Patent ID: 7315501

Claim:
Apparatus for the writing and reading of a holographic recording medium, preferably an optical card ( 2 ), in the form of two-dimensional bit-maps having a recording medium holding and/or positioning mechanism ( 4 ), movable or fixed read and write optics ( 9 ), the write optics comprising a polarised writing light source ( 20 ), polarising selector means ( 23 ) for separating and/or combining a reference beam and an object beam, object beam modulating means ( 25 ), polarisation wave plate ( 24 , 35 ), an objective lens ( 27 , 47 ) for imaging the object beam onto a recording layer, and further the read optics comprising a polarised reading light source ( 21 ), and a polarising selector ( 23 ′) and/or spatial filtering means for separating and/or combining the reference beam and an image beam, a light detector ( 29 ) and an objective lens ( 28 , 47 ) for imaging the image beam onto the light detector ( 29 ), characterised in that the wavelength of the reading light source ( 21 ) is different from the writing light source ( 20 ), recording data in a multiple recording layer, of the thickness of 250-1000 nm; the writing source ( 20 ) is of the kind of polarization holography; the read optics comprise wavelength distortion correcting means for correcting the distortion of the reconstructed image caused by the difference in the wavelength of the reading and writing light wherein the wavelength distortion correcting means of the read optics comprise an aspherical plastic objective lens ( 48 ).