A Hole in the shadow, is that possible?

Hi everybody !
I try to make a 2D rogue like style game. I would like to use unity dynamic light, but for performace reason, I try to make a low cost light system.
Info : I use a lot of spriteRenderers.

So my question is : is that possible to make a hole in Ambiant color/shadow , to reveal a part of the map ?

I tried to make a shader with grabpass and a hand made lightmap system but i dont think i am on the right way..
Thanks for help


(and sorry for poor english lol)