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Hand Me Some Information: Using Finger Heat for Signals

Photo: A theremin, by Carro Willis, via Flickr (cropped).

In research reminiscent of power emitted from fingers in sci-fi movies, Chinese scientists have made use of intelligent design using the heat naturally emitted by human hands. 

The human body runs on about 100 watts of energy. Some of that energy radiates in the infrared, what we know as “body heat.” It can be easily seen in infrared images, and includes infrared energy emitted by the hands and fingers. Sign language already confirms that information can be communicated by hands in space. The hearing-impaired have learned how to interpret the configurations, but who has interpreted the heat? Though invisible to the human eye, IR is a form of light. Can electromagnetic energy from fingertips convey information as well?

Handy Music

In 1919, a 23-year-old cellist, scientist and Russian spy named Leon Theremin found by accident that moving his hands near a gas meter produced squealing sounds. After amusing his colleagues with the ability to make music by moving his hands in the air, he created the theremin, an electronic instrument played literally “by hand” without touching anything. It works by hand motions interrupting electric fields generated by two antennas that control pitch and volume. Its ethereal sounds fascinated listeners as the player manipulated his or her hands to create very expressive music. 

Modern composers have written classical works for theremin, and producers have put its ghostly sounds to effective use in early sci-fi movies. But for most conductors, the instrument was a non-serious fad like the glass harmonica and it is rarely heard on classical radio stations. Once in a while it makes an appearance. There are societies for theremin enthusiasts, and the instruments are still sold by the Moog company begun by Robert Moog, a pioneer of electronic synthesizers, who got his start selling theremins. They cost about $350 on Amazon. This is one way the hand’s natural emissions convey information.

Handy Information

Now, a team of eight scientists from Shanghai Jiao Tong University in China has tested the human hand’s IR light as a remote informational device in multiple applications. Their work is published by Shun An et al., “Human hand as a powerless and multiplexed infrared light source for information decryption and complex signal generation,” in PNAS. It is not really powerless; the infrared emissions are powered by the food we eat. Assuming subjects did not skip meals to the point of stopping their emissions, though, hands are handy IR generators that do not require batteries or external power supplies. The researchers know that artificial IR has found many applications in engineering design. Why not use natural IR light? One of the graphics in the paper even shows IR light beaming out of the fingers.

Among all the natural light sources, biological systems emit IR radiation spontaneously and represent an interesting class of natural IR light sources. Recently, several studies are reported on the IR radiation from biological systems, such as the effective thermal management of various insects [butterflies, ants, and beetles] through the regulation of IR radiation emitted from these biological systems. Compared to engineered light sources or other nonbiological natural light sources, biological light sources can bring the advantages associated with the biological systems into engineered systems: chemical sustainability, structural multifunctionality, and intelligent controllability. [Emphasis added]

How Cool Is That?

Because the human hand is so flexible and controllable by intelligent beings, its emissions in the IR range offer new ways to engineer systems for a variety of applications.

In this work, we demonstrate that the human hand is not just a natural and powerless IR light source, but also a multiplexed light source with each finger serving as an independent light source. The relative position and number of light sources are controllable and adjustable at will by changing the hand gestures. Based on this mechanism, we show that the human hand can be integrated into various engineered functional systems…. The integration of the human hand into these engineered functional systems can not only enhance sustainabilitybut also increase the level of intelligence and controllability of these systems due to the direct involvement of human brains in the systems. 

Hands are natural IR emitters, and they run on hamburgers! The hand’s emissions, the team found, are very transmittable through air and fit within the detection window of IR receivers. The hand has additional advantages over electronic emitters:

The human hand not only can serve as a single integrated IR light source but also can serve as a multiplexed IR light source due to its unique composition. Each hand consists of five fingers, each of which can serve as an independent IR light source and can be controlled and grouped at will. With such integrated and relatively independent components, the human hand can also serve as a flexible and multiplexed IR light source.


One interesting application involves encryption. The team created a QR code pattern that is invisible to the unaided eye, composed of materials with different IR reflectivities. When the reader puts his hand over it, IR light from the hand adds 25 percent more contrast, and the pattern becomes visible, even at different temperatures. A short video shows an invisible image appearing “by hand,” as if someone turned on the lights.

This trick would work when no one is looking for a message. It’s a form of steganography, or sending hidden messages in plain sight. By varying the reflective material, images can even be made in different colors. One practical use is watermarking. “Such capability in multicolor coding provides additional levels of information encryption/decryption and makes unauthorized decoding even more challenging than the single-color coding.” 

Pressing a fingerprint into the substrate can make the pattern unclonable, offering additional protection from theft. This also allows for unbreakable encryption. “With the further manipulation of the composition and microstructures of the coding patterns,” the team says, “the unclonable features can be further enhanced.”

Things get really interesting when the fingers are used as independent (multiplexed) IR light sources. Finger gestures over diffraction gratings can trigger the formation of characters with an IR detector. 

Compared to the decryption using the whole hand as the single light source, the freedom of using any of the five fingers of the hand will offer much more space in information encryption/decryption, and only a specific gesture can serve as the matching key to unlock the encrypted information.

A wrong finger position would fail to decrypt the message. This increases the security of the encryption/decryption key.

New Sign Language

The authors demonstrated doing addition with “finger light” reflected off three diffraction gratings of different periods and orientations. The reflected patterns were then read by a detector and interpretated on a 2×3 matrix with dominos-like arrangements representing characters. Each character is made by moving the relative positions of the thumb and fingers. The hand is “a complex articulated object with more than 20 degrees of freedom,” they note.

How might this be used? Sign language recognition is sometimes compromised by the ambient lighting in the room used by the signer, or by the complexity of the background. Using finger IR illumination on diffraction gratings, interpreted by a reader, the authors were able to signal all 26 letters using only 3 fingers — a much simpler way than the American manual alphabet that relies on up to 9 positions for each of the 5 fingers. The new method was also easier to distinguish from the background, as it produced a distinct Braille-like 2×3 quad that can be interpreted by a device to spell out letters without having to watch the complex finger movements of the signer. The method works in dim light, too. In other ways, the team demonstrated the superior differentiation capability of the IR method.

Flexible Hands For Flexible Information

In general, hand-emitted IR creates opportunities for flexible communication, because “The relative position and number of light sources are controllable and adjustable at will by changing the hand gestures.” It works for encryption, for signal detection and recognition, and for sign language. The method depends on a difference between body temperature and ambient temperature, but a quick rubbing of the hands together takes care of that, they point out. But do users to carry diffraction gratings around? In the future, authors say, the “detector and gratings can possibly be integrated into a wearable device and worn on the wrist to fix the position between gratings and the hand and enhance the accuracy of finger positioning relative to the gratings.”

A Fascinating Nexus 

This paper brings a fascinating nexus between human body design and information design. Because hand conformations are integrated with human intelligence, the possibilities seem endless for sustainable and controllable systems. But why stop with hands? “The findings may also help further open up the possibility of using not only the human hand but also potentially other human components into functional systems and processes.”