CHAPTER 5 DISPERSIVE PRISMS AND GRATINGS

George J . Zissis En y ironmental Research Institute of Michigan Ann Arbor , Michigan

5 . 1 GLOSSARY

A p prism angle B prism base

D p angle of minimum derivation d grating constant E irradiance N number of slits n refractive index p order number

RP resolving power r angles

W prism width b angle g angle

5 . 2 INTRODUCTION

Spectroradiometers (Fig . 1) are radiometers designed specifically to allow determination of the wavelength distribution of radiation . This category of measurement systems usually consists of those in which separation of the radiation into its spectral components , or dispersion , is accomplished by the use of an optical element possessing a known functional dependence on wavelength—specifically , prisms and dif fraction gratings . (Interferometers can also provide spectral dispersion as is discussed in the chapter on Interferometer Instruments by P . Hariharan . )

5 . 3 PRISMS 1 , 2 , 3

The wavelength dependence of the index of refraction is used in prism spectrometers . Such an optical element disperses parallel rays or collimated radiation into dif ferent angles from

5 .1

5 .2 OPTICAL ELEMENTS

FIGURE 1 Basic spectroradiometer .

the prism according to wavelength . Distortion of the image of the entrance slit is minimized by the use of plane wave illumination . Even with plate wave illumination , the image of the slit is curved because not all of the rays from the entrance slit can traverse the prism in its principal plane . A prism is shown in the position of minimum angular deviation of the incoming rays in Fig . 2 . At minimum angular deviation , maximum power can pass through the prism . For a prism adjusted to the position of minimum deviation ,

r 1 5 r 2 5 A p / 2 (1) and

i 1 5 i 2 5 [ D p 1 A p ] / 2 (2)

where D p 5 angle of minimum deviation for the prism A p 5 angle of the prism

r 1 and r 2 5 internal angles of refraction i 1 and i 2 5 angles of entry and exit

The angle of minimum deviation D p varies with wavelength . The angular dispersion is defined as dD p / d l , while the linear dispersion is

dx / d l 5 F dD p / d l (3)

where F is the focal length of the camera or imaging lens and x is the distance across the image plane . It can be shown 1 that

dD p / d l 5 ( B / W )( dn / d l ) (4)

DISPERSIVE PRISMS AND GRATINGS 5 .3

FIGURE 2 Elementary prism spectrometer schematic . W is the width of the entrance beam ; S p is the length of the prism face ; and B is the prism base length .

where B 5 base length of the prism W 5 width of the illumination beam n 5 index of refraction

dx / d l 5 F [ B / W ][ dn / d l ]

The resolving power RP of an instrument may be defined as the smallest resolvable wavelength dif ference , according to the Rayleigh criterion , divided into the average wavelength in that spectral region . The limiting resolution is set by dif fraction due to the finite beam width , or ef fective aperture of the prism , which is rectangular . Thus ,

RP p 5 B [ dn / d l ] (5)

If the entire prism face is not illuminated , then only the illuminated base length must be used for B .

5 . 4 GRATINGS

A grating is an n -slit system used in Fraunhofer dif fraction with interference arising from division of the incident , plane wave front . Thus it is a multiple beam interferometer .

p l 5 d (sin θ 1 sin f ) (6)

5 .4 OPTICAL ELEMENTS

where p 5 order number ( 5 0 , 1 , 2 , . . . ) of the principal maxima d 5 the grating constant or spacing (the distance between adjacent slits) f 5 angle of incidence θ 5 angle of dif fraction w 5 width of any one slit

The most common case is f 5 0 , so that

p l 5 d sin θ (7)

and the irradiance distribution is

E 5 E o h sin (( π w sin θ ) / l ) / (( π w sin θ ) / l ) j 2

3 h sin (( N π d sin θ ) / l ) / sin (( π d sin θ ) / l ) j 2 (8)

where N is the number of slits or grooves . This equation is more often written as :

E 5 E 0 [(sin b ) / b ] 2 [(sin N g ) / sin g ] 2 (9)

which can be considered to be

E 5 (constant) 3 (single-slit dif fraction function)

3 ( N -slit interference function) (10)

These considerations are for unblazed gratings . For a dif fraction grating , the angular dispersion is given (for angle f constant) by

dD g / d l or d θ / d l 5 p / ( d cos θ ) (11)

The resolving power is given by RP g 5 pN (12)

5 . 5 PRISM AND GRATING CONFIGURATIONS AND INSTRUMENTS

Classical

There are several basic prism and grating configurations and spectrometer designs which continue to be useful . One of the oldest spectrometer configurations is shown in Fig . 3 . 1 Reflective interactions and prism combinations are used in Figs . 4 , 5 , and 6 . Dispersion without deviation is realized in Figs . 7 and 8 , while half-prisms are used in Fig . 9 in an arrangement which uses smaller prisms but still attains the same beam width . A few classical prism instrumental configurations are shown in Figs . 10 , 11 , and 12 . Multiple-pass prism configurations are illustrated in Figs . 13 and 14 . 4 , 5

A well-known example of a single beam double-pass prism infrared spectrometer was the Perkin-Elmer Model 112 instrument shown in Fig . 15 . Infrared radiation from a source is focused by mirrors M 1 and M 2 on the entrance slit S 1 of the monochromator . The radiation beam from S 1 , path 1 , is collimated by the of f-axis paraboloid M 3 and a parallel beam traverses the prism for a first refraction . The beam is reflected by the Littrow mirror M 4 , through the prism for a second refraction , and focused by the paraboloid , path 2 , at the corner mirror M 6 . The radiation returns along path 3 , traverses the prism again , and is returned back along path 4 for reflection by mirror M 7 to the exit slit S 2 . By this double dispersion , the radiation is spread out along the plane of S 2 . The radiation of the frequency interval which passes through S 2 is focused by mirrors M 8 and M 9 on the thermocouple

DISPERSIVE PRISMS AND GRATINGS 5 .5

FIGURE 3 Bunsen-Kirchhof f spectrometer . An illuminated scale is reflected from the prism face into the telescope .

TC . The beam is chopped by CH , near M 6 , to produce a voltage (at the thermocouple) which is proportional to the radiant power or intensity of the beam . This voltage is amplified and recorded by an electronic potentiometer . Motor-driven rotation of Littrow mirror M 4 causes the infrared spectrum to pass across exit slit S 2 permitting measurement of the radiant intensity of successive frequencies .

Gratings can be used either in transmission or reflection . 6 Another interesting variation comes from their use in plane or concave reflection form . The last was treated most completely by Rowland , who achieved a useful combination of focusing and grating action . He showed that the radius of curvature of the grating surface is the diameter of a circle (called the Rowland circle) . Any source placed on the circle will be imaged on the circle ,

FIGURE 4 Wadsworth constant-deviation , prism-mirror arrangement . The beam enters the prism at minimum deviation and emerges displaced but not deviated from its original direction .

5 .6 OPTICAL ELEMENTS

FIGURE 5 Amici prism . The central ray D enters and leaves parallel to the base . The C and F rays are deviated and dispersed .

FIGURE 6 Pellin – Broca prism . The prism is equivalent to two 30 8 prisms , ABC and BED , and one 45 8 prism , DEC , but is made in one place . The beam shown , entering at minimum deviation , emerges at 90 8 deviation to its entrance direction .

FIGURE 7 Zenger prism . The central ray D is undeviated . The C and F rays are deviated and dispersed .

FIGURE 8 Wernicke prism . This arrangement is essentially two Zenger prisms , back-to-back .

DISPERSIVE PRISMS AND GRATINGS 5 .7

FIGURE 9 Young – Thollon half prisms . The passage of a beam at minimum deviation is shown .

with dispersion , if the rulings are made so that d is constant on the secant to the grating-blank (spherical) surface . The astigmatism acts so that a point source on a Rowland circle is imaged as a vertical line perpendicular to the plane of the circle . Rowland invented and constructed the first concave grating mounting , illustrated in Fig . 16 . 1

If dispersion is suf ficiently large , one may find overlapping of the lines from one order with members of the spectra belonging to a neighboring order . Errors and imperfections in the ruling of gratings can produce spurious images which are called ‘‘ghosts . ’’ Also , the grooves in a grating can be shaped so as to send more radiation along a preferred direction corresponding to an order other than the zero order . Such gratings are said to be blazed in that order . These issues and many more involved in the production of gratings by ruling engines were thoroughly discussed by Harrison in his 1973 paper ‘‘The Dif fraction Grating—An Opinionated Appraisal . ’’ 7

Six more grating configurations 1 which are considered to be ‘‘classics’’ are :

1 . Paschen - Runge , illustrated in Fig . 17 . In this argument , one or more fixed slits are placed to give an angle of incidence suitable for the uses of the instrument . The spectra are focused along the Rowland circle P P 9 , and photographic plates , or other detectors , are placed along a large portion of this circle .

FIGURE 10 Infrared spectrograph of the Littrow-type mount with a rock salt prism .

5 .8 OPTICAL ELEMENTS

FIGURE 11 Mirror spectrometer with two choices of the location of the image . Arrangement ( b ) leads to smaller aberrations than arrangement ( a ) and is used in the Czerny-Turner mount .

2 . Eagle , shown in Fig . 18 . This is similar to the Littrow prism spectrograph . The slit and plate holder are mounted close together on one end of a rigid bar with the concave grating mounted on the other end .

3 . Wadsworth , shown in Fig . 19 . The Rowland circle is not used in this mounting in which the grating receives parallel light .

4 . Ebert - Fastie , shown in Fig . 20 . The Ebert-Fastie features a single , spherical ,

FIGURE 12 Pfund mirror . The use of a plane mirror to avoid astigmatism in the use of a paraboloidal mirror .

DISPERSIVE PRISMS AND GRATINGS 5 .9

FIGURE 13 Double-pass monochromator .

FIGURE 14 Perkin-Elmer Model 99 double-pass monochromator .

FIGURE 15 Perkin-Elmer Model 112 single-beam double-pass infrared spectrometer .

5 .10 OPTICAL ELEMENTS

FIGURE 16 Rowland mounting of the concave grating . The grating plate-holder bar , which slides on the two perpendicular ways , is shown in two positions , GP and G 9 P 9 . The slit SI and Source S remain fixed .

collimating mirror and a grating placed symmetrically between the two slits . The major advantage of the Ebert system is the fact that it is self-correcting for spherical aberration . With the use of curved slits , astigmatism is almost completely overcome .

5 . Littrow , shown in Fig . 10 . The Littrow system has slits on the same side of the grating to minimize astigmatism . An advantage of the Littrow mount , therefore , is that straight slits can be used . In fact , such slits may be used even for a spherical collimating mirror if the aperture is not too large . Its greatest disadvantage is that it does not correct for spherical aberration—not too serious a defect for long focal-length / small-aperture instruments . If an of f-axis parabola is used to collimate the light , aberrations are greatly reduced .

6 . Pfund , shown in Figs . 12 and 21 . This is an on-axis , Pfund-type grating instrument . 5 Incident infrared radiation , focused by a collimating lens on the entrance slit and modulated by a chopper , passes through the central aperture of plane mirror M 1 . Reflected by the paraboloidal mirror P 1 , it emerges as a parallel beam of radiation , which is reflected by mirror M 1 to the grating . The grating is accurately located on a turntable , which may be rotated to scan the spectrum . From the grating , the dif fracted beam , reflected by mirror M 2 , is focused by a second paraboloid P 2 through the central aperture of mirror M 2 to the exit slit . The emerging beam is then focused by the ellipsoidal mirror M 3 on the detector .

An of f-axis , double-pass grating instrument is illustrated in Fig . 22 . 6 Combinations of prisms and gratings are not uncommon . An illustrative and complex

prism-grating , double-monochromator spectrometer designed by Unicam Instruments , Ltd . is shown in Fig . 23 . 5 The prism monochromator had four interchangeable prisms , and the grating monochromator had two interchangeable gratings . The two monochromators ,

DISPERSIVE PRISMS AND GRATINGS 5 .11

FIGURE 17 Paschen-Runge mounting of the concave grating . Sl is the slit , G is the grating , and S is the light source .

FIGURE 18 Eagle mounting on the concave grating . Sl is the slit , G is the grating , S is the light source , and P is the plate holder .

5 .12 OPTICAL ELEMENTS

FIGURE 19 Wadsworth mounting of the concave grating . Sl is the entrance slit , G is the concave grating , M is the concave mirror , P is the plate holder , and AB is the rail for the plate holder . To minimize aberrations , one must locate the slit close beside the grating .

FIGURE 20 Ebert mounting of the plane grating designed by Fastie . Sl is the entrance slit , G is the grating , M is the concave mirror , and P is the photographic plate . The horizontal section is at the top and the vertical section is at the bottom .

DISPERSIVE PRISMS AND GRATINGS 5 .13

FIGURE 21 On-axis Pfund grating spectrograph .

ganged by cams which are linear in wave number , were driven by a common shaft . The instrument could be used either as a prism-grating double monochromator , or as a prism spectrometer by blanking the grating monochromator . Gratings , prisms , and cams could be automatically interchanged by means of push buttons . Magnetically operated slits , programmed by a taped potentiometer , provided a constant energy background . A star-wheel , time-sharing , beam attenuator was used in the double-beam photometer .

Contemporary

In recent years there has been more attention paid to total system design and integration for specific purposes and applications , as for analytical atomic and molecular spectro- scopy in analytical chemistry . Thus the conventional dispersive elements are often used in the classical configurations with variations . Innovations have come especially in designs tailored for complete computer control ; introduction of one- and two-dimensional detector arrays as well as new detector types (especially for signal matching) ; the use of holographic optical elements either alone or combined with holographic gratings ; and special data-processing software packages , displays , and data storage systems . This is the case also for interferometric systems as discussed in the chapter on Interferometer Instruments by P . Hariharan .

Some examples found by a brief look through manufacturers’ literature and journals

FIGURE 22 Of f-axis , double-pass grating spectro- graph .

TABLE 1 Examples of Prism / Grating Spectroradiometers

Manufacturer Comments

ARC (Acton Research Corp . ) , Acton , Mass . Czerny-Turner or Rowland systems with triple indexable Vac UV / IR gratings

ARIES (Acton Research Instrument & Equipment Services Inc . ) , QEI (Quantum Electronics Instruments Inc . ) , Concord , Mass .

Czerny-Turner variation with double or triple selectable gratings for 165-nm to 40- m m regions

Beckman Instruments Inc ., Fullerton , Calif . DU Series 60 and 70 modular construction , computer- controlled spectrophotometers for analytical applications

C VI Laser Corp ., Albuquerque , N . Mex . Digikrom Monochrometers , 1 / 8- , 1 / 4- , and 1 / 2-m Czerny-Turner grating systems , 186 nm – 20 m m

Cary / Varian Instrument Group , San Fernando , Calif . Cary 1 , 3 , 4 , and 5 spectrophotometers for UV-Vis-IR ; double beam , dual chopper / grating Littrow systems ; attachments (e . g ., reflectance) and applications software

CI Systems Ltd ., New York City , N . Y . and Israel CVF spectroradiometers for 0 . 4- to 20- m m scan Infrared Systems , Inc ., Orlando , Fla . CVF spectroradiometer Instruments SA , Inc ., J-Y Optical Systems , Edison , N . J . Monochrometers , spectrometers for UV-Vis-IR ,

holographic gratings in Czerny-Turner or concave aberration-corrected holographic gratings and Rowland mounts ; single and double pass ; imaging spectrographs

LECO Corp ., St . Joseph , Mich . ICP (Inductively Coupled Plasma) spectrometer system with Pachen-Runge mount concave grating followed by an Echelle and a linear detector array

Leeman Labs , Inc ., Lowell , Mass . ICP system with a fixed echelle grating followed by a prism with crossed order dispersion and scanned photomultipliers or detector arrays

McPherson , Division of SI Corp ., Acton , Mass . Double / triple monochrometers , spectroradiometers using gratings and / or prisms in Seya-Namioka , Czerny-Turner (C-T) , crossed C-T , or Rowland configurations

Minirad Systems , Inc ., Fairfield , Conn . CVF and discrete filters in spectroradiometers for field measurements , 0 . 2 to 30 m m

Optometrics Corp ., Ayer , Mass . Monochrometers , prism or grating , Ebert-Fastie systems for UV-Vis-NIR

Optronic Laboratories , Inc ., A Subsidiary of Kollmorgen Corp ., Orlando , Fla .

Spectroradiometers , UV-Vis-IR for precision measurements ; filter wheels , gratings , and prisms in single / double monochrometer configurations

Oriel Corporation , Stratford , Conn . Scanning monochrometers , rotation filter wheels , and detector array instruments

Perkin-Elmer Corporation , Norwalk , Conn . Complete sets of UV-Vis-IR spectroscopic systems using gratings and prisms , or FT-IR , with software and hardware for computer control , and accessories for microscopy , reflectance measurement , etc .

Shimadzu Scientific Instruments , Inc ., Columbia , Md . UV-Vis-NIR spectroscopic systems using holographic gratings in Czerny-Turner mounts in single- and double-beam configurations , computer-controlled , with accessories for analyses

SPEX Industries , Inc ., Edison , N . J . UV through IR grating spectrometers , 1 / 2- and 1 / 4-m , with CCD or PDA multichannel detectors

Thermo Jarrell Ash Corp ., A Subsidiary of Thermo Instrument Systems , Inc ., Franklin , Mass .

Monochromators and spectroscopic systems for analyses , UV-Vis-IR with gratings (in 1942 in Wadsworth , then in 1953 , Ebert , and now Paschen- Runge and crossed Czerny-Turner mounts) ; complete systems

DISPERSIVE PRISMS AND GRATINGS 5 .15

FIGURE 23 Unicam prism-grating double monochromator spectrometer .

such as Spectroscopy , Physics Today , Laser Focus , Photonics Spectra , and Lasers & Optronics , 8 are presented in Table 1 . Most of these systems are designed for analytical spectroscopy with techniques described in many texts such as Robinson’s Atomic Spectroscopy . 9

5 . 6 REFERENCES

1 . R . A . Sawyer , Experimental Spectroscopy , 3d ed . esp . Chapters 4 , 6 , 7 , and 11 , Dover Press , New York , 1963 .

2 . F . A . Jenkins and H . E . White , Fundamentals of Optics , 4th ed ., McGraw-Hill , New York , 1976 . 3 . E . Hecht , Optics : Second Edition , Addison-Wesley , Reading , MA , reprinted April 1988 . 4 . A . Walsh , ‘‘Multiple Monochromators II . Application of a Double Monochromator to Infrared

Spectroscopy , ’’ Journal of the Optical Society of America , Optical Society of America , Washington , DC , vol . 42 , 1952 , p . 95 .

5 . H . L . Hackforth , Infrared Radiation , McGraw-Hill , New York , 1960 , pp . 209 , 211 , 214 . 6 . A . H . Nielsen , ‘‘Recent Advances in IR Spectroscopy , ’’ Tech . Memo 53-2 , Of fice of Ordnance

Research , Durham , NC , December 1953 .

5 .16 OPTICAL ELEMENTS

7 . G . R . Harrison , ‘‘The Dif fraction Grating—An Opinionated Appraisal , ’’ Applied Optics , vol . 12 , no . 9 , 1973 , p . 2039 .

8 . See : Spectroscopy , especially issues of May and June 1990 , Aster Publishing Corp ., Eugene , OR . Physics Today , Annual Buyers’ Guide , 7 August 1990 , American Institute of Physics , 335 East 45th St ., New York . Laser Focus World and LFW’s The Buyers ’ Guide , 25th ed ., 1990 , PennWell Publishing Co ., Westford , MA . Photonics Spectra and The Photonics Directory , 4 vols ., 36th ed ., 1990 , Laurin Publishing Company Inc ., Pittsfield , MA . Lasers & Optronics and L & O ’ s 1 9 9 0 Buying Guide , Gorden Publications , Inc ., Morris Plains , NJ .

9 . J . W . Robinson , Atomic Spectroscopy , Marcel Dekker , Inc ., New York , 1990 .