A CAPTCHA is a program that can tell whether its user is a human or a computer. You've probably seen them colorful images with distorted text at the bottom of Web registration forms. CAPTCHAs are used by many websites to prevent abuse from "bots," or automated programs usually written to generate spam. No computer program can read distorted text as well as humans can, so bots cannot navigate sites protected by CAPTCHAs.

Telling Humans and Computers Apart Automatically

A CAPTCHA is a program that can generate and grade tests that humans can pass but current computer programs cannot. For example, humans can read distorted text as the one shown below, but current computer programs can't: CAPTCHA example

The term CAPTCHA (for Completely Automated Turing Test To Tell Computers and Humans Apart) was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University. At the time, they developed the first CAPTCHA to be used by Yahoo.

Recaptcha examples attached.

Topic attachments
I Attachment Action Size Date Who Comment
jpgjpg OCR_low.jpg manage 33.2 K 24 Aug 2009 - 11:43 ElaineFarrow OCR result of low quality scan
jpgjpg OCR_medium.jpg manage 96.7 K 24 Aug 2009 - 11:44 ElaineFarrow OCR result of medium quality scan
jpgjpg scan_low.jpg manage 86.3 K 24 Aug 2009 - 11:43 ElaineFarrow low quality scan
jpgjpg scan_medium.jpg manage 118.0 K 24 Aug 2009 - 11:44 ElaineFarrow medium quality scan
Topic revision: r2 - 24 Aug 2009 - 13:58:07 - ElaineFarrow
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