How to not be seen

 kw: book reviews, nonfiction, science, optics, visibility, invisibility

In a video you may have seen (watch it here before continuing; spoiler below),

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…titled "Selective Attention Test", you are asked to keep careful watch on certain people throwing basketballs.

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…Several seconds in, someone wearing a gorilla suit walks into the middle of the action, turns to the camera, beats its chest, then walks back out of the scene. When this is shown to people who've never heard of it, about half report seeing the "gorilla", and half didn't see it. 

This is called Inattentional Blindness. It is used by stage magicians, whose actions and talk in the early part of a performance direct the audience's attention away from what is happening right in front of them. A magician can't be content with misdirecting half of the audience; the goal is 100%. This is often achieved!

But what if someone wants to vanish from plain sight, without benefit of a flash of fire or smoke (the usual prop for a vanishing act)? Optical science researcher Gregory J. Gbur might have something to say about that in his book Invisibility: The History and Science of How Not to be Seen.

Much of the history Dr. Gbur draws upon is found in science fiction. It seems that every scientific discovery about optics and related fields was fodder for science fiction writers to imagine how someone could be made invisible. This cover image from a February 1921 issue of Science and Invention (edited and mostly written by Hugo Gernsback, later to write lots of science fiction and edit Amazing Stories) shows the rays from something similar to an X-ray machine making part of this woman invisible.

I looked for this cover image online and found an archive of S&I issues. However, the issues were apparently produced with various covers for different regions, and the version in the archive had a cover touting a different application of X-rays. However, the article on page 1074, referred to in the cover shown above, does discuss whether X-rays or something like them can be used to provide invisibility, and also shows another way that structures inside the body may be seen.

Here the "transparascope" makes certain tissues transparent, allowing the viewing of others. IRL, the development of CT scanning and MRI scanning, fifty-odd years later, were required to achieve such views. The invisibility beam of the cover image has so far proved elusive.

Invisibility sits in the broader realm of "how not to be seen." The book shows in detail that the technologies that have been developed to hide or cloak objects can only work perfectly over very narrow ranges of light wavelength (and by analogy, waves in water and other media), and usually a narrow range of viewing angle. Is perfection needed? That depends…

In the late 1960's I worked for a defense contracting company, mainly as an optical technician. I was loaned to a related project as an experimental subject. The team was gathering data on the limits of human vision, detecting the contrast between a lighted object in the sky (an aircraft) and the sky. This was the Vietnam War era. 

The experimental setup was a room with one wall covered with a screen on which versions of "sky blue" were projected. At the center was a hole and various targets were set in this hole. They simulated the look of a dark or darkish object in the sky, and each target had several lighted spots, little lamps. The lamps' color and brightness could be adjusted. I was instructed to tell what I could see. The first day I was there, the background target was black, and the lamps were small and bright. The targets had differing numbers of lamps and their brightness would be adjusted to reduce the visibility of the overall target. This tested acuteness of vision; how many lamps on a certain size target would "fuzz together" and seem to illuminate its entire area? 

For most people, the "fuzz" angle is 1/60th of a degree. When you look up at a Boeing 737 at 30,000 ft elevation, its length of about 130 feet means it subtends and angle of about 1/4 degree. It would take two rows of 25 lamps along the fuselage, and at least 10 lamps, or 10 pairs of lamps, along each wing, to counter-illuminate it and reduce its visibility. That's a lot. A B-52 bomber is 20 feet longer and its engines are huge, like misplaced chucks of fuselage.

On another day, the target's background color was a blue color somewhat darker than the "sky". The target had the optimum size and spacing of lamps to seem of more-or-less uniform brightness, and the brightness and color of the lamps were varied. This tested our color acuity; how far could the colorimetry of the target-lamp combination vary to remain invisible or minimally visible?

This image simulates the second kind of target-lamp combination If you look at this image from a sufficient distance, the simulated target will nearly disappear, or for you it may vanish completely. This works best if you either take off your glasses or look through reading lenses, to defocus the image.

The average color and brightness of the simulated target are a close match to the surrounding sky-blue color. Thus, if an aircraft's belly is painted a medium blue, and a sufficient number of lamps are mounted on it and controlled by an upward-looking system, it can seem to vanish against the sky as long as it is high enough that the angular distances between the lamps is smaller than the circle of confusion (1/60th degree) of the eyes of an observer below.

This set of letter-targets is similar to a different test. Each letter has a little different color and brightness than the "sky". The 5 letters here make up the word "ROAST", but are not in order. For this test the sky color would be adjusted to see which letters were least and most visible. In both panels you will probably see three or four letters, but one or two that are not seen in one panel will be seen in the other.

In the end, it was all for nought. The sky is too variable, and human vision is also variable. There are three kinds of color blindness, and six kinds of "anomalous color vision"; any of these renders visible a target that "normal" eyes cannot see. It's kind of the opposite of those color-blindness tests with pastel "bubbles" that show the letter K to "normies" but the letter G to most color blind people. Also, wearing polarized glasses changes the perceived color of the sky, and tilting your head makes a dramatic difference in the color. Anyone with shades on would see the aircraft easily.

A further drawback of these tests was that no Asians' eyes were tested. In my regular job at the time, we were developing an infrared light source that Asians could not see. The near-infrared lamps used for night vision goggles and SniperScopes were invisible to Anglos, but quite visible to the Vietnamese. Several American snipers lost their lives when they turned on their SniperScope and a bullet came back instantly. What eventually worked was not a different light source but hypersensitive image amplification, the "starlight scope".

My wife is Asian. Certain items that look green to me she tells me are blue. Away from the green-blue boundary, she and I agree on the colors of objects.

The later chapters of Invisibility describe experiments and simulations that could lead to effective cloaking. There is even an appendix that shows a home tinkerer how to make a couple of kinds of visual cloaks that work in at least one direction. Full-surround cloaking is still out of reach, but who knows?

This book earns my "fun book of the year" award. Well written and very informative.