Zelda Lens of Truth effect in a 2D game with sprites

Hello, been looking into how to do a lens of truth effect where you cant see certain sprites unless you have a an item, made a post about it here

http://answers.unity3d.com/questions/1074359/make-a-depthmask-in-unity-2d-without-multiple-came.html

Cant seem to figure it out for 2d as almost everything relating to this is for 3D games, anyone got any ideas how i can have certain sprites only visible if you have an aura item or something, see the pic for what i mean

55854-depthmask.png

Try Unity - Scripting API: Physics2D.OverlapCircleAll and write a shader that changes alpha to 1 for all pixels within the overlap area?