Optical sensor for determining the focussing error in an optical illumination system embodying a projection lens

Claim 2

Optical sensor for determining the focussing error of an optical illumination system wherein a projection lens is used to focus a beam of radiant energy on a reflecting surface, said optical sensor comprising photodetector means having a predetermined detection area arranged for selectively receiving at least one radiant energy portion emerging from the illuminated portion of said reflecting surface and focussed by said projection lens in the form of a diffraction pattern; the body of said diffraction paTtern being substantially equal to the detection area of said photodetector means; said optical sensor further comprising electrical means connected to said photodetector means for supplying a voltage representative of said focussing error.

Claim 3

Optical sensor as claimed in claim 1, wherein said photodetector means comprise two fixed photodetector elements; said fixed photodetector elements being respectively arranged to receive from said projection lens sharp images of the focus of said beam for two positions of said reflecting surface delimiting the focussing range of said optical illumination system; said electrical means comprising differential amplifier means having differential inputs respectively connected to the respective outputs of said fixed photodetector elements.

Claim 4

Optical sensor as claimed in claim 1, wherein said photodetector means comprise a moveable photodetector element, vibrational means for displacing said moveable photodetector element along the optical axis of said projection lens, and an electrical a.c. generator for energizing said vibrational means; said moveable photodetector element being arranged in one of its extreme positions to receive from said projection lens a sharp image of the focus of said beam for a position of said reflecting surface corresponding to one end of the focussing range of said optical illumination system; said electrical means comprising synchronous detection means having one input connected to the output of said electrical a.c. generator and a further input coupled to the output of said moveable photodetector element.

Claim 5

System using concentrated illumination of a reflecting surface for diffusion of information bearing signals through the medium of a recorded track lying in said reflecting surface, said system comprising: a source of radiant energy, a projection lens for concentrating said radiant energy onto said reflecting surface, focussing means associated with said projection lens, optical sensor means for determining the focussing error of said projection lens in relation to said reflecting surface and feedback means returning to the control input of said focussing means the error signal supplied by said optical sensor means; said optical sensor means comprising photodetector means having a predetermined detection area arranged for selectively receiving at least one radiant energy portion emerging from the illuminated portion of said reflecting face and focussed by said projection lens in the form of diffraction pattern; the body of said diffraction pattern being substantially equal to the detection area of said photodetector means; said optical sensor means further comprising electrical means connected to said photodetector means for supplying said error signal.

Claim 6

System using concentrated illumination of a reflecting surface for diffusion of information bearing signals through the medium of a recorded track lying in said reflecting surface, said system comprising: a source of radiant energy, a projection lens for concentrating said radiant energy onto said reflecting surface, focussing means associated with said projection lens, optical sensor means for determining the focussing error of said projection lens in relation to said reflecting surface and feedback means returning to the control input of said focussing means the error signal supplied by said optical sensor means; said optical sensor means comprising photodetector means having a predetermined detection area arranged for selectively receiving at least one radiant energy portion emerging from the illuminated portion of said reflecting face and focussed by said projection lens in the form of diffraction pattern; the body of said diffraction pattern being substantially equal to the detection area of said photodetector means; said optical sensor means further comprising electrical means connected to said photodetector means for supplying said error signal.

Claim 7

Optical sensor as claimed in claim 3, wherein said vibrational means comprise a moving coil excitation motor.

Claim 8

System as claimed in claim 4, wherein a semireflective plate is arranged between said source and said projection lens.

Claim 9

System as claimed in claim 4, further comprising an auxiliary source of radiant energy feeding said projection lens; the radiation emitted by said further source having a wavelength differing from the wavelength of the radiation emitted by said source.

Claim 10

System as claimed in claim 4, wherein said feedback means comprise a low-pass filter; the frequency band of said low-pass filter being below the frequency range of said information bearing signals.