Holographic ribbon is polypropylene film with aluminium evaporated onto it and a diffraction grating pressed into the metal. The pressing does not come out as one continuous ruling: it breaks into domains a few tens of microns across, each holding its grooves at its own angle, which is why the film glitters as it turns instead of sweeping like a compact disc. The curl is the demonstration. A ribbon drawn over a blade sets into a helix, and a helix carries its surface normal the whole way round, so the entire spectrum sits on one piece of film at once.