"To
archive human knowledge and to make information more accessible to the
world, multiple projects are currently digitizing physical books that
were written before the computer age. The book pages are being
photographically scanned, and then, to make them searchable,
transformed into text using "Optical Character Recognition" (OCR). The
transformation into text is useful because scanning a book produces
images, which are difficult to store on small devices, expensive to
download, and cannot be searched. The problem is that OCR is not
perfect. reCAPTCHA improves the process of digitizing books by
sending words that cannot be read by computers to the Web in the form
of CAPTCHAs for humans to decipher. More specifically, each word that
cannot be read correctly by OCR is placed on an image and used as a
CAPTCHA. This is possible because most OCR programs alert you when a
word cannot be read correctly.
But if a computer
can't read such a CAPTCHA, how does the system know the
correct answer to the puzzle? Here's how: Each new word
that cannot be read correctly by OCR is given to a user in conjunction
with another word for which the answer is already known. The user is
then asked to read both words. If they solve the one for which the
answer is known, the system assumes their answer is correct for the new
one. The system then gives the new image to a number of other people to
determine, with higher confidence, whether the original answer was
correct. http://recaptcha.net/