1、Designation: C1648 06 C1648 12Standard Guide forChoosing a Method for Determining the Index of Refractionand Dispersion of Glass1This standard is issued under the fixed designation C1648; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revis
2、ion, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide identifies and describes seven test methods for measuring the index of refraction of glass
3、, with commentsrelevant 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, thenature of the test specimen that c
4、an 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-bandwidthlight. Dispersion is the physical phenomenon of the variation of refract
5、ive index with wavelength. The nature of the test-specimenrefers to its size, form, and quality of finish, as described in each of the methods herein. The test methods described are mostlyfor the visible range of wavelengths (approximately 400 to 780m); however, some methods can be extended to the u
6、ltraviolet andnear 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 (
7、Abbe type and Pulfrich type),1.1.1.4 Metricon2 system,1.1.1.5 Vee-block refractometers,1.1.1.6 Prism spectrometer, and1.1.1.7 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, and1.1.2.4 Method
8、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 responsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior
9、to use.1.3 WarningRefractive index liquids are used in several of the following test methods. Cleaning with organic liquid solventsalso is specified. Degrees of hazard associated with the use of these materials vary with the chemical nature, volatility, and quantityused. See manufacturers literature
10、 and general information on hazardous chemicals.2. Referenced Documents2.1 ASTM Standards:3E167 Practice for Goniophotometry of Objects and Materials (Withdrawn 2005)4E456 Terminology Relating to Quality and Statistics3. Terminology3.1 Definitions:3.1.1 dispersion, nthe physical phenomenon of the va
11、riation of refractive index with wavelength.3.1.1.1 Discussion2 Metricon is a trademark of Metricon Corporation 12 North Main Street, P.O.Box 63, Pennington, New Jersey 08534.3 For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. Fo
12、r Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4 The last approved version of this historical standard is referenced onwww.astm.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.
13、United States1The term, “dispersion,” is commonly used in lieu of the more complete expression, “reciprocal relative partial dispersion.” Adispersion-number can be defined to represent the refractive index as a function of wavelength over a selected wavelength-range;that is, it is a combined measure
14、 of both the amount that the index changes and the non-linearity of the index versus wavelengthrelationship.3.1.2 resolution, nas expressed in power of 10, a commonly used term used to express the accuracy of a test method in termsof the decimal place of the last reliably measured digit of the refra
15、ctive index which is expressed as the negative power of 10. Asan example, if the last reliably measured digit is in the fifth decimal place, the method would be designated a 10-5 method.3.2 Symbols:n = index of refraction = Abbe-number; a representation of particular relative partial dispersionsD =
16、Abbe-number determined with spectral lines D,C, and Fe = Abbe-number determined with spectral lines e,C, and FD = the spectral emission line of the sodium doublet at nominally 589.3 nm (which is the mid-point of the doublet that has linesat 589.0 nm and 589.6 nm)C = the spectral emission line of hyd
17、rogen at 656.3 nmF = the spectral emission line of hydrogen at 486.1 nme = the spectral emission line of mercury at 546.1 nmC = the spectral emission line of cadmium at 643.8 nmF = the spectral emission line of cadmium at 480.0 nm4. Significance and Use4.1 MeasurementThe refractive index at any wave
18、length of a piece of homogeneous glass is a function, primarily, of itscomposition, and secondarily, of its state of annealing. The index of a glass can be altered over a range of up to 110-4 (that is,1 in the fourth decimal place) by the changing of an annealing schedule. This is a critical conside
19、ration for optical glasses, thatis, glasses intended for use in high performance optical instruments where the required value of an index can be as exact as 110-6.Compensation for minor variations of composition are made by controlled rates of annealing for such optical glasses; therefore,the abilit
20、y 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 refractiveindex are usually adequate. The refractive indices of
21、glass ophthalmic lens pressings are held to 510-4 because the tools used forgenerating the figures of ophthalmic lenses are made to produce curvatures that are related to specific indices of refraction of thelens materials.4.2 DispersionDispersion-values aid optical designers in their selection of g
22、lasses (Note 1). Each relative partialdispersion-number is calculated for a particular set of three wavelengths, and several such numbers, representing different partsof the spectrum might be used when designing more complex optical systems. For most glasses, dispersion increases withincreasing refr
23、active index. For the purposes of this standard, it is sufficient to describe only two reciprocal relative partialdispersions that are commonly used for characterizing glasses. The longest established practice has been to cite the Abbe-number(or Abbe -value), calculated by:D 5nD 21!/nF 2nC! (1)where
24、 vD is defined in 3.2 and nD,nF, and nC are the indices of refraction at the emission lines defined in 3.2.D 5nD 21!/nF 2nC! (1)4.2.1 Some modern usage specifies the use of the mercury e-line, and the cadmium C and F lines. These three lines are obtainedwith a single spectral lamp.1 This guide is un
25、der the jurisdiction of ASTM Committee C14 on Glass and Glass Products and is the direct responsibility of Subcommittee C14.11 on Optical Properties.Current edition approved Oct. 1, 2006Oct. 1, 2012. Published February 2007November 2012. Originally approved in 2006. Last previous edition approved in
26、 2006 asC1648-06. DOI: 10.1520/C1648-06.10.1520/C1648-12.TABLE 1 Spectral Lines for Measurement of Refractive IndexAFraunhofer Line A C C D d e F F g G hElement K H Cd Na He Hg H Cd Hg H HgWavelength Nanometers 786.2B 656.3C 643.8D 589.3 587.6 546.1 486.1 480.0D 435.8 434.0 404.7A From Ref (4).B A l
27、ater reference (identification not available) lists 789.9 nm for the potassium A line, although referring to Ref (4). The Handbook of Chemistry and Physics lists 789.9nm as a very strong line, and it does not list a line at 786.2 nm at all.C The wavelength of the corresponding deuterium line is 656.
28、0 nm.D The two cadmium lines have been recognized for refractometry since Ref (4) was published.C1648 122e 5ne 21!/nF2nC! (2)where ve is defined in 3.2 and ne,nF, and nC are the indices of refraction at the emission lines defined in 3.2.e 5ne 21!/nF2nC! (2)4.2.2 A consequence of the defining equatio
29、ns (Eq 1 and 2) is that smaller -values correspond to larger dispersions. For-values accurate to 1 to 4 %, index measurements must be accurate to 110-4; therefore, citing -values from less accurate testmethods might not be useful.NOTE 1For lens-design, some computer ray-tracing programs use data dir
30、ectly from the tabulation of refractive indices over the full wavelength rangeof measurement.NOTE 2Because smaller -values represent larger physical dispersions, the term constringence is used in some texts instead of dispersion.5. Precision, Bias, and Accuracy (see Terminology E456)5.1 PrecisionThe
31、 precision of a method is affected by several of its aspects which vary among methods. One aspect is theability of the operator to repeat a setting on the observed optical indicator that is characteristic of the method. Another aspect isthe repeatability of the coincidence of the measurement scale o
32、f the instrument and the optical indicator (magnitude of dead-bandor backlash); this, too, varies among methods. A third aspect is the repeatability of the operators reading of the measurement scale.Usually, determinations for a single test specimen and for the reference piece should be repeated sev
33、eral times and the resultingscale readings averaged after discarding any obvious outliers.5.2 Bias (Systematic Error):5.2.1 Absolute MethodsTwo of the test methods are absolute; the others are comparison methods. The absolute methods arethe prism spectrometer and the apparent depth of microscope foc
34、us. These yield measures of refractive index of the specimen inair. In the case of the prism spectrometer, when used for determinations of 110-6, correction to the index in vacuum (the intrinsicproperty of the material) can be calculated from the known index of air, given its temperature, pressure,
35、and relative humidity. Theaccuracy of the apparent depth method is too poor for correction to vacuum to be meaningful. Bias of the prism spectrometerdepends upon the accuracy of its divided circle. The bias of an index determination must not be greater than one-half of the leastcount of reading the
36、scale of the divided circle. For a spectrometer capable of yielding index values accurate to 110 -6, the biasmust be not greater than 510-7. Bias of the apparent depth method depends on the accuracy of the device for measuring thedisplacement of the microscope stage; it is usually appreciable smalle
37、r than the precision of the measurement, as explained in 7.6.5.2.2 Comparison MethodsAll of the comparison methods rely upon using a reference material, the index of which is knownto an accuracy that is greater than what can be achieved by the measurements of the given method itself; therefore, the
38、bias of thesemethods is the uncertainty of the specified refractive index of the reference material, provided that the instruments scale is linearover the range within which the test-specimen and the reference are measured. The bias introduced by non-linearity of the scalecan be compensated by calib
39、rating the scale over its range with reference pieces having indices that are distributed over the rangeof the scale. A table of scale-corrections can be made for ready reference, or a computer program can be used; using this, the scalereading for a single reference piece is entered and then correct
40、ed indices are generated for each scale reading made for a set oftest specimens. For a single measurement, scale correction can be made by first measuring the test specimen and then measuringthe calibrated reference piece that has the nearest index. In this case, the scale is corrected only in the v
41、icinity where the readingsare made.5.2.3 Test SpecimenDeviations of a test specimen from its ideal configuration can contribute a bias. For 110-6 refractometry,specimen preparation must be of the highest order and specimens are tested for acceptability for use. Bias introduced by a testspecimen vari
42、es in its manifestation with the type of test method and nature of the deviation from ideal. This consideration isaddressed in the descriptions of individual test methods.5.3 AccuracyThe limiting accuracies of the several test methods are given. The operator must estimate whether and how mucha given
43、 test measurement deviates from that limit. The estimate should take into account the observed uncertainty of identifyingwhere to set on the optical indicator (see 7.6, for example) as well as the precision of such settings. Specific considerations aregiven in the descriptions of the test methods.NO
44、TE 3The Subcommittee did not conduct an Inter-laboratory Study (as normally required) to quantify the Precision and Bias of Methods discussedin this Standard. The cited accuracies of the test methods are based on experience.TEST METHODS6. Becke Line (Method of Central Illumination)6.1 Summary of the
45、 MethodNot-too-finely ground particles of the glass for testing are immersed in a calibrated refractiveindex oil and are examined with a microscope of moderate magnification. With a particle in focus, if the indices of the oil and theglass match exactly, the particle is not seen; no boundary between
46、 oil and glass is visible. If the indices differ, a boundary is seenas a thin, dark line at the boundary of the particle with either the particle or the oil appearing lighter. The line appears darker asthe indices differ more; however, which material has the higher index is not indicated. When the f
47、ocal plane of the microscope ismoved above or below the plane of the particle (usually by lowering or elevating the stage of the microscope), one side of theboundary appears lighter and the other side appears darker than the average brightness of the field. When the focus is above theC1648 123plane
48、of the glass particle, a bright line next to the boundary appears in the medium of higher index. This is the “Becke line”;conversely, when the focus is below the plane of the particle, the bright line appears in the medium of lower index. Successivechanges of oil, using new glass particles, lead by
49、trial and error to a bracketing of the index of the particle between the pair of oilsthat match most closely (or to an exact match). Visual interpolation can provide resolution to about one fourth of the differencebetween the indices of the two oils. The physical principle underlying the method is that of total internal reflection at the boundary,within the medium of higher index. This is illustrated by a ray diagram, Fig. 1(a). Visual appearances are illustrated in Fig. 1(b),Fig.1(c), and Fig. 1(d), where different d
