“He's mad,” said Kemp; “inhuman. He is pure selfishness. He thinks of nothing but his own advantage, his own safety. I have listened to such a story this morning of brutal self-seeking! He has wounded men. He will kill them unless we can prevent him. He will create a panic. Nothing can stop him.”
                                            - H.G. Wells, The Invisible Man

Invisibility has secured its place on the scientific vice list. The quest to attain invisibility, the entire discipline of alchemy and the Fountain of Youth: these are topics most often found in science fiction movies and novels. But these pursuits are not just for Trekkies anymore. While popular culture has categorized these ideas neatly into the History and the Sci-Fi Channels, they maintain their universal appeal by hinting at our own questions of humanity and immortality. What would happen in a world where any metal could be turned to gold? Do we really want to live forever? What aspects of ourselves would be revealed if we could go unseen?

Various attempts at providing the illusion of invisibility have popped up in nearly all aspects of culture. From the ninja adage to "Be one with your surroundings," to the development of optical cameras and reflectors, the science of invisibility seems to permeate our lives. The two techniques that we'll look at illustrate both the common sense and complex attempts at invisibility. You may or may not believe that they work, but you will not understand either of them.

Or at least, we don't.

Low Tech: Camouflage.

The most basic type of camouflage is worn on the battlefield, where concealment is good for obvious reasons. Conventional camouflage has two basic elements: color and pattern. The trick to this type of concealment is to color the material with dull hues that match the predominant colors of the surrounding environment. The pattern of the material is also critical. The point of the seemingly random pattern we see in camouflage clothing is to hide the contour of the body. The visually disruptive pattern actually helps your brain to connect the lines of the colored blotches with the lines of trees, ground, leaves and shadows.

As science defines the mental processes of perception when viewing camouflage, taken from this paper:

A number of psychologists studying visual perception investigated the accuracy of edge localization under conditions of varying blur and varying contrast. Research by Watt & Morgan, Mather and Morgan, Morgan et al. and Georgeson & Freeman clearly show that for blurred 1-dimensional edges there is a shift in perceived edge location away from the zero crossing. The magnitude of the shift increases with the increase in magnitude of both the edge blur and edge contrast. Their proposed model of the shift is based on non-linearity of perceived brightness, with the saturation function (1) where c is a constant.

We didn't understand it either.

Continuity is key in visually deciphering something. Imagine a stack of ten Legos. If all of the Legos are colored red, you perceive the pile as one unit. However, if the bottom five Legos are red, and the top five Legos are blue, you may perceive the pile as two separate units. We tend to recognize some things as a separate object if it has one continuous color, so a person who is wearing one single color is much more likely to stand out than a person wearing a jumble of colors. In the jungle (or any other environment), you perceive the jumble of colors in camouflage material as many small components of the surrounding foliage.

It is this strange habit of the brain that allows a person in camouflage to go undetected even though they are in plain sight. Once you have spotted a camouflaged person, they stand out, and it may seem odd that you didn't notice them earlier. This is because your brain is now processing the visual scene differently: it is looking for a single person. Makes you wonder ... why do you ninjas wear all black?

High Tech: The Personal Cloaking Device.

If you took a camera and put it on your back, and then hooked it up to a monitor that you held in front of you, people would be able to, in a sense, see right through you, right? Yes, but of course no one would actually say that you were invisible. It's more of a stupid trick that this author tried at parties as a desperate attempt to impress girls. Not only does it not work on women, but it doesn't make you anywhere near invisible. The next step is to find clothes that take the place of the camera and the monitor. What if your jacket, instead of just large amounts of alcohol, allowed people to see right through you?

This has been the goal of Susumu Tachi's team and their cloaking system. Unfortunately, however, Tachi's cloak (which is more like a shiny raincoat) is more gimmick than practical prototype. Nonetheless, from the right angle and under controlled conditions, the cloak seems to make the wearer some sort of ghost. A camera behind the wearer feeds the background images through a computer to a projector, which then paints them on the jacket as though it were a movie screen. The wearer appears translucent, so long as they are being viewed at the right angle and as long as it's not too bright outside.

Tachi, a professor of computer science and physics at Tokyo University and also the founder of the Virtual Reality Society of Japan, designed the coat using microscopic reflectors which act as the movie screen. They can even reflect images if the material is wrinkled. Tachi is uncertain of the technology's practical uses, but he likes to think that one day we will have glass bottom airplanes and surgeons who would be able to see through their hands while they are performing surgery. Or, and this is a real stretch, it just might be used to help those on the battlefield kill even more people. Everything has a military application.

Conclusion: The Next Step.

“He's mad,” said Kemp; “inhuman. He is pure hornyness. He thinks of nothing but his own libido, his own urges. I have listened to such a story this morning of brutal male adolescents! He is an aroused man. He will photograph them unless we can prevent him. He will create a panic. Nothing can stop him.”
                                               R.U. Horny, “The Invisible Man Goes to the Locker Room”

Why do we really want to be invisible? Without trying to break this down into some kind of Freudian psycho-babble, I present this case study, the ultimate expression of invisibility.