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PRODUCT DATA of 09: Optical Materials
Material09: Optical Materials
General InformationThe classical meaning of the word "glass" is extended in this Standard to cover "organic glass" and some crystalline optical materials.
Use in SpacecraftGlasses, inorganic as well as organic, appear as optical elements: e.g. windows, lenses, prisms, solar-cell covers and filters.
Main CategoriesOptical materials can be grouped as:
  • inorganic glasses - such as silicates, alumino-silicates and boro-silicates;
  • organic "glasses" - polymers based on acrylic and methacrylic polymers, poly-carbonate and some polystyrene grades;
  • crystalline optical materials - pure silica, sapphire and transparent fluorides
Processing and AssemblyInorganic glass parts are mainly assembled by means of flanges and gaskets or adhesives. Glass-to-metal welds are possible. Assembly shall be rigid enough to provide accurate alignment but shall also be designed to cope with thermal expansion and provide suitable damping.
Organic glasses are easily machined: this operation can be performed on inorganic glasses by using special techniques (e.g. ultrasonic machining).
PrecautionsGlasses are transparent only to a certain wavelength range and shall be chosen in accordance with the mission requirements. Inorganic glasses are sensitive to mechanical and thermal shocks. Organic glasses are easily scratched and lose their polish.
Assembly methods are the most important points in the design of parts containing glass and particular attention shall be given to matching the thermal expansion coefficient of the optical material with that of its mounting.
Hazardous and PrecludedCanada Balsam and other similar products shall not be used in the assembly, since they are liable to produce contaminants.
Organic glasses should not appear in high-precision equipment except as plain windows or light-pipes.
Effects of Space environment
  • Vacuum exposure does not affect inorganic glasses or most organic glasses. The main danger comes from bonding agents, optical coupling agents and other assembly materials which can contaminate the optics by yielding condensable products. A contaminated optic is, in general, very difficult to clean.
  • Radiation is the most harmful factor to be considered for glasses. Some inorganic glasses are damaged by doses of the order of 10 Gray of ionizing radiation (1 Gray = 1 J kg^1 of absorbed energy): the damage is a loss of transparency in certain wavelength ranges due to colour-centre formation. UV is less harmful, at least for inorganic glasses. Particle radiation can also distort the shape of optical elements. Plastics can be damaged by particle and UV radiation. The result is, in general, a “yellowing”, and the damage under sunlight can be auto-accelerated by the increase in temperature due to higher absorption.
  • Temperature: Thermal shock can lead to fracture in inorganic glasses. Also, distortion can be noted in precision optics when the assembly is not designed to compensate correctly for the low expansion of these glasses and the high expansion of metal mountings. Organic glasses soften at quite low temperature (80 ºC to 100 ºC frequently) and have rather high expansion coefficients.
  • Atomic oxygen can attack organic glasses.
Some Representative ProductsIn the case of inorganic glasses pure silica should be used. This is sold by many European firms under many different trade names, for example:

Optical glasses are mainly designated by reference numbers from manufacturers like SCHOTT (D), CORNING (USA) and PILKINGTON (UK).

For solar-cell covers, the main sources are still OCLI (USA) and PILKINGTON (UK). Some are manufactured with an electrically conductive surface such as ITO by the same manufacturers.

Optical solar reflectors (OSR) based on silica/silver/inconel or silica/aluminium are manufactured by OCLI. PILKINGTON produces OSR’s based on cerium glass/silver-nickel-chrome.

Filters aremade by BALZERS(CH), SCHOTT(D),ASTRIUMSAS(F),M.T.O. (F), BARR and STROUD (UK) and THALES (UK).

Organic glasses based on acrylic and methacrylic polymers are well known: PLEXIGLAS from ROHM and HAAS (D), PERSPEX from ICI (UK). Polycarbonates like MAKROLON (BAYER, D) can also be considered as well as several polystyrene grades.