Why does the Four-Layer disc only have 128GB capacity? Why didn’t the capacity double again?

The capacity per layer is different in the Single Layer/Dual Layer, Triple Layer and Quadruple Layer formats:

	Single Layer	Dual Layer	Triple Layer	Quadruple Layer
Total Capacity	25GB	50GB	100GB	128GB
Capacity per Layer	25GB		33.4GB	32GB
Minimum Mark Width	.149 µm		.112 µm	.117 µm

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