A very common problem with digital signage window displays is reflectivity of the window glass, particularly in outdoor environments with direct sunlight.
The issue is twofold: First, your signage is not, and will never be, brighter than the sun. Second, the surface of the window is reflective, picking up every primary, as well as secondary (or reflective) light source which ultimately wreaks havoc with your window display.
One possible solution is installing anti-reflective film on the exterior surface of the window. Light diffusive film can be an ally in making your display pop better to passing viewers.
Another solution is LED signage, which is very effective (at distance) but with pixels more obvious at close up range.
We are interested in your thoughts on solutions for the ongoing battle with the sun. What have your results been?