We’re coming across more and more examples where digital techniques reach limitations and certain tasks are passed on to Humans. Amazon were among the first with their Mechanical Turk.
reCAPTCHA is a smart little applet that can be used to prevent abuse from spam-bots. It works because a OCR program is not able to read distorted text. Nothing new so far. But reCAPTCHA displays texts from digitalization projects that were not readable by the OCR program:
"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
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
image to a number of other people to determine, with higher confidence,
whether the original answer was correct."
Currently they are helping to digitize books for the Internet Archive. Quite Smart, eh?