Moirés between repetitive, non-periodic layers
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In the preceding chapters we basically restricted ourselves to the study of periodic layers and their superpositions. However, moiré effects are not only limited to the superposition of periodic layers, and, indeed, they often occur between other types of repetitive structures. Typical examples are the moiré effects which may be generated between families of curvilinear gratings (such as concentric circles), between straight gratings with variable spacings, or between any other repetitive geometric structures. Another example, that we have already implicitly used several times in previous chapters, is that of screen gradations, i.e., dot screens having a constant frequency but varying dot sizes and shapes (like in Figs. 4.1, 4.4, etc.).