Influence of substrate thickness on optical disk readout.

I consider the influence of different substrate thicknesses on the signal obtained from an optical disk. Two solutions are presented for the reading of different disk types. In one situation a substrate thickness change for disks with equal density is made inoffensive by introduction of a combined amplitude-phase mask in the pupil of the readout objective. In a second situation, where the different thickness is accompanied by a change in optical density, a specific selection in the far field of the detected radiation has proven to be adequate.