ASTM C1648-12(2023)

Standard Guide for Choosing a Method for Determining the Index of Refraction and Dispersion of Glass

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:
where vD is defined in 3.2 and nD, nF, and nC are the indices of refraction at the emission lines defined in 3.2.  
4.2.1 Some modern usage sp...
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 μm 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 re...

General Information

• Status: Published
• Publication Date: 30-Jun-2023
• ICS: 81.040.30 - Glass products
• Technical Committee: C14 - Glass and Glass Products
• Drafting Committee: C14.11 - Optical Properties

Standards Content (Sample)

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.
Designation: C1648 − 12 (Reapproved 2023)
Standard Guide for
Choosing a Method for Determining the Index of Refraction
1
and Dispersion of Glass
This standard is issued under the fixed designation C1648; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.2 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This guide identifies and describes seven test methods
responsibility of the user of this standard to establish appro-
for measuring the index of refraction of glass, with comments
priate safety, health, and environmental practices and deter-
relevant to their uses such that an appropriate choice of method
mine the applicability of regulatory limitations prior to use.
can be made. Four additional methods are mentioned by name,
1.3 Warning—Refractive index liquids are used in several
and brief descriptive information is given in Annex A1. The
of the following test methods. Cleaning with organic liquid
choice of a test method will depend upon the accuracy
solvents also is specified. Degrees of hazard associated with
required, the nature of the test specimen that can be provided,
the use of these materials vary with the chemical nature,
the instrumentation available, and (perhaps) the time required
volatility, and quantity used. See manufacturer’s literature and
for, or the cost of, the analysis. Refractive index is a function
general information on hazardous chemicals.
of the wavelength of light; therefore, its measurement is made
1.4 This international standard was developed in accor-
with narrow-bandwidth light. Dispersion is the physical phe-
dance with internationally recognized principles on standard-
nomenon of the variation of refractive index with wavelength.
ization established in the Decision on Principles for the
The nature of the test-specimen refers to its size, form, and
Development of International Standards, Guides and Recom-
quality of finish, as described in each of the methods herein.
mendations issued by the World Trade Organization Technical
The test methods described are mostly for the visible range of
Barriers to Trade (TBT) Committee.
wavelengths (approximately 400 μm to 780 μm); however,
some methods can be extended to the ultraviolet and near
2. Referenced Documents
infrared, using radiation detectors other than the human eye.
3
1.1.1 List of test methods included in this guide: 2.1 ASTM Standards:
1.1.1.1 Becke line (method of central illumination),
E167 Practice for Goniophotometry of Objects and Materi-
4
1.1.1.2 Apparent depth of microscope focus (the method of als (Withdrawn 2005)
the Duc de Chaulnes),
E456 Terminology Relating to Quality and Statistics
1.1.1.3 Critical Angle Refractometers (Abbe type and Pul-
3. Terminology
frich type),
2
1.1.1.4 Metricon system,
3.1 Definitions:
1.1.1.5 Vee-block refractometers,
3.1.1 dispersion, n—the physical phenomenon of the varia-
1.1.1.6 Prism spectrometer, and
tion of refractive index with wavelength.
1.1.1.7 Specular reflectance.
3.1.1.1 Discussion—The term, “dispersion,” is commonly
1.1.2 Test methods presented by name only (see Annex A1):
used in lieu of the more complete expression, “reciprocal
1.1.2.1 Immersion refractometers,
relative partial dispersion.” A dispersion-number can be de-
1.1.2.2 Interferometry,
fined to represent the refractive index as a function of wave-
1.1.2.3 Ellipsometry, and
length over a selected wavelength-range; that is, it is a
1.1.2.4 Method of oblique illumination.
combined measure of both the amount that the index changes
and the non-linearity of the index versus wavelength relation-
ship.
1
This guide is under the jurisdiction of ASTM Committee C14 on Glass and
Glass Products and is the direct responsibility of Subcommittee C14.11 on Optical
3
Properties. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved July 1, 2023. Published July 2023. Originally approved contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
in 2006. Last previous edition approved in 2018 as C1648 – 12 (2018). DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/C1648-12R23. the ASTM website.
2 4
Metricon is a trademark of Metricon Corporation 12 North Main Street, P.O. The last approved version of this historical standard is referenced on
Box 63, Pennington, New Jersey 08534. www.astm.org.
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