Optical read system

Claim 1

Apparatus for optically detecting markings of a magnetically marked element which markings are capable of modulating certain wavelengths in the spectrum of light, and for reducing spurious modulations present in an output signal which are produced by scratches, discontinuities and stresses in the magnetically marked element and by variations in the light source, comprising: a source for generating a beam of light having at least two wavelengths within separate regions of the spectrum, said beam impinging upon a magnetically marked element to detect the markings thereof, only one of said wavelength regions of light being modulated by said markings of said magnetically marked element, the other of said wavelength regions being unmodulated by said markings of said element, but both of said wavelength regions being modulated by the scratches, discontinuities and stresses present in said magnetically marked element and by variations in said source of light; means for separating that modulated region which is modulated by the magnetic markings upon said marked element from said wavelength region which remains unmodulated by said magnetic markings, said means being positioned to receive said beam of light after it has impinged upon said magnetically marked element; and means responsive to said separated beams for separately detecting and differentially comparing said beams, whereby the modulations produced by the scratches, discontinuities and stresses in said magnetically marked element and by variations in said light source are reduced to provide an improved output signal.