Captcha stands for: “Completely Automated Public Turing test to Tell Computers and Humans Apart”. It refers to a technology familiar to anyone who’s registered on a popular website – the “what word is shown on this image” challenge. As the “Turing test” alludes to, the purpose is to distinguish between humans and computers. There are problems with the visual check, and not only for blind users. If the images come from a pool, there is only a limited amount of them, and it would be easy to build up a database.
I’d be curious to know the attitudes and impact on mainstream users. Do most users understand why they are being asked to perform a ridiculously simple test? Can it be explained away with a glib “This is required for security purposes”? The test is so straightforward, and part of a larer registration process, so it possibly won’t turn many users away. But it certainly adds to generic resistance to register with any site in the first place.