An analyzer for a fluid within a pipe (2) has a tubular radiation transmission window (7) which is coaxially disposed between two sections of the pipe which contains the fluid (3). The window (7) comprises a beryllium tube (8) and a tubular guard (9) which fits tightly around the beryllium tube and is made of a material having a higher stiffness than beryllium. The tubular guard (9) has two diametrically-opposed through holes (10) formed therein through which radiation can pass. The radiation transmission window (7) may further comprise a ceramic coating (24) which is formed on the inner surface of the beryllium tube. The ceramic coating (24) protects the beryllium tube (8) from wear due to abrasive substances within the fluid. The ceramic coating is made of a material comprising light atoms such as alumina, silicon carbide, MgO, silicon dioxide, and diamond.