Millimeter-wave (mm-wave) imaging techniques are already a popular solution for imaging through dust and fog. While mm-wave offers excellent penetration to dust when compared with infrared or optical sensing, the longer wavelengths create many problems associated with the specular response of surfaces at mm-wave. Generally, at mm-wave, the geometry and orientation of the target object has a larger influence on captured contrast than material properties by several orders of magnitude. While these effects can be somewhat mitigated with a radar imager, there is still a large contrast dependence on beam-target angle, and images are still entirely derived from geometry instead of material compositions.
Active radiometers enable mm-wave remote imaging based on material compositions instead of geometry by deriving contrast from thermal and thermodynamic properties of the object being remotely observed. This added capability is extremely advantageous in imaging through a dust storm or heavy fog. Active radiometry is essentially a remote sensing approach where the thermal and thermodynamic properties of an object are evaluated by remotely heating the target with some form of directed energy (also a mm-wave source), while radiometry is performed to monitor spatially separated temperatures. The excitation used to raise and lower the temperature of the remote object is not continuous excitation, but instead is modulated with various waveforms so that transient (thermodynamic) properties of the object can be evaluated remotely.
Heat capacity is one of the simplest thermal properties and describes the rise in temperature for a given quantity of energy absorbed. This is similar to “specific heat capacity,” but does not factor the mass term. By simply transmitting continuous mm-wave power into the object and measuring the temperature rise with the observing radiometer, the specific heat can be estimated. The radiometer and exciter need to operate at quite different frequency bands so that interference does not occur, and the coupling between the exciter and radiometer is dominated by thermal transfer, not electromagnetic (harmonics and spurs) coupling. For this sensing approach, the exciter is located at the lower 30 to 40 GHz Ka band where high-power (>10 W) amplifiers are available to deliver useful quantities of power for heating. The radiometer will operate in the 75 to 110 GHz W-band range to provide better resolution as the setup optics will be less diffracted at shorter wavelengths.
Thermal conductance describes how quickly heat is transmitted through a material. This can be remotely measured by pulsing the exciter at a frequency comparable to the thermal constants of the object under observation (several Hz for most materials). By focusing radiometers at offset positions, the propagation rate of the exciter’s energy through the object material can be directly estimated from the time difference of the heat pulses at each radiometer. Again, with further development of raster optics for this approach, the thermal conductance of the remote object can be contrasted to provide a raster image of a remote target based on its thermal conductance at each point in the image.