If you’ve signed up for any sort of online service there is a high likelihood you’re very familiar with CAPTCHA. CAPTCHAs are “used by many websites to prevent abuse from ‘bots,’ or automated programs usually written to generate spam.” They essentially try to verify whether a user filling out a text form is human or a bot. Have you ever wondered why you are sometimes prompted with two CAPTCHA words instead of one? Google explains:
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.
Currently, we are helping to digitize old editions of the New York Times and books from Google Books.
CAPTCHAs can be frustrating at times, but take solace in knowing that once entered correctly, you’ve just completed a tiny bit of community service.