Measurement of electromagnetic energy flow through a sparse particulate medium: A perspective

First-principle analysis of the functional design of a well-collimated radiometer (WCR) reveals that in general, this instrument does not record the instantaneous directional flow of electromagnetic energy. Only in special cases can a sequence of measurements with a WCR yield the magnitude and direction of the local time-averaged Poynting vector. Our analysis demonstrates that it is imperative to clearly formulate the physical nature of the actual measurement afforded by a directional radiometer rather than presume desirable measurement capabilities. Only then can the directional radiometer be considered a legitimate part of physically based remote sensing and radiation-budget applications. We also emphasize the need for a better understanding of the nature of measurements with panoramic radiometers. -- Highlights: ► Functional design of well-collimated radiometers is analyzed based on first principles. ► It is shown that WCRs do not record the instantaneous directional flow of electromagnetic energy. ► Only in special cases can a WCR yield the local time-averaged Poynting vector. ► Need for better understanding of measurements with panoramic radiometers is emphasized