Browse Prior Art Database

Illumination and Imaging for Optical Character Recognition

IP.com Disclosure Number: IPCOM000073926D
Original Publication Date: 1971-Feb-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 3 page(s) / 27K

Publishing Venue

IBM

Related People

King, JH: AUTHOR

Abstract

High intensity illumination may be obtained at the surface of a document over a relatively small area simultaneous with projection of an image of the illuminated area with a maximum degree of contrast, irrespective of the surface texture of the document. It is assumed that some form of printed information appears on the surface of the document. Specular reflection from individual fibers in paper, from glossy surfaces of some inks, and from the glossy surface of photographic print papers normally causes the contrast ratio in the image to be severely reduced with the consequent reduction in signal to noise ratio at the output of a photodetector in an optical character recognition system.

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Illumination and Imaging for Optical Character Recognition

High intensity illumination may be obtained at the surface of a document over a relatively small area simultaneous with projection of an image of the illuminated area with a maximum degree of contrast, irrespective of the surface texture of the document. It is assumed that some form of printed information appears on the surface of the document. Specular reflection from individual fibers in paper, from glossy surfaces of some inks, and from the glossy surface of photographic print papers normally causes the contrast ratio in the image to be severely reduced with the consequent reduction in signal to noise ratio at the output of a photodetector in an optical character recognition system. High contrast in the image field is achieved by illuminating the desired area with linearly polarized light and incorporating a linear analyzer (polarizer) at an appropriate point of the optical path of the imaging system, with its axis of polarization oriented 90 degrees with respect to the axis of polarization of the illumination falling on the document.

By virtue of the fact that any light which is specularly reflected from any surface retains a major component having the same direction of linear polarization as that of the incident light, the crossed analyzer in this imaging system highly attenuates any light which may be specularly reflected from the document surface and thus, essentially, any light which is specularly reflected from the surface of the document does contribute to the image. However, materials like paper and other materials of heterogeneous composition, where a large percentage of the particle sizes are less than the optical wavelength will, when illuminated with linearly polarized light, diffusely scatter the incident illumination and at the same time essentially completely depolarize the scattered light. Thus the linear analyzer in this imaging system will pass on-the-order-of up to 50% of the scattered light emanating from the document and entering the entrance pupil of the imaging system and the light appearing in the image field will have a polarization 9O degrees relative to the polarization of the source of illumination. Thus the so-called glinting and sparkle effect due to specular reflection from paper fibers, glossy ink surface, and glossy surfaces of photographic print paper is essentially completely removed from the image. The type of illumination and imaging system described has application to optical character and pattern recognition where it is desirable to minimize background noise in the image...