Optical Camouflage: Invisibility Cloak Components
All right, so you have your video camera, computer, projector, combiner and wondrous reflective raincoat. Just how does augmented-reality technology turn this odd shopping list into a recipe for invisibility?
First, let's take a closer look at the raincoat: It's made from retro-reflective material. This high-tech fabric is covered with thousands and thousands of small beads. When light strikes one of these beads, the light rays bounce back exactly in the same direction from which they came.
To understand why this is unique, look at how light reflects off other types of surfaces. A rough surface creates a diffused reflection because the incident (incoming) light rays scatters in many different directions. A perfectly smooth surface, like that of a mirror, creates what is known as a specular reflection -- a reflection in which incident light rays and reflected light rays form the exact same angle with the mirror surface.
In retro-reflection, the glass beads act like prisms, bending the light rays by refraction. This causes the reflected light rays to travel back along the same path as the incident light rays. The result: An observer situated at the light source receives more of the reflected light and therefore sees a brighter reflection.
Retro-reflective materials are actually quite common. Traffic signs, road markers and bicycle reflectors all take advantage of retro-reflection to be more visible to people driving at night. The movie screens found in most modern commercial theaters also take advantage of this material because it allows for high brilliance under dark conditions. In optical camouflage, the use of retro-reflective material is critical because it can be seen from far away and outside in bright sunlight -- two requirements for the illusion of invisibility.