# Explaining the diffraction pattern of a lightbulb filament

This really nice video from the Royal Institution channel on YouTube has a gorgeous shot of the diffraction pattern caused in laser-pointer light by the helically-coiled filament of a light bulb:

There's multiple interesting features going on here and I would be interested in a more in-depth explanation of what causes them, that goes beyond the simple "because diffraction". As explained in the video, a helical coil can be seen roughly as two collections of line segments, which explains the cross-like structure, but there is also

• light on the outside of the cross but not inside the narrower angle,
• a definite double-period structure on the lower-left arm of the cross,
• a bi-periodic structure in the light on the obtuse angle of the cross,
• an offset between the cross arms at the vertex, I should think, and
• a global ring structure superposed on everything,

and that's just from a first glance. What causes these features, and what can they say about the structure of the filament?

(If that's too complicated, I would be happy with a good reference to a readable introduction to the diffraction patterns caused by helical structures.)

• Hmmm ... there are at least three size-scales available: the wire diameter, the axial spacing of the helix and the diameter of the helix. But where to go from there eludes me. Commented Apr 22, 2017 at 21:14
• Your picture is reproduced as part of a homework question due 10/2/15 (which means that you have missed the deadline) physics.indiana.edu/~simasgrp/p575f15/hw/hw3_combined_f15.pdf and a statement made that the theory can be found in this paper which I cannot access Cochran, W., Crick, F.H.C., and Vand, V., “The structure of synthetic polypeptides. I. The transform of atoms on a helix”, Acta Cryst. 5, 581-586 (1952) Commented Apr 27, 2017 at 12:19
• @Farcher Nice find. Happy to accept a summary as an answer. Commented Apr 27, 2017 at 13:13