WWW- The explosion of pornography on the Internet has spawned a huge business in parental controls and other blocking tools. Now the effort is repeating in the wireless world.

As cellphones become multimedia players with bigger screens and Web-browsing capability, the pornography industry increasingly is targeting its content at the mobile devices. And at a time when adult content on the Web is proliferating, more teenagers are at risk of seeing it: A Pew Internet study last summer found that about 45% of teens own a cellphone and one in four cellphone-owning teens have used their phone to connect to the Internet.

Many technology companies have begun developing real-time monitoring and filtering applications to block adult content from being viewed by underage cellphone users. Experts in blocking software say identifying porn on cellphones is much more difficult than on computers because wireless Web sites usually have very little text and the images are much smaller.

“You have so little information to work with,” says Alistair Allan, chief executive officer of RuleSpace LLC, a filtering-software maker based in Portland, Ore. “You’ll either miss it or overblock.”

So far the leaders in the field aren’t the same companies dominating comparable tools for the wired Internet, like McAfee Inc. and Symantec Corp. Rather they are smaller companies like RuleSpace and Blue Coat Systems Inc., based in Sunnyvale, Calif.

To be sure, all major U.S. cellphone carriers offer free parental-control services. For example, Cingular Wireless customers can choose to allow their children access only to certain sites listed on Cingular’s phone menu and deny them access to outside Web sites. Cingular is jointly owned by AT&T Inc. and BellSouth Corp.

But none of the phone companies has deployed sophisticated systems that can identify and filter pornographic content in real time, which has been the norm in the wired Internet world. That leaves cellphone users with a choice between exposure to all kinds of Web sites or blocking all access to the Web. So third parties are trying to come up with various software approaches to sell to service providers.

RuleSpace, for one, has developed a recognition system that it says identifies pornographic language or images on a Web page and can distinguish those from nonadult content. If the word “breast” appears together with the word “cancer,” for example, the system won’t block the page. But if it has “breast” and “teen” in the text, it probably will be blocked as porn.

The system also tries to identify inappropriate pictures by scanning skin tones, lighting, body position and facial expression. Professional porn sites usually have very bright lighting while amateur sites are often darker, says Mr. Allan of RuleSpace. The company claims its text recognizer has a 99% accuracy rate in correctly identifying adult sites and the image recognizer has a 90% accuracy rate.

When children try to access filtered sites, including some social-networking sites, the RuleSpace system can generate a message to the user saying, “You’re trying to access a site that your parent has said is inappropriate to you. You have the option to go somewhere else or request to go there.” If the child decides to go to the deemed inappropriate site, the parent will receive a message, asking if access should be allowed.

RuleSpace, which has been developing blocking technology for the wired Internet for years, started modifying its product for wireless devices two years ago. It sells its wired Internet product to carriers like BellSouth and leading Internet companies such as Yahoo Inc., but so far doesn’t have any customers for its wireless product. Mr. Allan says RuleSpace is talking to many major wireless carriers.

Blue Coat Systems recently was selected by Britain’s Vodafone Group PLC, the world’s largest cellphone service provider by revenue, to provide a content-filtering service. It is being installed on the networks of all of Vodafone’s 17 operating companies. Customers will be given the option of choosing to activate the service or not.

Blue Coat’s filtering system includes a large database of pornographic sites and can identify adult content in real time by looking at words and links on Web pages. On porn sites, for example, the word “sex” tends to appear more than once on a single page, and there typically will be links to other porn sites, the company says. The system can tell if phones are registered to underage users and block them from accessing those sites.

“Nobody wants to see porn on the playground,” says Steve Mullaney, an executive at Blue Coat.