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This paper proposes a three-layer electromagnetics model for investigating the interaction of millimetre wave radiation with human skin and clothing as a means to improve security screening technology. Progress in this spectral band is only possible with a deep understanding of the interaction of this radiation with the human body, the clothing and threat items; the proposed model in this paper is implemented to provide us with this information. The model consists of three layers namely, 1) air (a semi-infinite layer and lossless layer), 2) clothing layer (finite thickness layer ranging from 0.0 mm-1.2 mm), and 3) skin layer (semi-infinite layer). Three types of clothing are investigated by the model, these being: 1) Fleece, 2) Denim, and 3) Leather. Simulation results indicate that a clothing layer, in direct contact with human skin, enhances the transmission of radiation between air and skin. This is due to the clothing layer acting as an impedance matching transformer. The simulation results indicate that the power transmission coefficient varies with the frequency and thickness of the clothing layer. For the Fleece, the power transmission coefficient was found to increase fractionally, ranging from 0.05 to 0.112 at 90 GHz when the thickness of the fleece varied between 0.5mm and 1.2 mm. For the Denim, the difference in the transmission coefficient was found to increase with the thickness of the Denim, up to a factor of 0.095, when the thickness of the Denim layer varied from 0.5 mm to 1.2 mm at 60 GHz. For the beige leather, the variation in the transmission coefficient between different thicknesses was substantial and it could be up ~10.0% when the thickness of the material varied from 0.5 mm to 1.0 mm at 60 GHz. However, when the thickness of the leather was 1.2 mm, the maximum value of the transmission coefficient of 0.83 was achieved at around 45 GHz. Simulation results obtained from the three-layer model presented in this paper are in good agreement with the measurements reported in the literature reviews at specific frequencies. The importance of having such a validated model is that security screening system parameters can be optimised with a minimal amount of costly trials and experimentation.
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