1、Designation: C 1648 06Standard Guide forChoosing a Method for Determining the Index of Refractionand Dispersion of Glass1This standard is issued under the fixed designation C 1648; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, th
2、e year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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 of glass, with
3、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 that can be pro
4、vided,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 refractive index
5、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 780m); however, somemethods can be extended to the ultraviolet an
6、d 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 (Abbe type and
7、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 (seeAnnexA1):1.1.2.1 Immersion refractometers,1.1.2.2 Interferometry,1.1.2.3 Ellipsometry, and1.1.2.4 Method of oblique illum
8、ination.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 and health practices and determine the applica-bility of regulatory limitations prior to use.1.3 War
9、ningRefractive 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 manufacturers literature andgeneral info
10、rmation on hazardous chemicals.2. Referenced Documents2.1 ASTM Standards:3E 167 Practice for Goniophotometry of Objects and Mate-rials4E 456 Terminology Relating to Quality and Statistics3. Terminology3.1 Definitions:3.1.1 dispersion, nthe physical phenomenon of the varia-tion of refractive index wi
11、th 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 function of wave-length over a selected wavelength-range; that is, it i
12、s acombined measure of both the amount that the index changesand the non-linearity of the index versus wavelength relation-ship.3.1.2 resolution, nas expressed in power of 10, a com-monly used term used to express the accuracy of a test methodin terms of the decimal place of the last reliably measur
13、ed 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 refraction1This guide is under the jurisdiction of ASTM Committee C14 on
14、 Glass andGlass Products and is the direct responsibility of Subcommittee C14.11 on OpticalProperties.Current edition approved Oct. 1, 2006. Published February 2007.2Metricon is a trademark of Metricon Corporation 12 North Main Street, P.O.Box 63, Pennington, New Jersey 08534.3For referenced ASTM st
15、andards, 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.4Withdrawn.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West C
16、onshohocken, PA 19428-2959, United States.n = Abbe-number; a representation of particular relativepartial dispersionsnD= Abbe-number determined with spectral lines D, C,and Fne= Abbe-number determined with spectral lines e, C8,and F8D = the spectral emission line of the sodium doublet atnominally 58
17、9.3 nm (which is the mid-point of the doublet 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 nmC8 = the spectral emission line of cadmium at 643.8 nm
18、F8 = the spectral emission line of cadmium 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 o
19、f up to1310-4(that is, 1 in the fourth decimal 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 1310-6. Compensation for
20、 minorvariations of composition are made by controlled 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 i
21、nternal stress and less rigorous methods of test forrefractive index are usually adequate. The refractive indices ofglass ophthalmic lens pressings are held to 5310-4because thetools used for generating the figures of ophthalmic lenses aremade to produce curvatures that are related to specific indic
22、esof refraction of the lens materials.4.2 DispersionDispersion-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 migh
23、t be used when designing morecomplex optical systems. 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 longe
24、st established practice has been tocite the Abbe-number (or Abbe n-value), calculated by:nD5 nD1!/nF nC! (1)4.2.1 Some modern usage specifies the use of the mercurye-line, and the cadmium C8 and F8 lines. These three lines areobtained with a single spectral lamp.ne5 ne1!/nF8 nC8! (2)4.2.2 A conseque
25、nce of the defining equations (Eq 1 and 2)is that smaller n-values correspond to larger dispersions. Forn-values accurate to 1 to 4 %, index measurements must beaccurate to 1310-4; therefore, citing n-values from less accu-rate test methods might not be useful.NOTE 1For lens-design, some computer ra
26、y-tracing programs usedata directly from the tabulation of refractive indices over the fullwavelength range of measurement.NOTE 2Because smaller n-values represent larger physical disper-sions, the term constringence is used in some texts instead of dispersion.5. Precision, Bias, and Accuracy (see T
27、erminology E 456)5.1 PrecisionThe precision of a method is affected byseveral of its aspects which vary among methods. One aspectis the ability of the operator to repeat a setting on the observedoptical indicator that is characteristic of the method. Anotheraspect is the repeatability of the coincid
28、ence of the measure-ment scale of the instrument and the optical indicator (magni-tude of dead-band or backlash); this, too, varies amongmethods. A third aspect is the repeatability of the operatorsreading of the measurement scale. Usually, determinations fora single test specimen and for the refere
29、nce piece should berepeated several times and the resulting scale readings aver-aged after discarding any obvious outliers.5.2 Bias (Systematic Error):5.2.1 Absolute MethodsTwo of the test methods areabsolute; the others are comparison methods. The absolutemethods are the prism spectrometer and the
30、apparent depth ofmicroscope focus. These yield measures of refractive index ofthe specimen in air. In the case of the prism spectrometer, whenused for determinations of 1310-6, correction to the index invacuum (the intrinsic property of the material) can be calcu-lated from the known index of air, g
31、iven its temperature,pressure, and relative humidity. The accuracy of the apparentdepth 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
32、 the least count ofreading the scale of the divided circle. For a spectrometercapable of yielding index values accurate to 1310-6, the biasmust be not greater than 5310-7. Bias of the apparent depthmethod depends on the accuracy of the device for measuringthe displacement of the microscope stage; it
33、 is usually appre-ciable smaller than the precision of the measurement, asexplained in 7.6.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 786.2B656.3C643.8D589.3 587.6 546.1 486.1 480.0D435.8 4
34、34.0 404.7AFrom Ref (4).BA later 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.CThe wavelength of the correspon
35、ding deuterium line is 656.0 nm.DThe two cadmium lines have been recognized for refractometry since Ref (4) was published.C16480625.2.2 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 ac
36、hievedby the measurements of the given method itself; therefore, thebias of these methods is the uncertainty of the specifiedrefractive 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
37、intro-duced by non-linearity of the scale can be compensated bycalibrating the scale over its range with reference pieces havingindices 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 sca
38、le reading fora single reference piece is entered and then corrected indicesare generated for each scale reading made for a set of testspecimens. For a single measurement, scale correction can bemade by first measuring the test specimen and then measuringthe calibrated reference piece that has the n
39、earest index. In thiscase, the scale is corrected only in the vicinity where thereadings are made.5.2.3 Test SpecimenDeviations of a test specimen from itsideal configuration can contribute a bias. For 1310-6refracto-metry, specimen preparation must be of the highest order andspecimens are tested fo
40、r acceptability for use. Bias introducedby a test specimen varies in its manifestation with the type oftest method and nature of the deviation from ideal. Thisconsideration is addressed in the descriptions of individual testmethods.5.3 AccuracyThe limiting accuracies of the several testmethods are g
41、iven. The operator must estimate whether andhow much a given test measurement deviates from that limit.The estimate should take into account the observed uncertaintyof identifying where to set on the optical indicator (see 7.6, forexample) as well as the precision of such settings. Specificconsidera
42、tions are given in the descriptions of the test methods.NOTE 3The Subcommittee did not conduct an Inter-laboratory Study(as normally 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
43、 Line (Method of Central Illumination)6.1 Summary of the MethodNot-too-finely ground par-ticles of the glass for testing are immersed 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 matc
44、h exactly, the particle is not seen;no boundary between oil and glass is visible. If the indicesdiffer, a boundary is seen as a thin, 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 mater
45、ial has the higher index is not indicated. When thefocal plane of the microscope is moved above or below theplane of the particle (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
46、 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 ofo
47、il, using new glass particles, lead by trial and error to abracketing of the index of the particle between the pair of oilsthat match most closely (or to an exact match). Visualinterpolation can provide resolution to about one fourth of thedifference between the indices of the two oils. The physical
48、principle underlying the method is that of total internalreflection at the boundary, within the medium of higher index.This is illustrated by a ray diagram, Fig. 1(a). Visual appear-ances are illustrated in Fig. 1(b), Fig. 1(c), and Fig. 1(d), wheredifferent densities of cross-hatching indicate dark
49、er parts of thefield of view. Although calibrated indices are provided for theC- and F-lines, enabling an estimate of a dispersion-value, itmust be taken not to be very accurate.6.2 Advantages and LimitationsThis method uses thesmallest amount of specimen-material and it has the simplestand least expensive method of sample-preparation. Costs ofapparatus and materials, too, are moderate, as is the timeneeded to make a determination; however, the accuracy of themethod is limited to about 5310-4(index-values are lessaccurate for n 1.70).NOTE 4A related tes
