# Translations:Frequency spectrum/5/en

The Fourier transform need not apply only to frequency in cycles per second and time in seconds. The Fourier transform also can be used for spatial measurements. For example, consider the case of a coherent spatial distribution across a receiver aperture. In such a case, a relationship exists between the transmission pattern of the system in terms of the sine of the angle of projection and the distribution of the field along the aperture of the system (Robinson, 1967^{[1]}). The temporal frequency (i.e., the number of cycles per unit time) is the Fourier dual for the time (i.e., the number of time units). In the same way, the spatial frequency, or wavenumber, is the Fourier dual for the space variable. The wavenumber gives the number of cycles per unit distance. In this chapter, we deal only with time and temporal frequency, so we shorten the expression *temporal frequency spectrum* to *frequency spectrum*.

- ↑ Robinson, E. A., 1967, Statistical communication and detection: Hafner Publishing Co.