1、Designation: C1648 12 (Reapproved 2018)Standard 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
2、 of revision, 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 methodsfor measuring the index of refraction
3、of glass, with commentsrelevant to their uses such that an appropriate choice of methodcan be made. Four additional methods are mentioned by name,and brief descriptive information is given in Annex A1. Thechoice of a test method will depend upon the accuracyrequired, the nature of the test specimen
4、that can be provided,the instrumentation available, and (perhaps) the time requiredfor, or the cost of, the analysis. Refractive index is a functionof the wavelength of light; therefore, its measurement is madewith narrow-bandwidth light. Dispersion is the physical phe-nomenon of the variation of re
5、fractive index with wavelength.The nature of the test-specimen refers to its size, form, andquality of finish, as described in each of the methods herein.The test methods described are mostly for the visible range ofwavelengths (approximately 400 to 780 m); however, somemethods can be extended to th
6、e 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 ofthe Duc de Chaulnes),1.1.1.3 Critical Angle Refractometers
7、 (Abbe type and Pul-frich type),1.1.1.4 Metricon2system,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 AnnexA1):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 thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory
9、 limitations prior to use.1.3 WarningRefractive index liquids are used in severalof the following test methods. Cleaning with organic liquidsolvents also is specified. Degrees of hazard associated withthe use of these materials vary with the chemical nature,volatility, and quantity used. See manufac
10、turers literature andgeneral information on hazardous chemicals.1.4 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendat
11、ions issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:3E167 Practice for Goniophotometry of Objects and Materi-als (Withdrawn 2005)4E456 Terminology Relating to Quality and Statistics3. Terminology3.1 Definitions:3.1.1 disper
12、sion, nthe physical phenomenon of the varia-tion of refractive index with wavelength.3.1.1.1 DiscussionThe term, “dispersion,” is commonlyused in lieu of the more complete expression, “reciprocalrelative partial dispersion.” A dispersion-number can be de-fined to represent the refractive index as a
13、function of wave-length over a selected wavelength-range; that is, it is acombined measure of both the amount that the index changesand the non-linearity of the index versus wavelength relation-ship.1This guide is under the jurisdiction of ASTM Committee C14 on Glass andGlass Products and is the dir
14、ect responsibility of Subcommittee C14.11 on OpticalProperties.Current edition approved Aug. 1, 2018. Published August 2018. Originallyapproved in 2006. Last previous edition approved in 2012 as C1648 12. DOI:10.1520/C1648-12R18.2Metricon is a trademark of Metricon Corporation 12 North Main Street,
15、P.O.Box 63, Pennington, New Jersey 08534.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4The last approved v
16、ersion of this historical standard is referenced onwww.astm.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization estab
17、lished in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.13.1.2 resolution, nas expressed in power of 10, a com-monly used term used to express the accuracy of a t
18、est methodin terms of the decimal place of the last reliably measured digitof the refractive index which is expressed as the negativepower of 10. As an example, if the last reliably measured digitis in the fifth decimal place, the method would be designated a10-5method.3.2 Symbols: n = index of refr
19、action = Abbe-number; a representation of particular relativepartial dispersionsD= 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 atnominally 589.3 nm (which is the mid-point of the dou
20、blet thathas lines at 589.0 nm and 589.6 nm)C = the spectral emission line of hydrogen 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
21、at 480.0 nm4. Significance and Use4.1 MeasurementThe refractive index at any wavelengthof a piece of homogeneous glass is a function, primarily, of itscomposition, and secondarily, of its state of annealing. Theindex of a glass can be altered over a range of up to110-4(that is, 1 in the fourth decim
22、al place) by the changingof an annealing schedule. This is a critical consideration foroptical glasses, that is, glasses intended for use in highperformance optical instruments where the required value of anindex can be as exact as 110-6. Compensation for minorvariations of composition are made by c
23、ontrolled rates ofannealing for such optical glasses; therefore, the ability tomeasure index to six decimal places can be a necessity;however, for most commercial and experimental glasses,standard annealing schedules appropriate to each are used tolimit internal stress and less rigorous methods of t
24、est forrefractive index are usually adequate. The refractive indices ofglass ophthalmic lens pressings are held to 510-4because thetools used for generating the figures of ophthalmic lenses aremade to produce curvatures that are related to specific indicesof refraction of the lens materials.4.2 Disp
25、ersionDispersion-values aid optical designers intheir selection of glasses (Note 1). Each relative partialdispersion-number is calculated for a particular set of threewavelengths, and several such numbers, representing differentparts of the spectrum might be used when designing morecomplex optical s
26、ystems. For most glasses, dispersion in-creases with increasing refractive index. For the purposes ofthis standard, it is sufficient to describe only two reciprocalrelative partial dispersions that are commonly used for char-acterizing glasses. The longest established practice has been tocite the Ab
27、be-number (or Abbe -value), calculated by:D5 nD2 1!/nF2 nC! (1)where vDis defined in 3.2 and nD, nF, and nCare the indicesof refraction at the emission lines defined in 3.2.4.2.1 Some modern usage specifies the use of the mercurye-line, and the cadmium C and F lines. These three lines areobtained wi
28、th a single spectral lamp.e5 ne2 1!/nF2 nC! (2)where veis defined in 3.2 and ne, nF, and nCare the indicesof refraction at the emission lines defined in 3.2.4.2.2 A consequence of the defining equations (Eq 1 and 2)is that smaller -values correspond to larger dispersions. For-values accurate to 1 to
29、 4 %, index measurements must beaccurate to 110-4; therefore, citing -values from less accuratetest methods might not be useful.NOTE 1For lens-design, some computer ray-tracing programs usedata directly from the tabulation of refractive indices over the fullwavelength range of measurement.NOTE 2Beca
30、use smaller -values represent larger physicaldispersions, the term constringence is used in some texts instead ofdispersion.5. Precision, Bias, and Accuracy (see Terminology E456)5.1 PrecisionThe precision of a method is affected byseveral of its aspects which vary among methods. One aspectis the ab
31、ility of the operator to repeat a setting on the observedoptical indicator that is characteristic of the method. Anotheraspect is the repeatability of the coincidence of the measure-ment scale of the instrument and the optical indicator (magni-tude of dead-band or backlash); this, too, varies amongm
32、ethods. A third aspect is the repeatability of the operatorsreading of the measurement scale. Usually, determinations fora single test specimen and for the reference piece should berepeated several times and the resulting scale readings aver-aged after discarding any obvious outliers.5.2 Bias (Syste
33、matic Error):5.2.1 Absolute MethodsTwo of the test methods are abso-lute; the others are comparison methods. The absolute methodsare the prism spectrometer and the apparent depth of micro-scope focus. These yield measures of refractive index of theTABLE 1 Spectral Lines for Measurement of Refractive
34、 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 786.2B656.3C643.8D589.3 587.6 546.1 486.1 480.0D435.8 434.0 404.7AFrom Ref (1).BA later reference (identification not available) lists 789.9 nm for the potassium A line, although referring to Ref (1
35、). 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.CThe wavelength of the corresponding deuterium line is 656.0 nm.DThe two cadmium lines have been recognized for refractometry since Ref (1) was published.C1648 12 (2018)2speci
36、men in air. In the case of the prism spectrometer, whenused for determinations of 110-6, correction to the index invacuum (the intrinsic property of the material) can be calcu-lated from the known index of air, given its temperature,pressure, and relative humidity. The accuracy of the apparentdepth
37、method is too poor for correction to vacuum to bemeaningful. Bias of the prism spectrometer depends upon theaccuracy of its divided circle. The bias of an index determina-tion must not be greater than one-half of the least count ofreading the scale of the divided circle. For a spectrometercapable of
38、 yielding index values accurate to 110-6, the biasmust be not greater than 510-7. Bias of the apparent depthmethod depends on the accuracy of the device for measuringthe displacement of the microscope stage; it is usually appre-ciable smaller than the precision of the measurement, asexplained in 7.6
39、.5.2.2 Comparison MethodsAll of the comparison meth-ods rely upon using a reference material, the index of which isknown to an accuracy that is greater than what can be achievedby the measurements of the given method itself; therefore, thebias of these methods is the uncertainty of the specifiedrefr
40、active index of the reference material, provided that theinstruments scale is linear over the range within which thetest-specimen and the reference are measured. The bias intro-duced by non-linearity of the scale can be compensated bycalibrating the scale over its range with reference pieces havingi
41、ndices that are distributed over the range of the scale. A tableof scale-corrections can be made for ready reference, or acomputer program can be used; using this, the scale reading fora single reference piece is entered and then corrected indicesare generated for each scale reading made for a set o
42、f testspecimens. For a single measurement, scale correction can bemade by first measuring the test specimen and then measuringthe calibrated reference piece that has the nearest index. In thiscase, the scale is corrected only in the vicinity where thereadings are made.5.2.3 Test SpecimenDeviations o
43、f a test specimen from itsideal configuration can contribute a bias. For 110-6refractometry, specimen preparation must be of the highestorder and specimens are tested for acceptability for use. Biasintroduced by a test specimen varies in its manifestation withthe type of test method and nature of th
44、e deviation from ideal.This consideration is addressed in the descriptions of indi-vidual test methods.5.3 AccuracyThe limiting accuracies of the several testmethods are given. The operator must estimate whether andhow much a given test measurement deviates from that limit.The estimate should take i
45、nto account the observed uncertaintyof identifying where to set on the optical indicator (see 7.6, forexample) as well as the precision of such settings. Specificconsiderations are given in the descriptions of the test methods.NOTE 3The Subcommittee did not conduct an Inter-laboratory Study(as norma
46、lly required) to quantify the Precision and Bias of Methodsdiscussed in this Standard. The cited accuracies of the test methods arebased on experience.TEST METHODS6. Becke Line (Method of Central Illumination)6.1 Summary of the MethodNot-too-finely ground par-ticles of the glass for testing are imme
47、rsed in a calibratedrefractive index oil and are examined with a microscope ofmoderate magnification. With a particle in focus, if the indicesof the oil and the glass match exactly, the particle is not seen;no boundary between oil and glass is visible. If the indicesdiffer, a boundary is seen as a t
48、hin, dark line at the boundary ofthe particle with either the particle or the oil appearing lighter.The line appears darker as the indices differ more; however,which material has the higher index is not indicated. When thefocal plane of the microscope is moved above or below theplane of the particle
49、 (usually by lowering or elevating the stageof the microscope), one side of the boundary appears lighterand the other side appears darker than the average brightness ofthe field. When the focus is above the plane of the glassparticle, a bright line next to the boundary appears in themedium of higher index. This is the “Becke line”; conversely,when the focus is below the plane of the particle, the bright lineappears in the medium of lower index. Successive changes ofoil, using new glass particles, lead by trial and error to abra
