System for making a light beam scan a flat carrier with autofocusing

Claim 1

In a signal play-back device having a substantially flat record carrier with first and second oppositely facing reflective surfaces and a signal recorded as variations in at least said second surface, a light source emitting a light beam for scanning the recorded signal variations and providing correspondingly varied reflected beam energy, and photo-detecting means for receiving said varied reflected beam energy and providing a corresponding output; an optical system for continuously focussing said light beam at a point on said second surface irrespective of movements of said record carrier in directions normal to said surfaces comprising first lens means directing said light bean from said source in a path extending substantially normal to said first surface for reflection by the latter and focussing the light beam reflected from said first surface at an image point remote from said first surface, second lens means focussing said image point onto said second surface and directing the reflection of said image point from said second surface to said photodetecting means, said light beam, as received by said first lens means from said light source, being a parallel light beam, said first lens means being dimensioned and positioned in respect to said record carrier and said light source for focussing said parallel light beam from the latter at a point which is spaced along said path from said first surface in the direction toward said first lens means for the full range of said movements of the record carrier in sid directions, and said first and second lens means having respective magnification factors N 1 and N 2 satisfying the following relation: N.sub.1 ·N.sub.2 = 1/√2.