Airport Passenger Screening Using Millimeter Wave Machines Compliance with Guidelines (2017) / Chapter Skim
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Appendix C: Radiation Physics Relevant to Advanced Imaging Technology
Appendix C: Radiation Physics Relevant to Advanced Imaging Technology
Pages 144-157
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From page 144... ...
INTRODUCTION TO ELECTROMAGNETIC RADIATION There are many different categories of electromagnetic waves, including X rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves. These are generally characterized by their energy, wavelength, and frequency.
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From page 145... ...
This is true when the wave is propagating in empty space and also when the wave is propagating inside most uniform materials. Electromagnetic waves move through space at a certain velocity and, therefore, transport energy through space at this velocity.
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From page 146... ...
This means that, if the detector is oriented to detect vertically polarized radiation, then it is essentially blind to horizontally polarized radiation. A complete characterization of the electromagnetic field at a certain point in space requires knowledge of both the horizontal and vertical field components.
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From page 147... ...
, microwave waves, and millimeter waves are often used interchangeably unless explicitly stated. A second important parameter used to describe the properties of electromag FIGURE C.2 The electromagnetic spectrum with millimeter waves falling between infrared and microwaves in the mm regime.
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From page 148... ...
, referenced to a value of 1 mW (one thousandth of 2 IEEE, 2005, "IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Fre quency Electromagnetic Fields, 3 kHz to 300 GHz," IEEE Std C95.1™-2005.
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From page 149... ...
This is even more true in the millimeter wave range of the spectrum, where the energy of a single photon is even smaller. For a millimeter wave electromagnetic field of −40 dBm/cm2 at a frequency of 30 GHz, the number of photons per second per square centimeter is about 5 × 1015 -- a very large number, even for this exceedingly weak (low-power)
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From page 150... ...
Because all objects emit blackbody radiation at all times, we are continuously bathed in this infrared radiation for every moment of our lives. For example, a room temperature black body emits about 1 µW (10−6 W)
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From page 151... ...
In general, the velocity ratio depends on the frequency of the light, which means that the fractions reflected and transmitted also do. However, this dependence is usually fairly weak within the range of frequencies visible to human eyes, so we do not often see materials that exhibit visible color changes as a result of this effect.
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From page 152... ...
Figure C.3 shows a representation of reflection, transmission, and absorption for a thin slab of material, as discussed above. As noted, the absorption strength of liquid water is strong at frequencies in the microwave and millimeter wave ranges.
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From page 153... ...
Generally, this heating effect is by far the dominant mechanism for transferring energy from millimeter waves to tissue -- energy that could lead to health implications for exposure to microwaves and millimeter waves; no other energy transfer mechanisms for the human body have been identified in this region of wavelengths. In contrast, exposure to shorter wavelengths (e.g., ultraviolet radiation or X rays)
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From page 154... ...
This is why, when considering the health and safety impli cations of exposure to microwaves or millimeter waves, the overriding concern is that of heating. ANTENNAS AND WAVEGUIDES The above discussion of the interaction of electromagnetic waves with matter is generally applicable to materials in which all of the electrons are bound to atoms or are participating in chemical bonds and are, therefore, not free to move far from their starting points.
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From page 155... ...
The horn antenna shown at left is similar to the antennas used in the L3 AIT system to transmit millimeter waves toward the target. SOURCE: Images created using software from openEMS contributors, "Tutorials: Conical Horn Antenna," openEMS, http://openems.de/index.php?
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From page 156... ...
At lower frequencies, in the millimeter wave and microwave ranges, coaxial cables and small metal pipes are most commonly used. These are conductors that have been carefully designed in a configuration to maxi mize the transmission of electromagnetic radiation of a given frequency (or a given range of frequencies)
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From page 157... ...
Appendix C 157 than the energy generated by the electronic amplifier used as the source. This energy loss cannot be avoided and must be accounted for in the system design to guarantee that the signals emitted from the horn antenna are large enough to be useful in image formation.
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