An X-ray filter is a device to block or filter out some or all wavelengths in the X-ray spectrum.
X-ray filters are used to block low-energy X-rays during medical x-ray imaging. Low energy X-rays are more likely to be absorbed by the patient's soft tissues. This causes non-stochasic radioactive effects, and does not contribute to image quality.
X-ray filters are used in X-ray crystallography, where crystalline lattice spacings can be determined using Bragg diffraction. The filters allow only a single X-ray wavelength to penetrate through to a target crystal, allowing the resulting scattering to determine the diffraction distance.
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... X-ray crystallography X-rays Bragg diffraction. ...