Patent ID: 7292515

Claim:
An information storage system, comprising: an optical record carrier having a stack of information layers alternating with spacer layers; a scanning device focusing a radiation beam to a scanning spot on a one of the information layers; and a detection device receiving radiation reflected by the one of the information layers, wherein the detection device produces an information signal representing information read from the one of the information layers and a focus error signal representing the axial distance between the scanning spot and the one of the information layers, the focus error signal produced by the detection device forms an S-curve as a function of the axial displacement of the scanning spot from the plane of the one of the information layers, and the minimum distance between a first and second information layers in the optical record carrier is greater than 1. 5 nS p , where n is the refractive index of the spacer layer between the first and second information layers and S p is the peak-to-peak distance between extrema on the S-curve.