The understanding of physical principles is based on matching physical observations with mathematical constructs. This matching is the remit of physics and exploitation of this for wealth creation is the purpose of engineering.
The interaction of millimetre wave radiation with any media is dependent on the physical structure of the media, collectively described by its refractive index. At the fundamental level millimetre wave radiation is carried by the photon particle, which in sufficient numbers constitutes an electromagnetic wave. The photons interact with matter by exchanging energy with charged particles in the media through various mechanisms.
The development of a sensor to exploit the property of a media is dependent on measured signals being larger then the noise.
Signals may be scalar or vector quantities and if spatially varying can form an image or tensor. Harmonically varying signals, as in the case of travelling (millimetre) waves, may be sampled to obtain phase and amplitude or detected to extract the intensity. Signals containing frequency domain information may be analysed in a spectrometer. Multiple signals can be combined interferometrically to provide highly sensitive measurement capabilities.
True noise is purely random in nature (originating from thermal fluctuations and/or finite number of charge carriers in the receiver system) and may be minimised by signal integration over a period of time.