periodicals such as newspapers). This final database contains bibliographic information for each of

periodicals such as newspapers). This final database contains bibliographic information for each of these 129 million editions (Ref. $1). The country of publication is known for 85.3% of these editions, authors for 87.8%, publication dates for 92.6%, and the language for 91.6%. Of the 15 million books scanned, the country of publication is known for 91.5%, authors for 92.1%, publication dates for 95.1%, and the language for 98.6%. I.2. Digitization We describe the way books are scanned and digitized. For publisher-provided books, Google removes the spines and scans the pages with industrial sheet-fed scanners. For library-provided books, Google uses custom-built scanning stations designed to impose only as much wear on the book as would result from someone reading the book. As the pages are turned, stereo cameras overhead photograph each page, as shown in Figure $1. One crucial difference between sheet-fed scanners and the stereo scanning process is the flatness of the page as the image is captured. In sheet-fed scanning, the page is kept flat, similar to conventional flatbed scanners. With stereo scanning, the book is cradled at an angle that minimizes stress on the spine of the book (this angle is not shown in Figure $1). Though less damaging to the book, a disadvantage of the latter approach is that it results in a page that is curved relative to the plane of the camera. The curvature changes every time a page is turned, for several reasons: the attachment point of the page in the spine differs, the two stacks of pages change in thickness, and the tension with which the book is held open may vary. Thicker books have more page curvature and more variation in curvature. This curvature is measured by projecting a fixed infrared pattern onto each page of the book, subsequently captured by cameras. When the image is later processed, this pattern is used to identify the location of the spine and to determine the curvature of the page. Using this curvature information, the scanned image of each page is digitally resampled so that the results correspond as closely as possible to the results of sheet-fed scanning. The raw images are also digitally cropped, cleaned, and contrast enhanced. Blurred pages are automatically detected and rescanned. Details of this approach can be found in U.S. Patents 7463772 and 7508978; sample results are shown in Figure $2. Finally, blocks of text are identified and optical character recognition (OCR) is used to convert those images into digital characters and words, in an approach described elsewhere (Ref. $2). The difficulty of applying conventional OCR techniques to Google’s scanning effort is compounded because of variations in language, font, size, paper quality, and the physical condition of the books being scanned. Nevertheless, Google estimates that over 98% of words are correctly digitized for modern English books. After OCR, initial and trailing punctuation is stripped and word fragments split by hyphens are joined, yielding a stream of words suitable for subsequent indexing. 1.3. Structure Extraction After the book has been scanned and digitized, the components of the scanned material are classified into various types. For instance, individual pages are scanned in order to identify which pages comprise the authored content of the book, as opposed to the pages which comprise frontmatter and backmatter, such as copyright pages, tables of contents, index pages, etc. Within each page, we also identify repeated structural elements, such as headers, footers, and page numbers. Using OCR results from the frontmatter and backmatter, we automatically extract author names, titles, ISBNs, and other identifying information. This information is used to confirm that the correct consensus record has been associated with the scanned text. HOUSE_OVERSIGHT_017012