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How Invisibility Cloaks Work


Optical Camouflage: More Invisibility Cloak Components
As you can see in this image, the experience closely resembles walking directly in front of a movie projection screen, only with a real background.
As you can see in this image, the experience closely resembles walking directly in front of a movie projection screen, only with a real background.
AP Photo/Shizuo Kambayashi

For the rest of the setup, the video camera needs to be positioned behind the subject to capture the background. The computer takes the captured image from the video camera, calculates the appropriate perspective and transforms the captured image into the picture that will be projected onto the retro-reflective material.

The projector then shines the modified image on the garment, by shining a light beam through an opening controlled by a device called an iris diaphragm. This diaphragm is made of thin, opaque plates, and turning a ring changes the diameter of the central opening. For optical camouflage to work properly, this opening must be the size of a pinhole. Why? This ensures a larger depth of field so that the screen (in this case the cloak) can be located any distance from the projector.

Finally, the overall system requires a special mirror to both reflect the projected image toward the cloak and to let light rays bouncing off the cloak return to the user's eye. This special mirror is called a beam splitter, or a combiner -- a half-silvered mirror that both reflects light (the silvered half) and transmits light (the transparent half).

If properly positioned in front of the user's eye, the combiner allows the user to perceive both the image enhanced by the computer and light from the surrounding world. This is critical because the computer-generated image and the real-world scene must be integrated fully for the illusion of invisibility to seem realistic. The user has to look through a peephole in this mirror to see the augmented reality.

On the next page, we'll look at how this whole system comes together.