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Standard Guide for Choosing a Method for Determining the Index of Refraction and Dispersion of Glass
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NORM herausgegeben am 1.8.2018
Bezeichnung normen: ASTM C1648-12(2018)
Anmerkung: UNGÜLTIG
Ausgabedatum normen: 1.8.2018
SKU: NS-856414
Zahl der Seiten: 15
Gewicht ca.: 45 g (0.10 Pfund)
Land: Amerikanische technische Norm
Kategorie: Technische Normen ASTM
Keywords:
Abbe-number, dispersion, glass, index of refraction, refractive index, refractometer,, ICS Number Code 81.040.30 (Glass products)
Significance and Use | ||||
4.1 Measurement—The refractive index at any wavelength of a piece of homogeneous glass is a function, primarily, of its composition, and secondarily, of its state of annealing. The index of a glass can be altered over a range of up to 1×10-4 (that is, 1 in the fourth decimal place) by the changing of an annealing schedule. This is a critical consideration for optical glasses, that is, glasses intended for use in high performance optical instruments where the required value of an index can be as exact as 1×10-6. Compensation for minor variations of composition are made by controlled rates of annealing for such optical glasses; therefore, the ability to measure index to six decimal places can be a necessity; however, for most commercial and experimental glasses, standard annealing schedules appropriate to each are used to limit internal stress and less rigorous methods of test for refractive index are usually adequate. The refractive indices of glass ophthalmic lens pressings are held to 5×10-4 because the tools used for generating the figures of ophthalmic lenses are made to produce curvatures that are related to specific indices of refraction of the lens materials. 4.2 Dispersion—Dispersion-values aid optical
designers in their selection of glasses (Note 1). Each relative partial
dispersion-number is calculated for a particular set of three
wavelengths, and several such numbers, representing different parts
of the spectrum might be used when designing more complex optical
systems. For most glasses, dispersion increases with increasing
refractive index. For the purposes of this standard, it is
sufficient to describe only two reciprocal relative partial
dispersions that are commonly used for characterizing glasses. The
longest established practice has been to cite the Abbe-number (or
Abbe ν-value), calculated by: Note 1: For lens-design, some computer ray-tracing programs
use data directly from the tabulation of refractive indices over
the full wavelength range of measurement.
Note 2: Because smaller ν-values represent larger physical
dispersions, the term constringence is used in some texts instead
of dispersion.
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1. Scope | ||||
1.1 This guide identifies and describes seven test methods for measuring the index of refraction of glass, with comments relevant to their uses such that an appropriate choice of method can be made. Four additional methods are mentioned by name, and brief descriptive information is given in Annex A1. The choice of a test method will depend upon the accuracy required, the nature of the test specimen that can be provided, the instrumentation available, and (perhaps) the time required for, or the cost of, the analysis. Refractive index is a function of the wavelength of light; therefore, its measurement is made with narrow-bandwidth light. Dispersion is the physical phenomenon of the variation of refractive index with wavelength. The nature of the test-specimen refers to its size, form, and quality of finish, as described in each of the methods herein. The test methods described are mostly for the visible range of wavelengths (approximately 400 to 780 μm); however, some methods can be extended to the ultraviolet and near infrared, using radiation detectors other than the human eye. 1.1.1 List of test methods included in this guide: 1.1.1.1 Becke line (method of central illumination), 1.1.1.2 Apparent depth of microscope focus (the method of the Duc de Chaulnes), 1.1.1.3 Critical Angle Refractometers (Abbe type and Pulfrich type), 1.1.1.4 Metricon2 system, 1.1.1.5 Vee-block refractometers, 1.1.1.6 Prism spectrometer, and 1.1.1.7 Specular reflectance. 1.1.2 Test methods presented by name only (see Annex A1): 1.1.2.1 Immersion refractometers, 1.1.2.2 Interferometry, 1.1.2.3 Ellipsometry, and 1.1.2.4 Method of oblique illumination. 1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. 1.3 1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee. |
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2. Referenced Documents | ||||
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