CAPTCHA

I am a visually impaired computer user, and I am very irritated with such companies on the web that use 
CAPTCHA where a user needs to type in the letters and or numbers they see from an image in order to register or email a company with a question!
there could be a simple solution to this, if companies insist on using 
CAPTCHA then they should use mathematical 
CAPTCHA where a simple arithmetic problem is presented and must be solved in order to submit or register!