CAPTCHA (acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart”) is a challenge-response test that is utilized to determine whether or not the user is human or an automated script (also known as robot). This method is popular to prevent spam generated in form submissions, and is usually based upon an image that contains alpha numeric characters that are hard to read by an automated script, but not by a human. The image below is a typical image used in CAPTCHA.
Since the objective of CAPTCHA is preventing that automated scripts interpret the content of the challenge, it’s generally not accessible, and page readers are not able to read those challenges for people with sight disabilities.
Some CAPTCHA services offer audio as an alternate version of the image, as shown in the illustration down below. Visually impaired users have, then, the option of listening to the challenge and provide the answer. However, some people argue that managing those options is cumbersome and from time to time they cause the form to reload and losing the information they put in.
There is a type of CAPTCHA that offer better accessibility (TextCaptcha), which provides question challenges in text mode that require logical answers, like “What is 1 + six?” or “Which of sock, library, cake or red is a color?”. The problem with this method is that some robots can actually parse that content and provide an answer, but on the other hand, it could be hard to use by people with cognitive disabilities.
In conclusion, there is not a perfect option for CAPTCHA, they all present some challenges for the different types of disabilities. For that reason, it is recommended to give users an alternate way to send the information in the web form, like providing a telephone number they can utilize to call and have a person help them get the information/service they need.