MadSci Network: Engineering

Re: Are the infrared signatures emitted by the Human Body unique?

Date: Sun Feb 25 15:16:15 2007
Posted By: Phillip Henry, Staff, Physics, Lockheed Martin & Florida Tech
Area of science: Engineering
ID: 1171797223.Eg

Excellent question Adarsh. Infrared radiation are photons with 
wavelengths that vary from a little less than 1 micron to about 1 
millimeter. All objects with a temperature above absolute zero emit these 
photons as a function of the temperature and material characteristics.

Can the infrared signature of a human body be person-specific? First what 
is infrared signature? Its the characteristic irradiance of an object in 
the infrared spectrum - and may include both emission and reflection. Is 
it unique? Its not certain that the infrared signature would be 100% 
unique to an individual. It is however likely to be quite variable. On 
most humans, that infrared signature will be modified by attire. 
Different fabrics and clothing styles will modify/transmit the internal 
heat of the body differently. For example, this will sometimes allow the 
novelty of discriminating a person's gender by their heat signature. 
However, that would not be 100% reliable. 

Since photons are just light at a lower frequency, the person's shape 
would be resolved to within the resolution limits of the sensor system. 
Furthermore, the person's particular state of health, skin and 
circulatory system as well as that person's metabolic and emotional state 
as well as their physiologic response to external conditions (extreme 
cold, for example) will cause variations in thermal signature which could 
be unique. However, it would seem unlikely that these minor differences 
would be consistent enough to provide a unique identification test for 
the sensor. So to the extent that a person's general shape and general 
features are unique in the test group, one can descriminate individuals. 
Still, there are infrared differences between humans and other species. 
And certain physiological conditions can manifest themselves in 
observable variations of a person's thermal emission. Thus the use of 
themal imaging for medical diagnosis is a developing field.

So it is doubtful the human infrared signature would be consistently 
unique enough to differentiate between individuals in general. Perhaps 
where some external information is added, such as limiting the group out 
of which the person would need to be indentified and by adding a priori 
data (such as knowledge of the person's attire), it might be possible. 
But that would be very limiting. Differentiating between humans and other 
animals and other objects in the scene is more practical as is use of 
thermal imaging where a physiologic response could be detected (say in 
conjunction with a polygraph test). And use of infrared imaging in 
medicine is an emerging technology which offers promise.

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