Light diffraction and the principle of causality

Diffraction formulas for non-monochromatic light are written in the formalism of linear response theory to stress the role of the causality principle. The analysis yields a formula containing a space integral of a time convolution. An example shows when the order of space and time integrations can be exchanged: this is possible only at the end of the optical transient. The system output can then be written as a time function of the space Fresnel or Fourier transform of the input. The white light interferometry experiments (channeled spectra) support the model: they result from it when transients are shorter than the average pulse duration.