Commonly used captcha programs require the computer user to input a series of randomly generated numbers or letters that appear on the screen and appear distorted.

The underlying principle is that humans can read the distorted image and bots can not. Other modes of captchas include identifying pictures, solving visual patterns etc. It is quite common for captcha to be used at the time of signing up for free services such as email accounts or at the time of initial registration to a web account or service.

Captchas such as the above, however, works at a disadvantage for visually impaired users. Providing an audio captcha along with a visual one is one way of handling such scenarios. However, an audio captcha, still excludes people who are both visual and hearing impaired.

Further Reading

Refer to Captcha Project for further reading on this subject.

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