These masks are two-dimensional analogues of fibrous molecular structures, such as microtubules and the helical protein collagen. The masks that correspond most closely to a crystal structure are those with a periodically repeating array of apertures that in principle could cover the whole plane. The simplest example is a square lattice, with a pinhole at every vertex. By this stage in my work I thought I had a sense of how diffraction works, and so I thought I knew what the output would look like. Figure 5 was not what I expected.
Crystallographers call it the finite-size effect.
In Figure 5 the small-scale features are artifacts of the boundary. Irregularities or imperfections in a crystal can be modeled by randomly displacing the pinholes in a lattice mask. In the resulting diffraction pattern the reciprocal lattice is still visible, but it is enveloped in a gray fog or dust cloud that indicates the essentially continuous spectrum of distances between apertures in the perturbed mask. The limiting case of this process is a mask made up of apertures sprinkled totally at random, which produces a diffraction pattern consisting entirely of dust. In the random mask, apertures can fall anywhere, but in a glass no two atoms can approach any closer than the sum of their radii.