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 in this regard, 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 enormous grain of salt.

Two types of contrast

Contrast is measured in 2 ways: On/Off and ANSI. Each measurement is important, but they measure different aspects of the display's 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 it is measured by first a full black signal and then a full white signal, it is sometimes referred to as sequential contrast. ANSI contrast also expresses a ratio between the darkest dark and the whitest white, but it is measured using a checkerboard pattern that shows black and white at the same time. For this reason, it is sometimes referred to a simultaneous contrast.

Because the on/off number is always higher, for marketing reasons manufacturers generally cite this figure. Figures for ANSI contrast are rarely published at all. There are two reasons for this. First, simply as a marketing device the ANSI contrast of a display is less valuable insofar as the number is always so much smaller. Second, unlike on/off contrast, the measured ANSI contrast is a function of both the display AND the room in which it is measured. This is because the checkerboard pattern used to measure it will throw a considerable amount of light into the room. Rooms with highly reflective surfaces will return a significant portion of that light back onto the screen, which washes out the dark squares and thus lowers the measured contrast.

Two types of on/off contrast

We must also consider two types of on/off contrast: native and dynamic. Native contrast refers to the inherent ability of the display to produce sequential differences between light and dark. Dynamic contrast refers to the display's ability to produce sequential differences between light and dark aided by a mechanical iris and some type of electronic signal processing. Generally, dynamic contrast is achieved by sophisticated software that senses the brightness of the program material, sometimes on a frame-by-frame basis, and then adjusts a mechanical iris that lies in the light path, closing it during dim scenes and opening it on brighter scenes. At the same time an iris closes, the better dynamic systems also electronically boost the signal strength so the image does not appear to change in its inherent light output compared to what it would have looked like in absence of the iris. This all works to significantly raise the contrast ratio, because during dark scenes the iris lowers the black level, whereas in bright scenes the iris opens to allow the full light output of the bulb. Fixed irises can also have a positive impact on contrast by virtue of the fact that they lower the black level more than they lower peak output, but their measured effect is not as dramatic as dynamic systems. The downside of a dynamic system is the complexity required to

  • detect the average brightness of a scene
  • adjust the iris to match the signal level
  • electronically boost the signal strength as the iris closes

All three of these processes must occur nearly instantaneously and with great precision, otherwise artifacts will appear. The two most common artifacts are "image pumping" and "brightness compression." Image pumping occurs when the iris reacts to changes in the brightness of the program material too slowly and the viewer can visibly detect the image getting brighter or darker as the iris lags behind the program material. Brightness compression occurs when the electronic boost of the image reduces detail in brighter objects.

All else being equal, native contrast is always preferable to an equivalent amount of dynamic contrast. The reason for this is that on/off contrast not only provides lower black levels for dark scenes, it will also greatly improve the perception of depth in almost all scenes. Dynamic contrast can match the effects of native contrast in lowering black levels, but it will never have the other benefits of high contrast that are visible in better illuminated scenes as well.

How do we get high contrast?

The ability of a display to produce a high contrast ratio depends on three factors: the type of display technology, the manner in which that technology is implemented, and the room in which the display is viewed.

DLPs provide the excellent ANSI contrast figures in the 400-800:1 range (front projectors are better than rear projectors). Oddly, CRTs, which perform so well with on/off contrast, offer poor ANSI contrast figures of 75-150:1. LCoS displays offer numbers somewhere in between DLP and CRT. The champs for ANSI contrast are plasma and LCD flat panels, which offer ANSI contrast figures that are much closer to the measured on/off contrast than what you find with the other display technologies.

Until very recently CRTs performed the best with on/off contrast. 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. In recent years, however, digital technology has finally caught up with the CRT. JVC in particular has developed a line of LCoS-based projectors that provide native on/off contrast that challenges CRT performance. Several LCD and DLP projectors have implemented dynamic irises that are very effective in improving on/off contrast. Finally, plasma flat panels have achieved native on/off contrast that approaches the JVC projectors, while at the same time offering stratospheric ANSI contrast as well.

LCD flat panels have recently begun to provide world-class contrast as well. This is difficult for LCD because this display technology relies upon backlighting, a light source behind the panel. This makes it difficult for LCDs to provide low black levels. However, a recent advance in LCD technology called "local dimming" provides this backlighting from thousands of individual LED light sources. This dramatically improves both ANSI and on/off 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. The weakness of this approach is that it is cost prohibitive to provide an LED for every pixel on the screen. Thus, a signal LED must illuminate an area of many pixels. When this area contains an abrupt change in light and dark, the light from the LED can spill over into the dark part causing a haloing effect. This is most evident on light/dark boundaries.

The best contrast performance will be achieved in a room that has as few reflective surfaces as possible. This is specially important for front projectors, which can flood a room with an enormous amount of light and which require a dim environment in order to avoid washing out the image on the screen. The single most important step an owner of a front projector can take to improve the perceived contrast on the screen is to install black, non-reflective material on the ceiling directly in front of the screen. Because flat panels are so much brighter, they can be viewed in well-lit rooms. Thus, the reflectivity in the room is much less important. However, even with flat panels it is a good idea to lessen as much as possible the amount of direct sunlight or artificial light that hits the screen.

Why is contrast important?

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, Sin City, or Alien on a display with poor on/off contrast. You won't like it.

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. In my experience, ANSI contrast is generally a less important specification than on/off contrast. If CRTs have shown us anything it is this. However, when ANSI contrast gets very high, as it does with some plasmas, and LCD flat panels, the effect can be startling.

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 broad variety of criteria, including (in addition to contrast):

  • Sharpness
  • Resolution
  • Color accuracy
  • Freedom from artifacts
  • Perceived Depth

Engineering good contrast is generally not cheap, so high contrast displays will usually offer great images in any case.