To determine the effects of spherical aberration due to defocus and interfe
rence of layer 0(L0) and layer 1(L1) in high-density dual layered-discs, 17
sets of discs structures are measured while maintaining thickness uniformi
ty of the space layer within +/-1.5 mum for one revolution. After measuring
the discs, an optimum thickness of 30 mum of the space layer is obtained.