Abstract: In a signal play-back device having a substantially flat record carrier or disc provided with first and second oppositely facing reflective surfaces and a signal recorded as variations in at least the second surface, and an optical system in which a first lens directs a laser light beam against the first record carrier surface for reflection by the latter and passage back through the first lens for focussing by the latter at an image point remote from the first record carrier surface, and a second lens effects a predetermined focussing of such image point at the second record carrier surface for scanning the recorded signal variations, with the light beam reflected from the latter, and having corresponding variations in its beam energy, passing back through the second lens toward a light detector for providing an output corresponding to such variations in beam energy, and in which the first and second lenses have magnification factors for maintaining the predetermined focussing of the image point at the secon