Index of Refraction of Fused-Quartz Glass for Ultraviolet, Visible, And
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Journal of Research of the National Bureau of Standards Vol. 53, No.3, September 1954 Research Pape 2531 Index of Refraction of Fused-Quartz Glass for Ultraviolet, Visible, and Infrared Wavelengths 1 William S. Rodney and Robert J. Spindler The index of refraction of fused-quartz gJass was determined for 24 wavelengths fr om 0.34669 to 3.5078 microns, ll sing the minimum-deviation method . The whole range \\"as covered with a single instrument. The variation in index between samples was determined; no definite variations in dispersion were observed. Possible relations bet wee n purity ancl index are disc ussed. 1. Introduction recent onglO , also produced by the General E lectric Co. T'wo other prisms, A and B, were made from The physical and optical properties of fused the product of the Heraeus Co. in German~ - and quartz glass make its increased use in the optical obtained by Stanley S. Ballard, who made them industry highly desirable whenever it can be readily available for this investigation. A fifth prism was produced in optical quality. In composition it is made by grinding and polishin g windows on the definitely reproducible as compared with optical periphery of a 6-in . disk recently manufactured bv glasses, it is extrem ely stable, and its coefficient of the Nieder Fused Quartz Co., Babso n Park, Mass. thermal expansion is remarkably low. Its trans Ana ther prism was made by placing windows on the miSSlon limits, both in tIle ultraviolet and the in peripher~T of a cylinder of H eraeus fused quartz frared, are nearly the same as for the crystalline (sample C), approximately 4 in . long and 2 in. in material and in addition it is free from do uble refrac diameter. T he seventh sample was made available tion. Some spec imens of this material, particularly by the Coming Glass vVorks; it is known to have from European range finders, are of excellent optical eXLremely good ultraviolet transmission character quality. istics but has a stronger absorption band around 2.6 The refractive index of fused-quartz O"lass has J1. than the other samples. All seven spec imens are beeninvestigated in the ultraviolet and visible regions of near optical quality and free of noticeable striae by Gifford and Shenstone and by Trommsdorff; also and bubbles, except the elisk, which has considerable some measurements were made in the visible region striae. Even ir. this case, however, precise dat.a were by Riedel, by Rinne, and by Watson. All of these obtainable. data, together with some D-line indices by other observers, are referenced, compared, and anal~T se d 3 . Instruments Used in Measurements by Sosman [1].2 For the infrared region some in dices of fused quartz have been computed from re The 'IV atts precision spectrometer was used for the fl ection data but experimentally determined data visible region and the Gaertner precision spec seem limited to three-decimal-place values [2). Some tromcter for the ultraviolct and infrared. The published data [3) indicate different indices for speci visible-region measurements were also repeated on mens obtained from different sources, presumably the Gaertner spectrometer for calibration purpo es. made by somewhat different methods. However, Descriptions of these instruments, with photographs, very little precise and self-consistent information has arc given in previous publications by the authors previously been obtained by use of identical pro [4, 5). Figure 1 is a schematic diagram of the cedures for different wavelength regions and for the Gaertner spectrometer showing some ch anges. di fl'eren t specimens. On both instruments the minimum-deviation method, with its desirable features of high aCCUl"acy and 2. Samples Investigated simplicity of index computations, was used. The spectra used in these measurements were the mercUl"Y and cadmium emissions for wavelengths Seven prisms of fused-quartz glass were used in out to approximately 2.5 J1. and the absorption bands this investigation. One was made over 20 years of polystyrene beyond this region to approximately ago from fused-quartz glass produced by the General 3.5 J1. Electric Co. Its indices in the visible region and its A lead-sulfide detector was used for the emission degree of homogeneity were carefully investigated [3), line spectra and a Golay pneumatic detector for wave and it is used as a refractive-index standard for the lengths in the absorption spectra of polystyrene. precise calibration of refractometers. A second T he incident flux was chopped at approximately 10 prism was made of domestic fused·-quartz glass of cis to stabilize the zero, and the resulting signal were amplified in both cases by a gated amplifier 1 An account of thi, work co ntai lting less ex perimental detail but giving the supplied for the Golay detector. The amplified essential resnlts is pntlished in J. Opt. Soc. Alll. 44,677 (1954). 2 Fignres in brackets indicate the li teraL ure references at the end of this paper. signal was recorded on a recording potentiometer. 185 A R lines of the visual spectrum as determined by the use of the detector. The spectra are then scanned to identify other wavelengths and to determine the approximate scale position of the minimally deviated beam for each known wavelength. Once an approximate position is determined, hand settings are made by observing; the scale position for maximum or minimum deflec M' tion of the recorder pen. The scale readings are averaged for greater precision. 5 . Data Index determinations were made on Heraeus. prisms A and B at or very near 24° and 31 ° C. The average temperature of the room was controlled within ± 0.2 deg C and was determined frequently by means of a thermometer having its bulb near the prism. Temperature coefficients were then com puted and small corrections were made to adjust to exact temperatures of 24° and 31 ° C. On the other prisms, index determinations were made near 24° C FIGUHE 1. Modified Gaertner infrared spectrometer . only. 81 and Sz, entrance and exit slits, respectively; S, global' source; IVC condensing The refractive indices of the Heraeus B prism at mirror; ~I /, collimator mirror; 'l"', telescope mirror; OJ chopper; D , detector; A, amplifier; R , recorder. The prism P is rotated at one-half the rota tion ra te of T 24 ° C have been represented by means of the dis by use of t he gears G I and G,. persion formula [6] (>- expressed in microns) 84.06224 2=') 979864+ 0.008777808 (1 ) 4. Experimental Procedure n -"' >-2- 0.010609 96.00000- >-2 The index of refraction for visible wavelengths is and the goodness of fit of the formula is shown by the first determined on the \Vatts instrument by the small residuals, B - c, as tabulated in table 1, and usual minimum-deviation method. The errors for by their nearly random distribution. The predomi these data do not exceed ± 1 X 10- 5• The deviation nance of negative residuals for the region 3 to 3.5/L angles are observed and recorded. With the aid of indicates that further improvement can be made. an auxiliary telescope the prisms are then leveled HO'wever, no least-squares solution was attempted and centered with respect to the optical axes of the because at >- = 3.4188j.L, where the maximum negative Gaertner instrument. A second telescope is used residual was obtained, there is observational uncer to set the prism, for a given line, usually the 0.6438 tainty because of the very broad nature of this j.L line of cadmium, at its minimum-deviation posi absorption band, which tends to make the wave tion. The table is now clamped in this position. length of the minimum difficult to determine. Note The mirror acting as the telescope objective is that a positive residual was obtained for the Corning brought into the beam so that an image of this line prism at this wavelength (last. column, table 1). falls on the exit slit, causing a deflection of the The indices computed by formula (1) are, in potentiometer pen. The scale position correspond general, better than the observed values, and accord ing to maximum deflection of the pen is observed ingly table 2 gives such indices for selected regular for several pointings. The instrument is now set intervals of wavelength. and clamped at the average of these readings. A In order to compare the indices for the various system of gears, designed to maintain minimum prisms, the indices as computed by formula (1 ) deviation by causing the prism table to rotate at have been subtracted from all othors. These one-half the rotation rate of the telescope and differences are listed in table 1 and plotted in figure 2. detector, is now engaged and the prism table clamps From the variation shown in this figuro, the dif released. ferences in index of different samples seem clearly The design of the Gaertner spectrometer, as established. It will bo noticed that the Heraeus, adapted for nonvisible radiation, does not allow Nieder, and Corning samples agree in general in double deviation, or even the position of the undevi refractivity with the data of Trommsdorff and of ated beam, to be measured. Consequently, it is Gifford. Gifford's data woro taken on the fused necessary to compute the average reading for the quartz glass made by Gifford and Shenstone, 'with undeviated beam. This is done by applying a few particular attempts at purity of material. It seems deviations, as observed visually on the Watts instru that the Heraeus B and Corning samples, may ment, to the scale readings on the Gaertner for possibly be freer of traces of other substances, minimum-deviation settings of the corresponding because the common basic impurities would tend 186 TABLE 1.