Invisibility Cloaks

Invisibility has been on humanity’s wish list at least since Amun- Ra, a deity who could disappear and reappear at will, joined the Egyptian pantheon in the earlier BC. With recent advances in optics and computing, however, this elusive goal is no longer purely imaginary. Susumu Tachi, an engineering professor at the University of Tokyo, demonstrated a crude invisibility cloak. Through the clever application of some dirt-cheap technology, the Japanese inventor has brought personal invisibility a step closer to reality.

Tachi’s cloak- a shiny raincoat that serves as a movie screen, showing imagery from a video camera positioned behind the wearer- is more gimmick than practical prototype. Nonetheless, from the right angle and under controlled circumstances, it does make a sort of ghost of the wearer. In addition, unlike traditional camouflage, it is most effective when either the wearer or the background is moving (but not both).

Professor Tachi’s cloak works by projecting an image on to itself, which is behind the wearer. A computer generates the image that is projected so the viewer effectively sees through the cloak. The key element in the development of the cloak is a new material called retro reflective material. This material allows you to see a three-dimensional image. This material is the key to our technology.

A retro-reflective material contains 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.

It strongly depends on how light reflects from other types of surfaces. A rough surface creates a diffused reflection because the incident (incoming) light rays get scattered 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 a process known as 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


Optical camouflage delivers a similar experience to Harry Potter’s invisibility cloak, but using it requires a slightly more complicated arrangement. First, the person who wants to be invisible (let us call her person A) dons a garment that resembles a hooded raincoat. The garment is made of a special material that we will examine more closely in a moment. Next, an observer (person B) stands before person A at a specific location. At that location, instead of seeing person A wearing a hooded raincoat, Person B sees right through the cloak, making person A appear to be invisible. The photograph on the right below shows you what Person B would see. If person B were viewing from a slightly different location, he would simply see person A wearing a silver garment (left photograph below).