We all know we shouldn't spend too long out in the sun, but unless you're keeping a strict eye on the clock, it can be difficult to keep track of exactly how much exposure you're getting.

Now a new wearable technology from cosmetics giant L'Oréal could help take the guesswork out of how much sun is too much. 'My UV Patch', unveiled at CES this week, is being marketed as the first-ever stretchable skin sensor designed to monitor UV exposure.

"Previous technologies could only tell users the amount of potential sun exposure they were receiving per hour while wearing a rigid, non-stretchable device," said Guive Balooch, Global Vice President of L'Oréal's Technology Incubator. "The key was to design a sensor that was thin, comfortable and virtually weightless so people would actually want to wear it."

The transparent adhesive is just 50 micrometres thick and adheres directly to any area of skin you want to monitor. Once in place, a photosensitive dye inside the patch changes colour when exposed to UV rays to indicate varying levels of sun exposure.

One of the limitations with My UV Patch is that you'll need a smartphone on hand to interpret the colours of the dye correctly. Using a mobile app that will become available for iOS and Android devices, users can snap a shot of the patch on their skin. The app will then analyse varying photosensitive dye squares in the image to determine how much UV exposure the wearer has received.

In other words, it's up to you to keep checking your UV levels – the patch isn't going to alert you on its own that you've had too much sun.

While needing to bring your smartphone along may diminish the appeal of the patch at first, they're only designed to be a temporary educational tool anyway. The wearable – which is expected to launch later in the year and will be distributed for free – can be worn for days at a time, with the idea being that after wearing it for a while, you'll get a sense of how long you can safely stay in the sun for.

After that you can leave your smartphone (and patch too) at home, secure in the knowledge – hopefully – that you shouldn't spend too long outdoors without protection.