MadSci Network: Physics

Re: Can fiber optics be used to make an object invisible?

Date: Wed Apr 15 10:40:13 1998
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Physics
ID: 892483790.Ph


There are at least four ways to attempt to make objects appear to be 
invisible to an observer: 1) by blocking light from illuminating an object 
(e.g. darkness); 2)  by blocking or redirecting the light reflected from an 
object away from an observer 3) by redirecting the light reflected from an 
object to make it appear to the observer to be in a different location than 
where it actually is located (ghost objects) and 4) to make the object 
blend into the background (camouflage). 

Magicians and entertainment parks such as Disneyland make objects appear 
and disappear in their displays by using these four techniques. Typical 
optical components used at Disneyland include white light and laser 
projectors, two way mirrors (e.g. optical beam splitters), lenses, 
holograms and high quality mirrors.

The military make objects invisible by painting them to blend with the 
background (camouflage) or to make them as mirror like as possible to not 
reflect energy back to the observer. The mirror like surfaces on the F-117 
Night Hawk stealth fighter surface have been  faceted to make the aircraft 
invisible to microwave radar (microwave mirrors) and it is colored black to 
make it hard to see optically at night (camouflage); also, the hot portions 
of the engines and exhaust are designed to block the infrared energy from 
reaching observers on the ground. The Night Hawk web pages are at the 
following URL:

There is a configuration of planar mirrors called the K-mirror with the 
edges of 3 planar mirrors forming a K shape. In my illustration below a 
light ray comes from the left (arrow) and reflects down off tilted mirror 
M1 (not shown at the correct 22.5 degree tilt angle) and then the ray 
reflects off of the bottom mirror M3 back up to the tilted mirror M2 (also 
not shown at the correct tilt angle) and the ray continues to the right 
toward the observer (O). Thus the light from the backgrown is bent around 
the object hidden at point (X). The most difficult part of the illusion is 
to make the observer not see the mirror edges. Magicians are good at this 
type of detail.

               M1      M2
                \     /    
    ---------->  \ X / --------------->  O)
                  \ /
            M3 __________

Fiber optics could be used in place of the K mirror. They offer a  very 
transparent, flexible, transmission line for light which could also bend 
light around object (X). Fiber bundles containing thousands of fibers 
called endoscopes have been designed for medical applications. Endoscopes 
transfer images from the inside of the human body to a physician for 
diagnostic and surgical applications. Even though fiber optic endoscopes 
are only about a meter long they cost thousands of dollars. However, even 
at the high cost their unique capabilities are finding widespread use in 

All of the optical functions that can be performed by beam splitters, 
mirrors and lenses can also be implemented in fiber optics; however, they 
are considerably more expensive and often are not as efficient as 
conventional optical components. The flexibility of fibers and their small 
size do offer applications such as endoscopes and fiber optic 
communications links that conventional optics cannot perform. Once again 
hiding the surfaces of the fibers from view would be very challenging.

The third type of invisibility presented at the beginning of this note uses 
fiber optics to confuse optical and microwave radars with ghost targets. A 
radar or LIDAR (light radar) transmits a beam of light pulses out many 
kilometers (miles) and looks for reflections from targets. The distance and 
speed of the target can be accurately measured by measuring the time it 
takes for a light pulse to travel to a target and to be reflected back to 
the LIDAR.
If several miles of optical fiber are coiled up in a small volume about the 
size of a large coffee mug, a compact optical "delay line" is formed. If 
the target has a lens that focuses a LIDAR pulse into a fiber optic delay 
line, the light pulse travels several extra miles inside the coffee cup 
before it is directed from the end of the delay line back toward the LIDAR. 
To the LIDAR this delayed light pulse looks like a large target  several 
miles farther away from where the actual target is located (a ghost target) 
and the strength of the ghost target is much greater than the light 
reflected from the real target. If several  fiber optic taps are made along 
the delay line  at different distances, the LIDAR will detect the returns 
from each of the taps producing several ghost targets (a ghost formation) 
and the LIDAR will have a difficult time determining which are  real and 
which are ghost  targets. There are many other variations of this ghost 
target technique that are used in both LIDAR and microwave radar 
countermeasures; however, they use all use the same basic principles.

Best regards, your Mad Scientist

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