19 4 MECHAN ICAL SYSTEMS Glass Glass is silica (sand) fused by heat with lime and an alkali like soda or potash. The alkalis help lower the melting point making it easier to fuse, and the lime is a stabilizer. Additives are sometimes used to influence the properties of the final product. Boron, for example, increases the thermal and electrical resistance while barium increases the refractive index. There are several types of glass, but a few of them are used in automated lighting more often than others. Borosilicate glass, better known under the trade name Pyrex, is used as a substrate for dichroic filters and glass gobos. A type of boro- silicate glass called barium borosilicate glass made by Corning (7059 glass), and Borofloat ® made by Schott, are used by several manufacturers for this purpose. They're made mostly of silica (SiO 2 , 70­80%) and boric oxide (B 2 O 3 , 7­13%), plus small amounts of alkalis and aluminum oxide. The lower alkali content gives it good chemical durability and thermal shock resistance. It also has a low coeffi- cient of expansion, which means that it doesn't expand and contract a great deal when it heats up and cools down. This property is very important because it prevents the optical thin-film coatings from separating from the substrate due to excessive expansion and contraction. Fused Quartz Pure SiO 2 glass, also known as fused quartz or fused silica, is manufactured by gasifying silicon and oxidating it to make silicon dioxide, then fusing the silicon dioxide dust to make glass. It has a remarkably low coefficient of thermal expan- sion and it's transparent to UV and near IR wavelengths. For these reasons it's used to make glass envelopes for halogen lamps and arc tubes for discharge lamps. Optical glass Optical glass, which is used to make lenses and critical optical elements, is manu- factured under very carefully controlled circumstances to accurately reproduce certain characteristics useful in the design of optical systems. Consequently, it can be very expensive. Optical glass is classified according to its index of refrac- tion, coefficient of dispersion, coefficient of optical transmission, purity, hardness, and more. There are certain additives such as calcium fluorite, phosphate, zinc, barium, lanthanum, and antimony that can be introduced in the manufacturing process to shape the characteristics of the final product. The two principle types of optical glass are crown glass and flint glass, but there are many variations of each. Additives are used in their construction to tailor the product to the application.