For an image to be visible, the image brightness must be greater than the natural or ambient brightness of the surface onto which the image is being projected, where the natural brightness depends on both the amount of ambient illumination and the surface reflectance. The ratio of image brightness relative to the natural projection surface brightness is called the luminance contrast ratio (LCR). An image is typically just-visible when the LCR is 1.5, meaning the image is 50% brighter than the surrounding surface. However "just-visible" is typically insufficient from the perspective of usability. To be usable, a minimum LCR of 2-4 is often required. For maximum usability (across a range of ambient conditions), an LCR of 25 or more is desirable

The brightness of the ground will depend on the ambient lighting conditions and surface reflectivity in the area. A well-lit city street directly under a street light will have much more light reflecting off of it than a country back road illuminated only by the moon. To maintain the same LCR, the displayed image must be much brighter in well illuminated conditions than in dark ones. Typically, a display with a higher LCR is desirable and the image will never appear too bright in DGP applications. Dynamic ground projectors should be designed to be visible in the brightest nighttime ambient conditions, and darker conditions will only improve the perceived brightness of the displayed image. Daytime brightness conditions are prohibitively bright for DGP applications to be visible.

The approximate ambient brightness levels in various lighting conditions are included in Table 2-1.