Bomco Inc. has developed ultrahigh-vacuum (UHV) windows that enable the use of optical pyrometry, ellipsometry, light scattering and reflectance-difference spectroscopy to obtain data in real time during molecular beam epitaxy operations. The Re-Entrant UV Window extends inside the vacuum chamber, saving space and providing protection to the window during UHV operations. Its flange is 4.50 in., the tube has a 2.25-in. OD and the unit is 4.50 in. The conventional UHV window extends outside the vacuum chamber and is available in 1.50- and 2-in. flange sizes. A protective stainless steel shield is available for both sizes.