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K9 glass, sometimes referred to as K9 crystal, is a variety of optical borosilicate crown glass.The letter "K" is a reference to the German word for "crown" (Krone) and the number 9 refers to the lead oxide content of the glass (9%).
A bundle of optical fibers. Fluoride glass is a class of non-oxide optical glasses composed of fluorides of various metals. They can contain heavy metals such as zirconium, or be combined with lighter elements like aluminium and beryllium.
The prismatic lenses of the belay glasses are arranged so as to bend light from above through total internal reflection into the observer's eye, allowing the belayer to observe the climber while maintaining a comfortable head/neck position. The design based on the eyeglasses frame provides for a split field of vision:
So-called "low-Tg-glasses" with a maximum transition temperature of less than 550 °C have been developed in order to enable new manufacturing routes for the moulds. Mould materials such as steel can be used for moulding low-Tg-glasses whereas high-Tg–glasses require a high-temperature mould material, such as tungsten carbide.
The prism is also less bulky than the double Porro design. The prism is sometimes simply called a "roof prism", although this is ambiguous, because other roof prisms exist, such as the Amici and Schmidt–Pechan designs. A variant of the Abbe–Koenig prism replaces the "roof" section of the prism with a single mirror-coated reflecting surface.
Some of these glasses (e.g. Germanium dioxide (GeO 2, Germania), in many respects a structural analogue of silica, fluoride, aluminate, phosphate, borate, and chalcogenide glasses) have physicochemical properties useful for their application in fibre-optic waveguides in communication networks and other specialised technological applications.