Measurements of electromagnetic energy flow are an integral part of solving various energy-budget and optical-characterization problems. For example, the physical state of a cloud of water droplets or ice crystals in the terrestrial atmosphere can be affected by an imbalance between the incoming and outgoing electromagnetic energy, while measurements of specific manifestations of electromagnetic energy flow with a suitable device can potentially be analyzed to infer useful information about the cloud. Conceptually similar problems are encountered in many other areas of science and engineering. It is therefore very important to understand clearly what specific measurement is afforded by an optical instrument and how to model this measurement theoretically.

Let us recall, for example, the energy-budget problem for a macroscopic volume element of an idealized liquid-water cloud discussed in Section 1.4. Suppose that we have at our disposal a Poynting-meter, i.e., a device that can determine both the direction and the absolute value of the time-averaged local Poynting vector. Then measuring ≪S(r,t≫ at a sufficiently representative number of points densely distributed over the boundary ΔS would enable one to evaluate the integral in Eq. (1.12) numerically and thereby quantify the degree of electromagnetic energy imbalance of the volume element ΔV.

Unfortunately, none of the existing photometers can, strictly speaking, be considered a Poynting-meter.