How dvd dual layer +R/-R discs work?

There are 2 layers attached to the substrate. The ';lower'; layer is semi-reflective and when the laser is at full power it can read through that layer to the upper layer. The ';upper'; layer is fully reflective, just like the fully reflective layer of a single layer disc. The burning starts at the inside of the upper layer and continues to the outside of that layer. It then switches to the outside of the lower layer and continues back to the inside of the lower layer. This is referred to as OTP (opposite track pitch) because one layer reads in-to-out and the other is out-to-in. You can also burn discs with both layers going from the center out, but it makes the layer break much more noticeable unless you author the disc properly.How dvd dual layer +R/-R discs work?
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