Home Theater Projector Buyer's Guide
What is Contrast?
Contrast is the difference in brightness between the brightest and darkest parts in an image. The greater the difference, the higher the contrast.
Why is Contrast so important?
With business projectors, lumen output is of primary importance, and contrast is a secondary concern. Home theater projectors are the exact opposite. Contrast is arguably the single most important measurable quality in a home theater projector. A high-contrast projector produces a picture with a deep black level and clearly defined shadow detail. Contrast, in essence, gives "depth" to video images. A projector with excellent contrast can make a two-dimensional image appear almost three-dimensional.
The contrast ratios noted on a projector's spec sheet can be reported in one of two ways. If it just says "Contrast," it usually indicates On/Off contrast, which is the ratio of the whitest white and the darkest black that the projector is capable of producing. If it says "ANSI contrast," the ratio has been determined by displaying a checkerboard pattern of white and black squares and measuring the relative brightness of each. On/Off contrast is always a much larger number, and more typically listed on projector spec sheets, but ANSI contrast is a somewhat more accurate representation of what your projector is actually capable of displaying in any given scene. Note that neither of these measurements tells the whole story, and only taking both numbers into account gives even a moderate approximation of a projector's capabilities. To really know what a projector is capable of in relation to other models, either find a way to see it in person or read our Projector Reviews.
What about a dynamic iris?
A dynamic iris is a device built into some projectors that sits between the lamp and the lens. Many times per second, the projector evaluates the overall brightness of the image being projected and then opens or closes the iris to allow more or less light through.
A good dynamic iris will improve on/off contrast. Dark scenes will appear darker, while bright scenes will appear brighter. The on/off contrast rating will be based on the whitest white with the iris opened, and the blackest black when the iris is closed. Dynamic irises have no effect on ANSI contrast, though, so a projector with a lower contrast rating may appear higher in contrast in any given scene. As with ANSI lumen ratings, it is best to take official contrast specifications with a grain of salt. They can be highly misleading.
Dark Room Needed for Best Results
You've noticed that commercial movie theaters are dark, including dark-colored, non-reflective ceilings and walls. That is because any front projection system looks its best when there is no light in the room; this includes stray light from the screen that reflects off the walls or ceiling. Once you introduce light into the room, that light will make blacks look more like dark gray. This reduces the contrast of the image, making it appear flat or washed out. This will happen no matter what the contrast capability of your projector is.
Though the ideal viewing room is dark, most people don't want to darken the walls and ceiling of a living room or multipurpose room just to get ideal theater conditions. Today's high-contrast gray screens help to improve black level when there is some ambient or reflected light in the room. For the best possible image quality, though, take whatever steps you can to eliminate ambient light and reduce the reflectivity of the room's walls and ceiling.