Friedrich-Alexander-Universität Erlangen-Nürnberg

Imaging and material characterization with millimeter waves

Imaging and material characterization with millimeter waves

Electromagnetic waves with wavelengths in the range of millimeters possess properties, which make them attractive for a number of technical applications. They offer a good compromise between penetration depth into certain materials and lateral resolution. Hence, they represent an interesting frequency band for the determination of both the geometrical structure and the material parameters of objects. Another advantage is their non-ionizing nature. Imaging systems employing millimeter waves can be found in wide fields of use, such as non-destructive testing, security and medical applications.

The imaging process is exemplarily sketched in the image to the right. An array of coherent millimeter wave transmitter (Tx) and receiver (Rx) elements is used to characterize the object-to-image. One by one, selected elements in the array act as illuminators for the object, while all other elements measure the scattered field for both amplitude and phase. Due to the fact that the scattered field is subject to diffraction effects during its propagation from the object to the array, the measured field data has to be postprocessed by the computing unit in terms of a digital focussing. Furthermore, aperture synthesis algorithms (SAR) as well as digital filters have to be applied with respect to resolution, accuracy and computation speed considerations. The final image represents the reflection properties of the object and allows for the detection of its shape. Defects can be identified by intensity and phase variations.

More detailed information about the object of interest can be extracted with a fully polarimetric imaging system, i.e. a system transmitting and receiving two orthogonal types of polarization. In this way, the full electromagnetic scattering properties of the object can be determined and the 3D space resolved material properties, for example the dielectric constant, can be derived.

The box with different objects (left) was sealed (middle) and imaged at a frequency of 80 GHz (wavelength ~ 3.7 mm).The resulting image (right) allows to “look” through the package material with non-ionizing millimeter waves.


  1. J. Adametz, F. Gumbmann and L.-P. Schmidt, “Inherent Resolution Limit Analysis for Millimeter-Wave Indirect Holographic Imaging,” in Proceedings of the German Microwave Conference, 2011
  2. H. P. Tran, F. Gumbmann, J. Weinzierl, and L.-P. Schmidt, “A Fast Scanning W-Band System for Advanced Millimetre-Wave Short Range Imaging Applications,” in Proceedings of the 3rd European Radar Conference, Sept. 2006, pp. 146–149.
  3. F. Gumbmann, H. P. Tran, J. Weinzierl, and L.-P. Schmidt, “Multistatic Short Range Ka-Band Imaging System,” in Proceedings of the German Microwave Conference, 2009
  4. S. Ahmed, A. Schiessl, and L.-P. Schmidt, “Near Field mm-Wave Imaging with Multistatic Sparse 2D-Arrays,” in Proceedings of the German Microwave Conference, 2009
  5. S. Ahmed, A. Schiessl, and L.-P. Schmidt, “A Novel Fully Electronic Active Real-Time Imager based on a Planar Multistatic Sparse Array”, IEEE Transactions on Microwave Theory and Techniques, Vol. 59, No. 12, Dec. 2011
  6. F. Gumbmann, and L.-P. Schmidt, „Millimeter-Wave Imaging With Optimized Sparse Periodic Array for Short-Range Applications”, IEEE Transactions on Geoscience and Remote Sensing, Vol. 49, No.10, Oct. 2011


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Optical Metrology
Optical Material Processing
Optics in Medicine
Optics in Communication and Information Technology
Optical Materials and Systems
and Computational Optics.