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The term "anomaly" in the study of grating diffraction generally refers to an abrupt change in the reflectivity of a given order within a relatively short range of wavelengths or angles of incidence. Much attention has been paid in the past to anomalies found for TM polarized incident light (magnetic field parallel to the grooves, the plane of incidence being perpendicular to the rulling direction): they are correctly reproduced by models based on the electromagnetic theory and different kinds of mechanism that originate them have been identified. For the opposite fundamental polarization case, TE (electric field parallel to the grooves), the corresponding anomalous behaviour does not appear to have been as thoroughly studied and there is much confusion in the existent literature as to what exactly is termed an anomaly, the prediction of the positions for which they may occur and fundamentally the physical mechanisms that give rise to them. In particular, there is a lack of experimental data obtained from reasonably well characterized gratings. This information is essential since anomalies for TE polarized light have been reported for deep, highly conducting gratings, a situation in which the calculation of the diffracted efficiencies is difficult and numerical results are not as reliable as in other regimes.
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