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Device for optically scanning a recording medium by focusing the best focus or the paraxial focus to the information layer
| Details |
Inventors: Braat, Josephus J. M.; Ubbens, Igolt P. D.; Coombs, James H.; Sonneveld, Jacob; Kroon, Jacobus P. C.; Jutte, Petrus T.;
Assignee: U.S. Philips Corporation (New York, NY)
Primary Examiner: Nelms; David C.
Assistant Examiner: Huber; P. W.
Attorney, Agent or Firm: Belk; Michael E.
An optical scanning device can scan two types of record carriers each having a transparent layer with a different thickness, in which a radiation beam scans an information layer of the record carrier through the transparent layer. When scanning a first type of record carrier the best focus of a scanning radiation beam is positioned on the information layer and when scanning a second type of record carrier the paraxial focus of the radiation beam is positioned on the information layer. |
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DETAILED DESCRIPTION It is an object of the invention to provide a device for optically scanning a record carrier, which has a simple construction and an improved utilisation of the optical power emitted by the radiation source.
A further object of the invention is to provide a method for scanning different types of optical record carriers without the above-mentioned disadvantages.
The first object is met by a device as described in the opening paragraph, which device is characterized according to the invention in that it comprises means for positioning the best focus of the radiation beam substantially on the first information layer and the paraxial focus of the radiation beam substantially on the second information layer.
The objective lens in the device according to the invention is designed for converging the radiation beam with a single vergence through the first transparent layer to a best focus on the first information layer.
A lens has, in general, three focal points: the paraxial focus, the best focus, and the marginal focus.
In the absence of aberrations, these three focal points coincide.
The best focus is the axial position where the intensity of the converging beam reaches its maximum value.
When the best focus should be positioned on the second information layer, the second transparent layer will introduce spherical aberration which gives a low quality of the focal spot.
The invention resides in the insight that the second information layer may be properly scanned by the aberrated converging beam, if, instead of the best focus, the paraxial focus of the converging beam is positioned substantially on the second information layer.
The paraxial focus is the axial position where the intensity of the converging beam reaches a sub-optimum value, and where rays close to the optical axis of the beam come to a proper focus.
The optimum position of the second information plane is in general near the paraxial focus, up to a few focal depths removed from it because of residual spherical aberration in the central part of the beam
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Optical Systems 
