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Coherence and interference play crucial roles in emission and absorption ofphotons to and from large systems with many atoms. Confusion has arisen becausenuclear X-ray physicists and atomic quantum-optics physicists do not understandone another's individual descriptions of related phenomena. Basic physics samefor all wave lengths from optical to nuclear gamma ray photons. But differentlanguages are used to describe this physics in different domains. Crucialparameters vary over many orders of magnitude and what is intuitive orcounterintuitive varies widely. Differences in parameters arising fromdifferences between coherent emission effects in different domains produce verydifferent results. Unified general treatment of the entire photon spectrummakes basic physics intelligible to all. In the optical region the meandistance between the scattering atoms is much longer than the photon wavelength, Dicke superradiant scattering is isotropic and multiple scattering,Fano couplings are important and the lifetimes of intermediate stats aresufficiently short to be negligible. In X-ray scattering the mean distancebetween atoms is comparable to the photon wave length, Dicke superradiance isconcentrated in a forward peak, multiple scattering and Fano effects arenegligible, lifetimes are measurably long, speedup shortening the lifetime isimportant and most of the radiation is not elastically scattered but lost toabsorption. Explicit calculations for a one-dimensional array shows the greatdifference between the case where the photon wave length is much shorter orcomparable to the distance between nearest neighbors. A full investigation ofthe angular distribution and speedup of the intensity for two andthree-dimensional scatterers give very different results for the two cases.

Directed Spontaneous Emission from $N$-atom Extended Ensemble