Optical camouflage is a kind of active camouflage. This idea is very simple. If you project background image onto the masked object, you can observe the masked object just as if it were virtually transparent.
RPT (Retro-reflective Projection Technology),
HMP (Head Mounted Projector)

Scientists closer to making objects invisible

WASHINGTON (AP) Harry Potter and Capt. Kirk would be proud. A team of American and British researchers has developed its own cloak of invisibility.

Well, OK, it's not perfect yet. But it is a start, and it did a pretty good job of hiding a copper cylinder.

In this experiment the scientists used microwaves to try to detect the cylinder. Like light and radar waves, microwaves bounce off objects making them visible and creating a shadow, though it has to be detected with instruments.

If you can hide something from microwaves, you can hide it from radar -- a possibility that will fascinate the military.

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