High-performance thin film polarizer for the UV and visiblespectral regions

J. A. Dobrowolski and A. Waldorf

The practical limitations on the performance of a conventional MacNeille prism polarizer can be overcomeif a liquid is used as the prism material. In particular it is possible to construct large aperture polarizerssuitable for use with high power lasers in the UV, visible and near IR spectral regions. The width of the ef-fective spectral region and the polarizing efficiency of the polarizer can be controlled almost at will. Thelosses in the transmitted beam can be low. The values of the optical thicknesses of the layers are not verycritical and the cost of the device is low compared to that of any other polarizer having the same aperture andperformance. Results will be given of numerical calculations and of experimental measurements.

1. IntroductionIn many optical processes and instruments plane-

polarized radiation is required. There are many dif-ferent ways of producing it. H. E. and J. M. Bennetthave written an excellent detailed review of thissubject.1 Briefly, the attributes desirable in a linearpolarizer are that it should:

(1) produce a beam of high degree of polarization;(2) be effective over a wide spectral region;(3) attenuate the polarized beam as little as pos-

sible;(4) be capable of being used with high power la-

sers;(5) have as large an aperture as possible;(6) be capable of being used in an imaging system;(7) not alter the direction of the incident beam;

and(8) be cheap and simple to construct.The shortcomings of the various linear polarizers used

in the past in the UV, visible and near-IR regions arelisted below.

Prism polarizers of various types are based on thephenomenon of double refraction observed in certaincrystals. They are expensive and severely restricted bythe size of birefringent crystals available. Cemented

The authors are with National Research Council of Canada, Divi-sion of Physics, Ottawa, Ontario KIA 0R6.

Received 19 September 1980.0003-6935/81/010111-06$00.50/0.

prisms cannot be used with lasers because with the highpower densities frequently encountered the glues de-teriorate and the polarizers become useless.

Dichroic polarizers are based on the property of somematerials of absorbing light polarized in one directionmore strongly than at right angles. Because they de-pend on absorption, they seriously attenuate the pola-rized beam and hence cannot be used with high powerlasers.

Brewster angle reflection polarizers are based on thefact that for any material there is an angle (the Brewsterangle) at which all the radiation reflected from a surfaceis completely polarized. But they attenuate the pola-rized beam seriously and change its direction.

Brewster angle transmission polarizers consist ofseveral plates in series at the Brewster angle to increasethe degree of polarization of the transmitted beam.However, with a reasonable number of plates the degreeof polarization is not high and, because of multiple re-flections between the various surfaces, they cannot beused in imaging systems.

In interference polarizers2 ' 3 the radiation fallsobliquely onto systems consisting of one or more layershaving carefully chosen thicknesses and refractive in-dices. The layers may be deposited onto plane-parallelplates (interference plate polarizers) or be embeddedbetween two prisms (interference prism polarizers).Usually they are effective over a narrow spectral region.The aperture of interference prism polarizers is limitedby the size and cost of prisms that can be constructed,and their degree of polarization is limited somewhat bythe residual birefringence of the prism material. Ce-mented interference prism polarizers cannot be usedwith high power lasers.

1 January 1981 / Vol. 20, No. 1 / APPLIED OPTICS 111

11. Theory of the MacNeille PolarizerThis is a special type of interference prism polarizer

whose action depends not only on interference in thinfilms but also on the Brewster angle phenomenon.4' 5 Inits usual embodiment it suffers from all the limitationslisted above for interference prism polarizers, exceptthat its effective spectral region is relatively wide.Before we show how it can be modified in an effort tomeet the desiderata for a linear polarizer set out in theintroduction, it is necessary to explain briefly the con-struction and operation of this device (Fig. 1). It con-sists of two prisms of refractive index np that are madeof glass, quartz, or some other suitable solid material ofhigh transparency and optical quality. A suitable thinfilm system consisting of alternate layers of two mate-rials with refractive indices nL,nH is deposited onto thehypotenuse of one of the prisms. The prisms are thenjoined to form a rectangular block using a transparentcement having a refractive index as close as possible tonp.

The angles of refraction in the three media, P,0 H,OL,satisfy Snell's law of refraction (Fig. 2):

/ THINIf/

CEMENT -

FILMS

11

Fig. 1. MacNeille polarizer.

Fig. 2. Angles of refraction within the multilayer.

np sinkp = nH sinOH = nL sinOL.

If, in addition, we can arrange that Brewster's law,COSOH COSOL

nH nL

(1)

(2)

holds for the interface between the two coating mate-rials, radiation polarized parallel to the plane of inci-dence will be fully transmitted (i.e., zero reflectance) nomatter what the thicknesses of the layers.

To satisfy Eqs. (1) and (2) simultaneously we re-quire

np sin~~~~~p n~~~nH ~(3)ip sinOp (n + nH)1/2

MacNeille polarizers are usually constructed with Op =450. Equation (3) is then used to find the refractiveindex of the prism material for a given pair of coatingmaterials.

The film thicknesses are selected to provide a highreflectance for the radiation polarized perpendicular tothe plane of incidence so that essentially none of thisradiation remains in the transmitted beam. Usuallyquarterwave stacks are used.

Ill. A Liquid Prism MacNeille PolarizerMacNeille polarizers have not been used with high

power lasers because of the optical cement. One wayto avoid the use of cements might be to optically contactthe two prisms. This technique has been used suc-cessfully to contact coated quartz plates during theconstruction of solid Fabry-Perot interference filters.6However, obtaining surfaces flat enough for opticalcontacting is very difficult on prisms.

Holmes7 described a MacNeille polarizer for thevisible and near-IR spectral regions in which the opticalcontact between the two prisms was achieved by meansof a thin film of liquid. This device was proposed foruse with high power lasers. An equivalent solution for

COATED PLATE

Fig. 3. Liquid prism MacNeille polarizer.

the UV should also be possible. A number of trans-parent liquids have been suggested in the past whoserefractive indices match those of quartz in the UV.8However, the use of a contact liquid does nothing toovercome the problem of reduction in the degree ofpolarization caused by the residual birefringence inprism materials, nor does it reduce the limitation of theaperture of the beam by the availability and cost of largeprisms.

In a modification of the MacNeille polarizer thatovercomes the above remaining shortcomings of theoriginal device, the multilayer coating is deposited ontoa thin plate that is then immersed at the appropriateangle in a cell that is filled with a suitable UV trans-parent liquid (Fig. 3). If the refractive index of theplate matches that of one of the coating materials,Brewster's law [Eq. (2)] will also be observed at thesubstrate/layer interface. The substrate may then beregarded as being part of the layer system. There arethen no restrictions on the value of the refractive indexnp of the liquid, providing that the angle of incidenceOp obtained from Eq. (3) is real. It will be shown thata polarizer built on this principle can be very flexibleindeed.

The use of a liquid medium in a suitable cell to forma polarizer seems to have been first proposed byGeffcken.9 He suggested the use of calcium fluoride

112 APPLIED OPTICS / Vol. 20, No. 1 / 1 January 1981

w / l --

w l bI

1.0

z

4

_I-

4t

I-)

0.1

0.01

0.001

0.000102 0.3 0.4 0.5 0.6 0.7 0.8 0.9

WAVELENGTH (inpm)

Fig. 4. Calculated transmittance for radiation polarized perpen-dicularly to the plane of incidence of four different multilayer sys-

tems.

solution for wavelengths down to 0.250 Am. Becauseat the time even the best UV coating materials wereslightly soluble, Geffcken deposited the coatings ontothe outside walls of the cell and utilized the reflectedbeam. There are at least two other references to liquidprism MacNeille-type polarizers in the literature, nei-ther of which is concerned with the UV or with lasers.Fugitt' 0 describes a polarized light source for under-water use, and Zechnall' 1 describes a polarized beamvehicle headlight, both consisting of coated plates im-mersed in water.

IV. Numerical ResultsAccording to a table of short wavelength transmission

limits of various solvents12 the liquid with the lowestcut-off wavelength (-0.10 m) is distilled water.

Our past experience has shown that HfO2 /SiO2coatings withstood use in excimer lasers operating atwavelengths >0.248 Am.

Multilayers produced at our laboratory were testedat the Los Alamos Scientific Laboratory. The best werefound to have damage thresholds of 212-J/cm2 to 10-nsec pulse width, 35-Hz 0.308-Aum radiation from aLumonics 861 Xe-Cl laser that was focused down to a0.7-mm diam spot size.13 We also found by experimentthat multilayer coatings made of these materials did notdeteriorate upon immersion in water. All our polarizersare therefore based on HfO2 and SiO2 although, quitelikely, other material combinations could be used too.

In the calculations on these polarizers we assumedthat the materials were nonabsorbing and that the re-fractive indices nL,np,nH were nondispersive and hadvalues 1.366, 1.492, and 2.117, respectively. This yieldsa value of Op = 63.50. These assumptions do not in-troduce any major errors into the results of the thin filmcalculations at wavelengths higher than 0.25 Aim, theonset of the absorption edge for the SiO2 /HfO2 multi-layers.

As stated before, if Eq. (2) is satisfied the transmit-tance of the multilayer for radiation polarized parallelto the plane of incidence will be unity, no matter whatthe thicknesses of the individual layers might be. It is

also well-known that the reflectance of a quarterwavestack in its high reflectance region is very insensitive tothickness errors. These two facts have an importantimplication for the manufacture of MacNeille polariz-ers; the multilayers can be made to be quarterwavestacks for normal incidence. The effect of this on theperformance at 0 = 63.50 is a somewhat reduced re-flectance and a slightly narrower rejection region forradiation polarized perpendicular to the plane of inci-dence.

Figure 4 shows the calculated transmittance curvesat Op = 63.50 for radiation polarized perpendicular tothe plane of incidence of multilayers of the type (LH)7

that are immersed in water. Here, using the standardconvention, L,H stand for layers with optical thick-nesses of X/4 of materials of refractive index nL,nH.The thicknesses were chosen so that for normal inci-dence of light the systems were quarterwave stackscentered at X 0.368, 0.532, 0.738, and 1.047 m, respec-tively.

In the actual polarizers the fused silica substrateforms the outermost layer of refractive index nL. Itsthickness is several orders of magnitude larger than thatof the remaining layers. This imparts a small ampli-tude, sine-wave-like modulation to the transmission ofthe multilayer that can be observed only with a highspectral resolution instrument. The curves of Fig. 4represent essentially the calculated upper limits on thetransmittance of a thirteen-layer polarizer for the un-wanted polarization, occurring at those wavelengths forwhich the optical thickness of the substrate correspondsto an odd number of quarterwaves.

Figure 5 shows the effect of the angle of incidence onthe transmittance of the polarizer for radiation polar-

,.0

0.8

1.0

0.8

wC-,z

C,z.-

1.0

0.8

1.0

0.8

I.0

0.8

! I I I I I I | 6.

'! I I I I . -0.2 0.3 0.4 0.5 0.6 0.7

WAVELENGTH (in /Lm)0.8 0.9

Fig. 5. Calculated transmittance for radiation polarized parallel tothe plane of incidence at several different angles.

1 January 1981 / Vol. 20, No. 1 / APPLIED OPTICS 113

I I I I I I I ' 61.0

I I I I I I ' 66.0;

I I I I i I I

I I I I I I I 5a 5.

I I I I I I I

I I I I I I I ' 63.5'

Fig. 6. Liquid prism MacNeille polarizer with two plates in series.

ized parallel to the plane of incidence. The calculationswere performed on the system centered at X = 0.738,um.The loss in transmittance is acceptable for all angles 61.0

0 < 66.0. There is no significant degradation of thetransmittance for the rejected polarization (see Fig.4).

If the effective spectral region of one of the abovesystems were not wide enough for a particular applica-tion, it should be possible to extend it by mounting twoor more coated plates in the same cell (Fig. 6).14 Thus,for example, if all four systems whose reflectances areshown in Fig. 4 were to be combined, a polarizer with aneffective spectral region of 0.22 < < 0.93 gim wouldresult. Another way of achieving the same effect is todeposit the multilayer systems on top of one another onone and the same substrate. 15

By combining two identically coated plates within thesame cell, theoretically it should be possible to reducethe unwanted polarization in the transmitted beam atthe center of the rejection region.

V. Experimental ResultsThe multilayer coatings were deposited in a standard

evaporation plant at a pressure of 3 X 10-5 Torr ontofused silica substrates heated to 1500 C. An electronbeam gun was used to evaporate the materials. Thesimple normal incidence extremum monitoring methodwas used to control the deposition process.

Three different types of cell were used in the exper-iments. The first two were simple rectangular boxesmade of aluminum and plastic, respectively, with fusedquartz windows cemented over holes in two oppositevertical sides. However, during high power laser teststhe plastic box started to melt where the reflected laserbeam struck its sides, even though it was filled withwater. A permanent cell, housing up to two coatedplates, was then built (Fig. 7). It provides for an accu-rate alignment of the plates and has exit windows for thereflected beams. These are necessary if all the powerin these beams is not to be dissipated within the polar-izer.

The spectral performance of the coatings was mea-sured on a Perkin-Elmer type 330 spectrophotometer.To obtain significant low transmission readings knownattenuators were placed in the reference beam of thisinstrument.

The results for a thirteen-layer polarizer designed foruse with a Xe-Cl excimer laser (X = 0.308 gim) areshown in Fig. 8. Also indicated in the figure is the way

in which the measurements were interpreted. Thus To,(T 1/T 2), and (T 3/T1 ) represent the transmittance of thewater-filled cell without plates, of a single coated plate,and of two crossed polarizer plates, respectively.

The sharp falloff in transmission below 0.22 gm is dueto the absorption of the water. The coatings start toabsorb significantly below 0.24 gtm. The transmissionof two crossed polarizers is of the order of 5 X 10-4.

This is in reasonable agreement with the theoreticallyexpected rejection: the mean of the transmittances forradiation polarized perpendicular to the plane of inci-dence of the layer systems (LH)7 and H(LH).6

Fig. 7. Special cell for a liquid prism MacNeille polarizer.

I0

wL0zI-

C'zfI-

0.1

0.0w

0.00I

0000l0.2 0.3

WAVELENGTH (inm)

-u <j1- T

2~EZEH2

1L1 -H I 1I1/ ll

0.4

Fig. 8. Measured performance of a thirteen-layer liquid prismMacNeille polarizer.

114 APPLIED OPTICS / Vol. 20, No. 1 / 1 January 1981

M�_ N 777 1

1.0

LU0

U,z

0.8

0.6

0.4

0.2

0.0

0.01

0001,/T,

0.00001

0.2 0.3 0.3 0.4 0.5 0.6

WAVELENGTH (in Am)

Fig. 9. Measured performance of two forty-seven- and fifty-two-layer wideband polarizers for the UV (a) and visible (b) spectral re-

gions.

The results for two forty-seven- and fifty-two-layerpolarizers, designed for the 0.20-0.38-gm and 0.35-0.65-gm spectral regions, are shown in Fig. 9. Eachconsists of three superimposed layer systems that weremonitored at 0.320,0.403,0.470 and 0.503,0.595,0.705gim, respectively. Because of the large number of layersinvolved, the absorption at wavelengths below 0.25 gmis considerable.

The dashed line in the lower part of Fig. 9(b) repre-sents the measured transmittance when two widebandvisible polarizer plates were crossed with two othersimilar plates.

Figure 10 demonstrates that by combining thewideband UV and visible polarizer plates of Fig. 9 in thesame cell, a device results that is effective over a 3:1range of wavelengths.

The performance of the polarizers shown in Figs. 9and 10 does not meet the theoretical expectations. Thisis probably due to stray reflected light from the cellwindows (which were not AR coated), from the alumi-num walls of the cell (which were not blackened), anddue to residual birefringence in the quartz windows andplates that were not made of high quality material.

Laser damage tests on the two UV polarizers wereperformed by R. S. Taylor and P. Cassard of the Laserand Plasma Physics Section, Division of Physics,NRCC. A Lumonics 861 excimer laser operating at X= 0.308um (Xe-Cl) was used for the tests. The beamenergy of this laser was 50 mJ and the pulse durationwas 4 nsec.

In single-shot laser damage tests a 0.5-m focal lengthlens was placed in the laser beam and the polarizer wasgradually moved into the focus of the lens. The mini-mum surface area tested was 0.085 cm 2 and the maxi-mum energy density was thus 0.6 J/cm2 . The maxi-mum power density was 150 MW/cm2 . Over the aboverange of conditions no single-shot damage was observedfor the two coatings.

In high average power damage tests a 0.13 cm 2 areaof the coatings was irradiated by the laser radiation for-2 min. The average power was raised by increasing

0.8

0.6

0.4

0.2

0.0

wOzI.

Uf)z

It

0.00I

0.0001

0.00001 0.2

n ~~~~~,// \ ~~~~~~~~~/ ~~ T.

I I I0.3 0.4 0.5 0.6

WAVELENGTH (in m)

Fig. 10. Measured performance of the polarizers of Fig. 9 placed inseries in the same cell.

the repetition rate. A maximum power level of 5 Wcould thus be attained. No damage was observed forthe thirteen-layer polarizer of Fig. 8 at 5 W (powerdensity of -40 W/cm 2). The damage threshold for theforty-seven-layer polarizer of Fig. 9(a) was 0.5 W (-4W/cm2 ) and 2.0 W (-16 W/cM2 ) for laser radiation in-cident on the substrate and coatings sides, respec-tively.

In tests in which power entered the polarizer at a rateof 200 W no change in transmission of two crossed po-larizers was observed during a 15 min period. Thetemperature coefficient of the refractive index of thewater is of the order of 1 X 10-4/C. It may be that withhigher power levels, or with the use of the polarizers forextended periods of time, stirring or circulation of watermight have to be considered if significant temperaturegradients are to be avoided.

VI. ConclusionsSince the devices built by us have now survived im-

mersion in water for over 4 months, we feel that we havedemonstrated the feasibility of the construction of liq-uid prism MacNeille polarizers for the UV and visibleparts of the spectrum that meet the desiderata listed inthe Introduction.

The materials used by us start to absorb significantlybelow 0.25 gim. The useful lower wavelength limit ofthe polarizers depends on the acceptable attenuationof the transmitted beam and also, when used with lasers,on the damage threshold at those wavelengths. But itmay be that in the IR the usefulness of the polarizerswill be limited by Brillouin scattering. With differentsubstrate, window, and coating materials, and with theuse of fully fluorinated hydrocarbon oils7 such asKEL-F3,1 6 extension of the spectral range to 7.0 gm isconceivable.

Although the polarizers that we have built so far havea clear aperture of only -2.5 cm (1 in.), in principle itshould not be difficult to scale up the device. Large

1 January 1981 / Vol. 20, No. 1 / APPLIED OPTICS 115

I

/11

T,

I I I I

to0

plates of fused silica are much easier to obtain and aremuch cheaper than the homogeneous blocks needed forthe construction of prisms of similar dimensions. Wehave also shown that the complexity of the multilayersystem can be reduced to a minimum.

The authors would like to thank G. R. Hanes forvaluable discussions. M. Ranger designed and con-structed the special polarizer cells. P. Brennan, G.Crampton, and B. Simpson prepared the multilayercoatings and carried out some of the measurements.

References1. H. E. Bennett and J. M. Bennett, "Polarization," in Handbook

of Optics, W. G. Driscoll and W. Vaughan, Eds. (McGraw-Hill,New York, 1978).

2. P. B. Clapham, M. J. Downs, and R. J. King, Appl. Opt. 8,1965(1969).

3. R. R. Austin, Proc. Soc. Photo-Opt. Instrum. Eng. 50, 143(1975).

4. S. M. MacNeille, U.S. Patent 2,403,731, 9 July 1946.5. M. Banning, J. Opt. Soc. Am. 37, 792 (1947).6. R. R. Austin, Opt. Eng. 11, 65 (1972).7. S. J. Holmes and F. J. Woodberry, U.S. Patent 3,704,934, 5 De-

cember 1972.8. J. A. Dobrowolski, "Coatings and Filters" in Handbook of Optics,

W. G. Driscoll and W. Vaughan, Eds. (McGraw-Hill, New York1978), p. 8.104.

9. W. Geffcken, German Patent 899,120, 7 Dec. 1953.10. R. B. Fugitt, U.S. Patent 3,743,380, 3 July 1973.11. R. Zechnall, E. Linder, A. Schmid, G. Raabe, K. Kerner, and R.

Socknick, U.S. Patent 3,935,444, 27 Jan. 1976.12. J. Grasselli and W. M. Ritchey, CRC Atlas of Spectral Data and

Physical Constants for Organic Compounds (CRC Press, WestPalm Beach, 1973).

13. B. Newnam, Los Alamos Scientific Laboratory; private commu-nication (1980).

14. R. S. Sokolova and T. N. Krylova, Opt. Spectrosc. 14, 213(1963).

15. A. F. Turner and P. W. Baumeister, Appl. Opt. 5, 69 (1966).16. Produced by the 3M Company. For an IR transmission spec-

trum, see Hummel and Scholl, Infrared Analysis of Polymers,Resins and Additives-An Atlas, Vol. I (Wiley, New York, 1969),Part 2, Spectrum No. 647.

Corporation Members continued from page 104

RUDOLPH RESEARCH CORPORATION, 40 Pier Lane, P.O. Box 1446, Fairfield,New Jersey 07006, is engaged in research, development, and manufactureof products utilizing polarized light as a measuring tool. In addition to custompolarized light instrumentation, the company manufactures and marketsstandard product lines of polarimeters, saccharimeters, and ellipsometersfor use in pure and applied research by universities, independent and gov-ernment laboratories, and industry. Routine automatic polarimeters areused for process control in all phases of the chemical and related industries.Manual and automatic saccharimeters are used by sugar growers, proces-sors, and refineries, as well as by governments, to regulate both domesticand international sugar trade. Ellipsometers find widespread applicationin the semiconductor integrated circuit industry for measurement of filmthicknesses in the angstrom region.

SCHOTT OPTICAL GLASS, INC., 400 York Ave, Duryea, Pennsylvania 18642,manufacturer of precision and commercial optical glasses and finished orsemifinished components, including color filter glass, finished filters (UV,IR, and visible), Interference filters, radiation shielding windows, CRTfaceplates, ophthalmic lens blanks, and others. The company maintainscomplete laboratories for fundamental and applied research.

SOPELEM, 125 Boulevard Davout, 75960 Paris, Cedex 20, France, designsand manufactures optical instruments and equipment. Sopelem providesfacilities for research and development, industrial optical manufacturing(spherical lenses, aspherical lenses, plane and prismatic optics, coatingand engraving), and custom design and manufacturing. Sopelem productsinclude metrology instruments, refractometers (fluid concentration mea-surement), microscopes, light intensification equipment, laser components,submarine periscopes, and hydraulic and lubrication systems.

SPECTRA PHYSICS, INC., 1250 W. Middlefield Rd, Mountain View, California94040, is active in the areas of gas laser research, development, and pro-duction of scientific and OEM, He-Ne, Ar-ion, and dye lasers, various ac-cessories, tunable diodes (through its Laser Analytics, Inc. subsidiary), andis active in the development and manufacture of electrooptic and digitalsystems and subsystems utilizing lasers. Such systems include supermarketU.P.C. symbol scanners, laser photocoagulators for medical applications,computer output on microfilm, construction guidance and alignment systems,liquid chromatographs, and lab automation data reduction computing sys-tems. The company also has a substantial research and development effortleading to the production of dielectric coatings and optical components tobe used throughout the UV, visible, and IR spectrum.

R. HOWARD STRASBAUGH, INC., 18460 Gothard St, Huntington Beach,California 92648, manufactures precision optical machines, and all typesof polishers, from the smallest to 380 cm in diameter, centering and edgingmachines, universal glass grinders, and a patented electronic spherometer.Manufacturing plant is in Huntington Beach, California, with a branch inWoodbury, Connecticut. The company manufactures ophthalmic surfacingmachines, generators, and cyclometers.

TINSLEY LABORATORIES, INC., 2448 6th St, Berkeley, California 94710,founded in 1926, is engaged in the design and manufacture of lens systems,precision aspheric components, Schmidt systems, metal and glass mirrors,as well as a standard instrumentation product group consisting of laser in-terferometers and autocollimators.

TROPEL, INC., 1000 Fairport Pk, Fairport, New York 14450, provides opticaldesign and consulting services, constructs prototype optical systems, andmanufactures lenses for a variety of OEM applications. Optical engineeringand development is concentrated in lens design and image analysis.Commercial instruments manufactured include laser accessory instruments,interferometers, and optical testing equipment.

TRW DEFENSE & SPACE SYSTEMS GROUP, Technical Information Center,1 Space Park S/1930, Redondo Beach, California 90278, is a researchand development firm specializing in systems engineering and advancedtechnology. A wide range of optical technologies are pursued in the areasof lasers, sensors, and coherent optics. Present laser projects include thedevelopment of HF, DF, 12, excimer, and free electron lasers as well as highpower optical components, beam handling and alignment systems. Ad-vanced ground and space based sensor and sensor systems are under de-velopment for environmental observations, altitude control, and satellitetracking. Coherent optics activities include optical processing and holo-graphic systems for ground and space applications.

TRW TECHNOLOGY RESEARCH CENTER, 2525 El Segundo Blvd., El Se-gundo, California 90245.

JOSEPH SCHNEIDER & COMPANY, 655 Bad Kreuznach, Postfach 947, WestGermany, manufactures lenses for photography, enlargement, graphic arts,cinematography, CCTV, and television and undertakes special lens designfor industrial applications. continued on page 135

116 APPLIED OPTICS / Vol. 20, No. 1 / 1 January 1981