On a Temporal‐Spatial Radiation Functional and its Measurement

A so‐called radiation functional of the electric field‐strength is introduced depending on time and space and on the temporal and spatial frequencies, which is applicable to the most general forms of radiation (e.g. non‐stationary, inhomogeneous, partially coherent). The time‐dependent spectrum and the specific intensity of radiation can be deduced as special forms of the radiation functional. The dispersion relations are incorporated. The measurement of the radiation functional by means of a linear optical system followed by square‐law detectors is dealt with; for quasi‐stationary and quasi‐homogeneous radiation the measurable function is proportional to the radiation functional. The propagation of the radiation functional in space is investigated, and, as a simple example, the diffraction by a grating of nonstationary radiation is dealt with.