The last time Ray Campbell tried to buy Cubs tickets online, Tickets.com asked him to enter the text in a distorted image in order to prove that he was not a robot programmed to automatically buy tickets for scalpers.

The only problem: Campbell couldn't read the text in the image. In fact, he couldn't see it at all -- he's been blind his entire life.

"All I want to do is buy tickets, and I can't do that because there's this verification and they have not provided an audio link to it," Campbell said.

For America's nearly 2 million blind or visually impaired Internet users, problems like these can prevent them from taking advantage of all the Internet has to offer.

"The two challenges with Web accessibility are not just being able to access the site, but being able to use the site," said Leah Gerlach, director of counseling at the Diecke Center for Vision Rehabilitation in Wheaton.

The growing use of multimedia video on Web sites creates a significant accessibility challenge, Gerlach said, adding that Internet video can confuse the screen reading software that blind and visually impaired people use to browse the Internet.

Screen reading software uses text-to-speech conversion, machines that translate on-screen text to Braille or a combination of both to present a Web page to a blind or visually impaired user.

Campbell is a technician at Chicago Lighthouse, an organization for the blind and visually impaired. A former software engineer at Lucent Technologies, he now takes calls from blind and visually impaired people across the U.S. and Canada and helps them solve computer problems and navigate Web sites.

Campbell identified what he said are the Web's three major accessibility problems: graphics without descriptive text, required plug-in installations and visual registration tests like the one Campbell encountered on Tickets.com, called CAPTCHAs -- an acronym for Completely Automated Public Turing test to tell Computers and Humans Apart.

Captchas, which many major sites require, are particularly troublesome in terms of accessibility.

And as interactive, multimedia Web sites become more prevalent, blind and visually impaired users might find themselves behind the curve as designers forgo accessible pages for glitzy ones and screen reading software lags behind, Gerlach said.

"We don't drive change. We have to follow it and keep up with it," Gerlach said. "We're always six months behind cutting edge because we have to be."