The US Army has put out a call for proposals from companies for wearable camouflage that has the ability to shift its colour according to the background, making the wearer invisible. It wants to test the best designs in just 18 months.

That doesn't seem far away at all, but actually we're closer to that kind of technology that most people realise. Metamaterials that can bend light around an object were first demonstrated nine years ago, and since then many researchers have showed off their own invisibility cloak designs.

The tricky thing is that their invisibility usually only works in the lab from certain angles or at specific wavelengths. The US Army has specified that prototype uniforms submitted must work from all angles, in all terrain and weather conditions, across a wide range of temperatures.

That's a big ask, especially as any power source required by the camouflage must last at least eight hours and weigh less than half a kilo. But some firms have said that they're on their way there - most notably a Canadian company called Hyperstealth Biotechnology, which supposedly demonstrated camouflage technology to the US army in 2014.

At the end of the day, any technology that gets commissioned isn't going to allow for total invisibility - instead it'll be compared with existing camouflage patterns. Any improvement will almost certainly be grounds for research to continue.