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Standard Features of Glass Quality |
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Optical glass quality indices tolerances are calculated on the basis of requirements for an optical system. Overvaluation of the requirements in excess of the necessary gives no marked improvement of the optical system quality, whereas considerably increases glass cost.
Values of given in the catalogue optical parameters are of nominal kind. Glasses of actual melts can have some deviations from the nominal values within the stated tolerances.
Standard features of optical glass quality:
For each of the above indices there is fixed tolerance system.
Refractive Index, Abbe-number
Five quality categories are standardized with the following allowable deviations delta ne and delta ve from the nominal: | category | ± delta ne (1*10-4) | delta ve (%) | | 1 | 2 | 0.2 | | 2 | 3 | 0.3 | | 3 | 5 | 0.5 | | 4 | 10 | 0.8 | | 5 | 20 | 1.6 | Standard glass shall have the refractive index reliably measured in the fourth decimal place.
The normal category of glass quality - 4. For normal quality glasses with ne >1.650, delta ne = ± 15·10-4, delta ve= ± 1.6%.
Variation of ne in a Batch of Blanks
It is defined by the maximum difference delta ne in the supplied batch of blanks. There are four fixed classes by ne value: | class | delta ne (1*10-4) | | A | 0.2 | | B | 0.5 | | C | 1.0 | | G | within the category stated when ordering | Optical Homogeneity
Optical homogeneity of glasses and other optical materials is understood as homogeneity of the refractive index within a volume of material and can be defined by the maximum difference of values n for different areas of a piece . Breach of the homogeneity can be caused by chemical and physical inhomogeneities.
Chemical inhomogeneities are striae which arise during glass melting and glass working and are conditioned through variations of glass composition because of incomplete melting or volatilization of some components, eating of glass melting furnace walls, and poor homogenization of melt.
Striae are transparent, vitreous inclusions, which are thread-like, cord-like or knot-like formations with threads split off from them.
Residual stress and structural non-uniformity account for variations in physical properties. Stress in glass initiates birefringence.
Measurement of the refractive index value across a blank's field is a very difficult and labor-consuming process, requiring precision quality surfacing and application of complex device equipment. Therefore, in practice indirect methods of homogeneity estimation are used via setting of a strict tolerance system for striae and birefringence ensuring high level of optical homogeneity for supplied glass.
Blanks for fabrication of optical components for high-accuracy collimators' objectives, for micro lithography, for measurement and astronomical devices , whose residual wave aberrations systems are comparable with distortions induced by glass inhomogeneity, are usually tested by interference methods judging by the wave-front distortion after a light beam passing a blank. For such components requirements for optical homogeneity are stipulated additionally when preparing the order.
Optical homogeneity categories on the refractive index are listed in Table: | optical homogenity categories on the refractive index | difference of refractive indices delta n in blank's volume, not more (1·10–6) | | 01 | 1 | | 1 | 2 | | 2 | 5 | | 3 | 10 | | 4 | 20 | | 5 | 50 | Integrated Attenuation Coefficient, Transmittance
According to the levels of the integrated attenuation coefficient muA and the transmittance coefficient tauA for the source A eight categories are set characterized by the limiting values shown in Table: | categories by the attenuation coefficient | attenuation coefficient muA (cm –1) | transmittance factor tauA in 1 cm layer | transmittance factor tauA in 2.5 cm layer | | 1 | 0.0002 - 0.0004 | 0.9995 – 0.9990 | 0.9988 – 0.9977 | | 2 | 0.0005 - 0.0009 | 0.999 – 0.998 | 0.997 – 0.995 | | 3 | 0.0010 - 0.0017 | 0.998 – 0.996 | 0.995 – 0.990 | | 4 | 0.0018 - 0.0025 | 0.996 – 0.994 | 0.990 – 0.985 | | 5 | 0.0026 - 0.0035 | 0.994 – 0.992 | 0.985 – 0.980 | | 6 | 0.0036 - 0.0045 | 0.992 – 0.990 | 0.980 – 0.974 | | 7 | 0.0046 - 0.0065 | 0.990 – 0.985 | 0.974 – 0.963 | | 8 | 0.0066 - 0.0130 | 0.985 – 0.970 | 0.963 – 0.928 | Birefringence
According to birefringence levels there are five categories defined by the difference of beam paths at the wavelength 550 nm measured in the area and direction shown in Table: | categories by birefringence | birefringence | | 1 | 2 | | 2 | 6 | | 3 | 10 | | 4 | 15 | | 5 | 50 | Striae
Optical glass is checked for striae with the direct-shadow method allowing to detect striae visually on a screen when viewing in a given direction and to estimate them by comparison with certified test samples of the first and the second categories of striae.
Depending on the direction of glasses viewing two classes are set: A - checking in two mutually-perpendicular directions; B - checking in one given direction.
If it is not specified in an order, the slab glass is checked in the direction perpendicular to the bottom and top surfaces of the slab. Blanks and pressed blanks are checked in the working direction.
For prisms terms of checking are specified additionally.
Terms of supply concerning striae for glass types LK7, CTK9, CTK12, KF7, as well as for other types when thickness of ordered blanks is more than 100mm shall be specified in the order.
Bubbles
Bubbles in optical glass and equated to them inclusions and knot-striae are formed during melting process.
Bubbles are gradated by the classes, which sets the allowable total area of cross-sections of bubbles exceeding 0.05 mm in size in every 100 cm3 of glass and the maximum allowable bubble diameter in mm. Bubbles of the size less than 0.05 mm are disregarded.
| class of bubbles | total bubbles cross-section area per 100 mm3 volume glass, not more (mm2) | | 1 | 0.032 | | 2 | 0.12 | | 3 | 0.25 | | 4 | 0.50 | | 5 | 1.00 | | 6 | 2.00 | | 7 | 4.00 | When ordering blanks of large sizes (more than 500 mm), quantity and size of bubbles shall be specified. Bubbles classes listed in the catalogue can be ensured as the highest for the given glass type.
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