Captcha ­- which stands for Completely Automated Public Turing test to tell Computers and Humans Apart -­ protects web sites against automatic and usually malicious programs that roam the internet trying to set up spam. They are tests that humans can pass but currently computers cannot.

The distorted image of a character sequence is the most commonly encountered form and usually appears as part of an online registration process.

If you cannot interpret the code and type it into a box, you are unable to register ­ but the majority of sites provide no alternative. Unless you crack the code, you are effectively locked out.

The problem with Captcha is that many humans cannot decipher the code either and nor can the technologies they commonly use, such as screen-reading software. If you have impaired vision, dyslexia or a learning disability, you may be unable to decipher the characters.

The accessibility armlock that Captcha represents is a problem for millions of users.
The most common alternative is an audio version of the graphical image, used by Google and Microsoft (but not Yahoo or YouTube), which provides a second chance at interpreting the code by speaking the characters aloud. But the audio distortion needed to obscure the code from malicious programs can make it almost impossible to hear.

Other alternatives are in the pipeline, including choosing descriptions of images from drop-down lists and asking the user to decide if a randomly rotated image is upright; but these still require some vision and the ability to deduce the image’s subject.

Other options use logic problems in text form ­ often too difficult for users with cognitive or learning disabilities ­ and a browser plug-in that is a useful tool but does not always work and demands Firefox as your browser.

Captchas are here to stay. But the only failsafe solution for those who require assistance is to have an obliging human on standby to wrestle with the
test on their behalf.

Robin Christopherson is head of accessibility at AbilityNet