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When shopping for projectors, most people make the decision based on cost per lumen — brightness vs. price. When creating projected AR installations, it also important to consider a third variable: contrast ratio.
Contrast ratio is the ratio between the maximum and minimum brightness, or in other words, the ratio between the brightest white and the darkest black. For example, a contrast ratio of 1000:1 means that the brightness of a completely white image is 1000 times greater than the brightness of a completely black image. Projectors with a high contrast ratio, such as 100,000:1, have darker black levels than projectors with lower contrast ratios, such as 1000:1. A higher contrast ratio also produces images with more saturated colors since the difference between black and white is greater.
Choosing contrast ratio is heavily dependent on the lighting conditions of your environment. If you are planning on projecting in a office space, or an area with a lot of ambient light, a projector with a lower contrast ratio will be optimal for your experience. On the other hand, if you are planning on projecting in a dark environment, a projector with a higher contrast ratio will be optimal for your experience.
Projectors with higher contrast ratios (deeper blacks) are typically marketed as "home cinema" projectors where the environment is very dark and black level is important. They tend to have larger enclosures to allow for more silent operation and cooling. Projectors with lower contrast ratios are typically marketed as "business" or "conference room" projectors, as they are optimized for the brightest possible image in high ambient light conditions. In conference room environments, black levels are less of a concern.
Because you will often not use the entire image of the projector, in dark environments you may end up seeing a "gray rectangle" around your scene. The projector is still casting a bit of light in those areas, because its contrast ratio is not high enough. Projectors with low contrast ratios will have a better price per lumen value, but keep in mind that when using them in dark settings, you will likely see the "gray rectangle" outline. Remember, a high contrast ratio (i.e., 100,000:1) means darker blacks, and a low contrast ratio (i.e., 1000:1) means brighter, more visible "gray" blacks.