Google said this week one of its least expensive innovations -- virtual reality headgear made of cardboard -- has become a huge hit.

And the tech giant said it has a new version of its cardboard gadget, which consumers can buy for as little as $4.

Playfully named Google Cardboard debuted ast year at the Internet giant's annual developers conference.

One simply folds the cardboard into goggles of a sort that have eye holes opening into a slot designed to hold smartphones that serve as display screens for immersive videos.

"It took off; people keep finding new and creative uses of Google Cardboard virtual reality," vice president of product management Clay Bavor said at a Google I/O developers gathering concluding on Friday.

"One guy even proposed to his girlfriend using Google Cardboard. I'm not sure how that worked, but I hope she said yes."

Bavor introduced a second-generation version of Cardboard that required less folding and was adapted to the popularity of large-screen smartphones.

A Cardboard software kit works with Apple or Android smartphones, meaning they can be used as screens in the viewers, which can be had for just a few dollars.

Hundreds of applications have been created to work with Cardboard, taking advantage of position-sensing capabilities in smartphones to give wearers a sense of looking around in virtual environments while turning their heads.

- Virtual field trips -

Bavor announced availability of an "Expeditions" version of Cardboard designed to let teachers take students on virtual field trips to places such as The Great Wall of China; undersea reefs, or Versailles.

Students wear Cardboard viewers while teachers using tablet computers guide virtual reality (VR) outings, according to Bavor's presentation.

"It lets teachers take classed on field trips to anywhere," Bavor said. "Hundreds of classes around the world have already gone on expeditions."

He said that Google is working with GoPro on a specialized camera rig and accompanying system for capturing video in 360-degrees and weaving the imagery into VR presentations.

Google-owned video sharing service YouTube will support VR videos people will be able to view using smartphones and Cardboard.

"Google is definitely democratizing virtual reality more than, say, Facebook," Current Analysis research director Avi Greengart told AFP at the gathering.

"It is very clever of Google to almost back into it by giving people something at almost no cost."

The chief executive of Facebook-owned VR head gear company Oculus Rift went on record this week estimating that getting going from the ground up with one of its systems could cost about $1,500 when it makes its market debut.

Brendan Iribe of Oculus was at a Code technology conference in California when he gave the ballpark figure, which included the cost of a computer with the processing power needed for rich, immersive, seamless video graphics.

Oculus, acquired by Facebook last year in a deal valued about $2 billion, has announced plans to begin shipping headsets to consumers early next year.

The headset, designed for immersive gaming and other applications, has built a strong following among developers and has won praise from analysts for limiting the motion sickness which affects users of VR gear.

"Virtual reality is going to transform gaming, film, entertainment, communication, and much more," Oculus said when revealing launch timing.

Facebook co-founder and chief Mark Zuckerberg has described buying Oculus as a long-term bet that making the social network's offerings more immersive would pay off in the future, enabling members to sort of "teleport" to distant places.

"VR will definitely have a place in gaming; whether it will have a place in general computing is anyone's guess," Greengart said.

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