The contrast of a projector (or any display) is the ratio of brightness in a full-white image to a full-black image. Ideally displays would have infinite contrast, where any black pixel displayed emits no light, but in practice this doesn’t happen. If a displayed contrast is too low, not as much detail can be seen in darker parts of the image. For a projected image with low contrast in a high ambient brightness environment, a dark border can sometimes be seen around the image, illuminating a grey box, or ‘postcard’ around the bright image in the center.

Figure 2-2 DGP projections with varying contrast

For DGP applications, the ambient lighting conditions will not be well controlled, so the contrast needs to be high enough to avoid making the background visible in dark ambient conditions. But overdesigning the contrast can lead to increased cost and size. Projectors for DGP applications should be designed for a contrast that will provide good enough performance while minimizing manufacturing cost. What contrast level is deemed ‘good enough’ is subjective and will also depend on the projection surface. Typically this is around 400:1 contrast as a minimum. 400:1 contrast in many dark ambient conditions will still be high enough to remove the background shadow from the image.