Extreme ultraviolet denotes short-wavelength ultraviolet between longer-wavelength ultraviolet and X-rays; precise band boundaries depend on convention. Its photons have sufficient energy to cause strong optical absorption in many materials that transmit visible light.
This makes beam transport an apparatus problem: an ordinary air path or visible-light window can absorb the beam, and suitable reflective optics can be more useful than an ordinary transmitting optical lens. Before interpreting a weak detector signal, compare a blocked-beam reading with the open-beam reading and account for wavelength-dependent detector quantum efficiency.
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