Page History

...

In additional to the fundamental wavelength, the generation of x-ray by electron passing through undulators also produces higher order radiation (only the odd orders in fact). For example, for a fundamental at 5keV, photons at 15keV, 25keV, etc. are also generated.

The intensity off of successive orders drops very quickly, but the 3rd harmonic is still intense enough to create spurious and unwanted signals. Because photons of higher energy penetrates more in materials, the rejection of these higher order is not straight-forward. This in fact make the problem worse if the attenuator is used, as the fundamental is more attenuated than the higher orders, hereby worsening the ratio of "good" photons to unwanted ones. Moreover, because they are multiple of the fundamental energy and thus share , they are shared similar diffraction conditions, and diffraction-based approach do not work.

...