Now you can measure everything under the sun with the technology you carry. A wristband that monitors exposure to UV rays allows users to soak up the light needed to make vitamin D without getting burned, and without the need for sunscreen.

"Vitamin D deficiency is pandemic around the world," says Karin Edgett, co-founder of UVA+B SunFriend. The vitamin is important for managing calcium levels in the body and maintaining healthy bones. In recent years it has also been implicated in reducing the risk of diabetes, arthritis and cancers such as breast cancer.

Although sunscreens protect from the harmful effects of the sun's rays – including skin cancer – they also stop the skin absorbing UVB rays, needed to make vitamin D. "There are huge numbers of people who are aware the sun is healthy, but don't know how much they need," Edgett says.

Safer rays

She hopes the UVA+B SunFriend will help people overcome this problem. Users input their skin sensitivity – estimated on a scale from 1 to 11 – and LED lights illuminate as their UV exposure increases. The lights begin to flash when it is time to get out of the sun. The device is also waterproof.

"UVB gives you vitamin D and UVA tends to give you more sun damage," Edgett says. The device lets you maximise UVB exposure whilst minimising damage from UVA. Once users begin to exceed the safe levels, they can either put sunscreen on or cover up. Edgett says she has had good response from parents who worry about the chemicals in sunscreen.

"This is an interesting gizmo and provided that the calibration is reasonably accurate, for both the UVB and for the sensitivity to sunlight, it could be a useful safeguard against sunburn," says Barbara Boucher who researches vitamin D at Queen Mary University of London.

But it might not give you much information about vitamin D levels, she says. "Worn on the wrist, it only tells you about sunlight on that wrist and not how much vitamin D one might make during exposure, since the rest of the arms could be covered up."

Get Sunsmart

The SunFriend, which costs $50, went on sale this week at the Wearable Technology Show in London, before general release in May.

The next version will have Bluetooth capability linked to a smart phone app, and the team are also developing a sensor that will differentiate between UVA and UVB.

Yinka Ebo, a spokesperson for the Cancer Research UK Sunsmart campaign, agrees that people need to avoid sunburn because it is the first physical sign of skin damage. This is the only device she has seen that allows people to adjust for their own skin sensitivity.

However, she cautions against foregoing sunscreen and other protective measures when the sun is strong. "We do all need some sun, but we don't know enough about how effective these devices are in helping people to avoid sunburn," she says.

Other UV tracking bands are due to be released later this year. The NetatmoJune, for instance, is a UV sensor disguised as jewellery.

Catch some rays the safer way (Image: Karin Edgett)