Creating a shadeless material in Cycles
In this recipe, we will create a shadeless material, which is a material that behaves as self-illuminated but does not actually emit any light on the nearby objects.
In the following screenshot, we can see the difference between a Plane with a shadeless material and a Plane with an emitting material:
At the top, the cloudy sky image is perfectly self-illuminated and visible, but it's neither affecting the Spheres or the Suzannes nor the floor Plane (nevertheless slightly visible because of a low intensity World).
This is the reason a shadeless material is perfect ...