X-rays can pass through many materials, and different materials block ('attenuate') the passage of X-rays to different degrees. As X-rays pass through an object, X-rays of a particular wavelength are attentuated more or less depending on the materials through which the X-rays pass and the thickness of the material. The attenuation of X-rays passing through the object makes a 'shadow pattern' which can be captured for study on photographic film, or by a reusable phosphor screen which can be read by a digital scanner, or directly by a digital detector. Each of these three techniques has advantages and disadvantages and all three are currently used at the Field Museum. The resulting X-radiographs allow conservators to look at features hidden below an object's surface. The different attenuation values can also be used to distinguish between materials which look the same under visible light but have different X-ray absorptions.
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