reCAPTCHA

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 cant read such a CAPTCHA, how does the system know the correct answer to the puzzle? Heres 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.

reCAPTCHA helps prevent automated abuse of your site (such as comment spam or bogus registrations) by using a CAPTCHA to ensure that only humans perform certain actions.