A Simple Grating Spectrometer for the Infrared

Robert J. Bell and T. E. Gilmer, Jr.

This paper describes a simple and relatively inexpensive modification of the Perkin-Elmer Model 112spectrometer for use as a grating instrument. The grating spectrometer has been used to 180 cm-l wave-numbers in single-pass operation and in double-pass use data have been obtained to 260 cm '. Thewavenumber spread at 200 cm-' was about 1.5 cm-'; this is gratifying in that the original thermocoupledetector was used.

Introduction

This grating monochromator design is based on thePerkin-Elmer Model 112 infrared spectrometer and isan inexpensive modification for grating use. It is pos-sible to change from a prism to a grating instrument orvice versa in a few hours simply by removing the prismand installing two easily aligned pieces. The instru-ment as built has a useful wavenumber range from 360cm-' to 180 cm-', can be used with a liquid heliumtemperature range dewar system, and has dry gasflushing capability. The band spread throughout thewavenumber range is about 1.5 cm-' for this single-beam, single-pass monochromator.

Lord and McCubbin' described a far infrared spec-trometer modification of the Perkin-Elmer Model 12-Cspectrometer which was for work to 50 cm-' wave-numbers. However, the easy and inexpensive modi-fication with the above characteristics also recommendsthe monochromator described in this paper.

The components of the Perkin-Elmer Model 112which need to be removed are: the prism in its holder,the plane mirror before the exit slit, and the planemirror and its holder in the source housing. Thechopper normally used with the Model 112 as well asthe double-pass mirrors may be left in place provided abaffle is used to block them from the radiation beam.It is not necessary to change the prism, Littrow mirror,or off-axis parabolic mirror adjustments between grat-ing and prism operation. It is not necessary to modifythe slits, slit drive, or wavelength drive mechanism.These are decided advantages in lowering modificationcosts and time.

Both authors were with Virginia Polytechnic Institute, Blacks-burg, Virginia, when this work was done. R. J. Bell is nowat Southwest Research Institute, San Antonio, Texas.

Received 28 April 1964.

Apparatus

Figure 1 is an optical diagram of the grating spectro-meter. The source is a globar rod operated at ap-proximately 200-W input, the same as in the prism in-strument. The part numbers referred to in this sec-tion are shown in Fig. 1.

A special rough surface chopper (part No. 2), de-signed for simple insertion, is described in anotherarticle by the authors. 2 The rough surface oscillatesabout a vertical axis between two fixed positions. Oneposition, with the spherical mirror, focuses the longwavelength radiation on the entrance slit, and in theother position the beam focus is moved to the side ofthe slit. This rough surface plate has the same di-mensions as the source plane mirror normally used inthe Perkin-Elmer instrument, and, with the properchoice of surface roughness, this chopper can also beused with the prism instrument. An additional ad-vantage of this mode of operation is that the beam ischopped before going to the sample and the dispersionelement, thus eliminating thermal effects in the sampleand the monochromator. The spherical mirror in thesource housing is in the same position for either gratingor prism operation.

The entrance slit (part No. 6) required no modifica-tion since two 2-mm slit-widths were sufficient for workto 180 cm-'.

The off-axis parabolic mirror (part No. 7) is in thesame position as in the prism instrument and, since thismirror is tedious to adjust in double-pass prism use, notchanging its alignment is an important advantage.

The grating (part No. 8) is in a special mount whichbolts into the same hole provided for the prism mount.Figure 2 is a sketch of the grating, grating mount, anddrive mechanism. The grating was a Bausch & Lombechellette with a 45-/i blaze wavelength and 260 45'blaze angle. By placing the grating in the specialmount forward of the former prism position, the Per-kin-Elmer monochromator housing cover could be used.

February 1965 / Vol. 4, No. 2 / APPLIED OPTICS 157

Fig. 1. Optical schematic of the modified Perkin-Elmer Model112 spectrometer. 1. globar source, 2. rough surface chopper, 3.spherical mirror, 4. removable stop, 5. cryostat with sample, 6.entrance slit, 7. off-axis parabolic mirror, 8. grating, 9. Littrowmirror mount, 10. reflection filter, 11. exit slit, 12. triple mirror

holder, 13. ellipsoidal mirror, 14. thermocouple detector.

The ellipsoidal mirror and thermocouple detectorwere left as aligned for the prism use with a 1-mm CsIwindow on the thermocouple vacuum housing.

Single-Pass Grating OperationFigure 3 is a typical trace of the signal-vs-wave-

number performance of the spectrometer. These datawere obtained for a 3.6-m path length at 30% relativehumidity at 250 C. The radiation filtering systememployed the rough surface chopper,2 one piece ofBaF2, and additional reststrahlen materials as indicatedin the caption. The slit-widths used are also listed inthe legend. The approximate wavenumber calibra-tion was based on atmospheric water vapor.5

Double-Pass Grating Operation

In order to establish the feasibility of double-passgrating operation the following experiment was per-formed.

All optical components were in the same positionsdescribed in the single-pass modification except asmentioned. An aluminum rough surface (400 grit)was in the rough surface chopper position in the sourcehousing with the beam focus fixed on the entrance slit.

Fig. 2. Sketch of grating, grating mount, and drive mechanism.

Also, by using a rigid rod coupled with the Littrow mir-ror mount to drive the grating, it was possible to leavethe Littrow mirror in adjustment for prism use.

The coupling rod from the Littrow mirror to thegrating mount has ball bearing swivels at each end andhas proven reliable as shown by excellent reproducibil-ity during several hundred hours of use. With theabove arrangement it was possible to rotate the gratingthrough about 18° with no modifications in the originaldrive system.

After returning to the off-axis parabolic mirror, thebeam is reflected from a reststrahlen filter into the exitslit. This filter is in a holder which was designed as areplacement for the mirror holder attached to theprism mount. For the entire 360 cm-' to 180 cm-'region a flat piece of barium fluoride had sufficientlyhigh reflectivity3 4 to be left in the beam at all times.Also, barium fluoride has its minimum reflectivity near22.5 , (i - 450) which is ideal for use with a 45-a blazewavelength echellette grating because it is effective ineliminating the second-order, second-harmonic radia-tion near the blaze angle.

The triple-mirror holder (part No. 12) furnished bythe Perkin-Elmer Corporation had mounted in it allthe reststrahlen materials needed between 340 cm-'and 180 cm-'. The reststrahlen materials, NaF,BaF2 , and NaCl, were used between 340 and 258 cm-',265 and 197 cm-', and 200 and 180 cm-', respectively,with the beam purity better than 95% at all wave-numbers as determined by using NaCl and KBr astransmission filters. 13,4

180 185 190 195 200 210 220 230 240 260 280 300WAVE NUMBER (CM-')

Fig. 3. Signal vs wavenumber for single-pass operation, showingatmospheric water vapor absorption. The wavenumber spreadwas approximately 1.5 cm-' from 180 cm-' to 340 cm-'. From178 cm-' to 197 cm-', NaCl and BaF2, 2.0-mm slit-widths; 197cm-' to 214 cm-', two pieces BaF2, 2.0-mm slit-width; 214 cm'to 265 cm-', two pieces BaF2, 1.4-mm slit-width; and 258 cm'

to 340 cm-', NaF and BaF2, 0.8-mm slit-width.

-U ;>W U E M BE R (CMWAVE NUBE (C-I)

Fig. 4. Signal vs wavenumber for double-pass operation, show-ing atmospheric water vapor absorption.

158 APPLIED OPTICS / Vol. 4, No. 2 / February 1965

320 340

50 2 i0 -- 280 290WAVE NUMBER (M-I)

330 340

II

I

I2

-o

-

I

j;t:S7-

I

II

7

The mask was removed from the Perkin-Elmer cylindri-cal chopper and mirror assembly, and this chopper wasused to interrupt the beam between grating passes.Instead of BaF2 in the mirror position before the exitslit, a plane first surface mirror was used.

For the test between 260 cm-' and 340 cm-' a NaFreststrahlen plate was inserted in the triple mirrorholder and the sample water data obtained with 1.5-mmslit-widths are shown in Fig. 4.

Further studies of the 200 cm-' region with a BaF2instead of NaF indicated stronger filtering would beneeded to increase the beam purity to better than 95%.

The authors wish to thank Luther Barnett of Vir-ginia Polytechnic Institute and his staff for assisting in

the making of the mechanical parts for the mono-chromator. Also we are indebted to Herbert Kohler ofSouthwest Research Institute and his staff for prepara-tion of the figures.

References1. R. C. Lord and T. K. McCubbin, Jr., J. Opt. Soc. Am. 47,

689 (1957).2. R. J. Bell and T. E. Gilmer, Jr., Appl. Opt. 4, 45 (1965).3. W. Kaiser, W. C. Spitzer, R. H. Kaiser and L. E. Howarth,

Phys. Rev. 127, 1950 (1960).4. Y. Yamada, A. Mitsuishi, and H. Yoshinaga, J. Opt. Soc.

Am. 52, 14 (1962).5. K. N. Rao, R. V. deVore, and E. K. Plyler, J. Res. Natl. Bur.

Std. A87, 351 (1963).

Mee~t5 t0ingnjs :Ca letnd arm

February3 OSA Pittsburgh Sect. Mtg., Spectrometer Design by

Charles H. Church, Westinghouse Electric Corp.4-5 12th Ann. Conf. Western Spectroscopy Assoc.,

Asilomar, California8-12 50th Norelco X-ray Analytical School on X-ray Dif-

fraction, Diffractometric and Spectrographic Tech-niques, Willard Hotel, Washington, D.C. PhilipsElectronic Instruments, 750 S. Fulton Ave., MountVernon, N. Y. 10550

11 OSA San Diego Sect. Mtg.17 OSA Chicago Sect. Mtg., Optical Staining by Walter

McCrone of McCrone Assoc. Anton Weigandt, 1434W. Catalpa Ave., Chicago 40

26-27 APS, K. K. Darrow, Columbia U., New York City 27March

1-5 16th Pittsburgh Conf. on Anal. Chemistry and Appl.Spectroscopy, Pittsburgh, Pennsylvania W. G.Fateley, Mellon Inst., 4400 Forbes Ave., Pittsburgh,Pa.

3 OSA Pittsburgh Sect. Mtg., speaker Thomas M.Donahue, U. of Pittsburgh

11 OSA San Diego Sect. Mtg., Information retrieval byW. Lewis Hyde

20 OSA Chicago Sect. Mtg., Optical Metrology, all dayseminar at IIT Anton Weigandt, 1434 W. CatalpaAve., Chicago 40

28-Apr. 2 ASP Ann. Mtg., Amer. Cong. on Surveying andMapping, Wash., D.C.

31-Apr. 2 Optical Society of America Spring Meeting, Statler-Hilton Hotel, Dallas, Tex. M. E. Warga, OSA,1155 16th St. N.W., Wash., D.C. 20086

April- Symp. on Inhomogeneity in Glass, Sheffield D. Hawks-

worth, Soc. of Glass Technology, Thornton, 20 HallamGate Rd., Sheffield 10, U. K.

- 14th Pugwash Conf. on Science and World Affairs,Italy J. Rotblat, 8 Asmara Rd., London, N.W. 2,U. K.

7 OSA Pittsburgh Sect. Mtg., What can a fish see? byS. Q. Duntley

7 OSA San Diego Sect. Mtg., An Astronautics spacesystem simulator by Robert Ackley of Astronautics

9-10 APS N.Y. State Section Symp. and Spring Mtg. onModern Developments in Magnetism, IBM Re-search Center, Yorktown Heights W. J. Turner,IBM Research Center, P.O. Box 218, YorktownHeights, N. Y. 10598

20-22 Internatl. Symp. on System Theory, PolytechnicInst. of Brooklyn Symp. Comm., Polytechnic Inst.of Brooklyn, 333 Jay St., Brooklyn, N.Y. 11201

21 OSA Chicago Sect. Mtg., Projection TV Systems byJohn R. Miles of John R. Miles Corp. AntonWeigandt, 1434 W. Catalpa Ave., Chicago 40

21-23 IEEE/OSA Symp. on Optimization Techniques,Carnegie Institute of Tech., Pittsburgh A. Levi,Dept. of Electr. Engr., CIT, Pittsburgh, Pa. 15213

21-23 3rd Internatl. IEEE Conf. on Nonlinear Magnetics,Wash., D.C. E. W. Pugh, IBM Components Div.,Poughkeepsie, N. Y. 12602

May5 OSA Pittsburgh Sect. Mtg., Interference Filters and

Thin Optical Films by R. J. Pegis, St. John FisherCollege, Rochester, N.Y.

5 OSA San Diego Sect. Mtg., Transformation of irbackgrounds to electrical noise by George Car-michael of Astronautics

5-7 1965 Microwave Theory & Techniques Symp., JackTar Harrison Hotel, Clearwater J. E. Pippin,Sperry Microwave Electronics Co., Box 1828, Clear-water, Fla.

13-14 Symp. on Signal Transmission & Processing, Colum-bia U., New York City Omar Wing, Dept. Elec.Engr., Columbia U., New York City 10027

19 OSA Chicago Sect. Mtg., Discussion of some papersgiven at ICO, Japan, by Philip N. Slater of IITAnton Weigandt, 1434 W. Catalpa Ave., Chicago 40

27-31 SPSE Ann. Conf. on Frontiers in Photography,Sheraton-Cleveland Hotel, Cleveland, Ohio SPSE,P.O. Box 1609, Main P.O., Washington, D.C.20013

23-27 IFIPS, 3rd Congress, N.Y.C. I. L. Auerbach, Auer-bach Corp., 1635 Arch St., Phila., Pa. 19103

24-June 1 IAMA Internatl. Conf. on Cloud Physics, Tokyo andSapporo, Japan H. Weickmann, M41eteorological Br.,Signal Corps Engr. Labs., Ft. onmouth, N.J.07701

31-June 2 5th Australian Spectroscopy Conf., Perth A. J.Parker, Dept. of Chem., U. of Western Australia,Nedlands, Australia

June- 8th Internatl. Color Mtg., Lucerne, Switzerland

Centre d'Information de la Couleur, 23, rue Notre-Dame-des-Victoires, Paris, France

- 20th Ann. Cong., Canadian Assoc. of Physicists,Vancouver, B.C.

2 OSA San Diego Sect. Mtg., annual address by out-going president, John M. Hood, Jr. of U.S.N.Electronics Lab.

9-11 MISFITS Conf. (Mellon Inst. Symp. on Far InfraredTranspose Spectroscopy), Pittsburgh W. G. Fateley,Mellon Institute, Pittsburgh, Pa.

13-18 ASTM 68th Ann. Mtg., Purdue Univ., Lafayette,Indiana

14-17 16th Ann. SAS Mid-America Symp. on Spectroscopy,Sheraton-Chicago, A. M. Palmer, Continental CanCo., 7622 S. Racine Ave., Chicago, Ill. 60620

14-18 Ann. Symp. Molecular Structure and Spectroscopy,Ohio State U. K. N. Rao, Dept. Physics, OSU, 174W. 18 Ave., Columbus, Ohio 43210

16 OSA Chicago Sect. Mtg., Activities and Responsibili-ties of Sears Color Control Lab. by N. R. Pugh ofSears Roebuck Anton Weigandt, 1434 W. CatalpaAve., Chicago 40

21-25 Program on Principles of Color Technology by FredW. Billmeyer, Jr., Rensselaer Polytechnic Inst.A. A. K. Booth, Rensselaer Polytechnic Inst., Troy,N.Y. 12181

February 1965 / Vol. 4, No. 2 / APPLIED OPTICS 159

W, �n M I em� ��KAM um I

Norbert J. Kreidl, director of Materials Research and Develop-ment at Bausch & Lomb, Inc., will help establish a graduateschool of ceramics in Rutgers University's new $5-million Engi-neering Center, with the appointment of Professor of Ceramicsfor the school year 1964-65. He will continue to direct MaterialsR&D on a part-time basis at B&L until returning to his full

duties in 1965.

James Ullom, previously manager of applied optics at Itek, is nowmanager of optical operations, with responsibility for opticalmanufacturing, for J. W. Fecker Division of American Optical

Company.

George A. Economou, director of Development at J. W. FeckerDivision of American Optical Company, has become manager of

Fecker's optical instrumentation.

H. Philip Hovnanian, manager of the Medical Science Depart-ment of Aveo R&D Division, has been elected a Fellow of the

Institute of Physics of London.

160 APPLIED OPTICS / Vol. 4, No. 2 / February 1965