What material is used in the x-ray tube window to allow photons to pass through?

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Transmitting x-rays efficiently requires a window that lets photons pass with minimal attenuation while withstanding vacuum and heat. Materials with a very low atomic number and density do this best, so a thin window made of beryllium allows the diagnostic x-ray beam to emerge with little loss, especially the lower-energy photons that are most easily absorbed. Beryllium’s combination of low absorption, plus good strength and heat tolerance, makes it ideal for the tube window. Glass would absorb more photons and can crack under vacuum and heat; aluminum attenuates more than beryllium; steel attenuates even more and is far heavier. So, beryllium is preferred.