Hey fogman, it works well, thanks!
I have another problem.
If i move the object that blocks the light between the lamp and an other object, the size of the shadow doesn't change.
When the blocking object is closer to the lamp, the size of the area covered by the shadow should grow, but it remains the same.
What is the reason? How could to resolve this problem, to get an shadow thats size varying depending on the distance between the lamp and the object?