aolba, If this is the shader tree you are using, ditch the opacity layer. This is only used if you are creating a decal-like layer on an object. It will not make the rest of the plate transparent. Keep shader one ("Whole Object" at the top of the tree) the same, but check the box at the bottom that says "No Light Interactions When Fully Transparent". This is very important as it will make your plane transparent. Take your opacity map and put it in the transparency channel with your color texture map. Invert the color of your transparency image as the transparent parts need to be white. Check the box at the bottom that says "No Light Interactions When Fully Transparent". Once again this will make it truly transparent in those areas you want it to be. Check it out and see what happens. Ken  ps My version of the shader tree is above.