Pyroelectricity: From Ancient Curiosity to Modern Imaging ToolSidney B. Lang

Citation: Physics Today 58, 8, 31 (2005); doi: 10.1063/1.2062916View online: https://doi.org/10.1063/1.2062916View Table of Contents: https://physicstoday.scitation.org/toc/pto/58/8Published by the American Institute of Physics

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Pyroelectricity: From Ancient Curiosity to Modern Imaging Tool

One of the least-known properties of solid materials, py-roelectricity is rigorously defined as the temperature

dependence of the spontaneous polarization in certainanisotropic solids.1⊗6 To appreciate the meaning of that def-inition and the nature of the pyroelectric effect, consider asimple example: a thin, parallel-sided sample of material,such as a tourmaline crystal or a ceramic disk of bariumtitanate, cut so that its crystallographic symmetry axis isperpendicular to the flat surfaces. The unit cells of pyro-electric materials have a dipole moment.The dipoles are packed so that the compo-nents of the dipole moment in each unit celladd up in the direction normal to the flatsurfaces. The dipole moment per unit vol-ume of the material is called the sponta-neous polarization PS. Always nonzero in apyroelectric material, PS exists in the ab-sence of an applied electric field and isequivalent to a layer of bound charge oneach flat surface of the sample.

Nearby free charges such as electrons orions will be attracted to the sample (see fig-ure 1). Imagine that conductive electrodes arethen attached to the surfaces and connectedthrough an ammeter having a low internalresistance. If the temperature of the sampleis constant, then so is PS and no current flowsthrough the circuit. But in most single crys-tals and ceramics, an increase in temperaturecauses the net dipole moment and, conse-quently, the spontaneous polarization to de-crease. The quantity of bound charge then de-creases, and the redistribution of free chargesto compensate for the change in bound chargeresults in a current flow—the pyroelectriccurrent—in the circuit.

If the sample had been cooled instead ofheated, the current’s sign would be reversed.Note that the pyroelectric effect is only ob-servable during the period in which the tem-perature changes. In an open circuit, thefree charges would simply remain on theelectrodes and a voltage could be measured.

A large number of pyroelectric materials exist, in-cluding minerals such as tourmaline, single crystals suchas triglycine sulfate, ceramics such as lead zirconate ti-tanate, polymers such as polyvinylidene fluoride, and evenbiological materials, such as collagen. For a detailed treat-

ment of the relations between variousproperties of a material and how thoseproperties contribute to the pyroelec-tric effect, see box 1.

Historical threadsThis treatment of pyroelectricity in

terms of a change in net dipole moment emerged in moderntimes. But as a phenomenon, the pyroelectric effect has beenknown for 24 centuries—the Greek philosopher Theophras-tus probably wrote the earliest known account.7 He de-scribed a stone, called lyngourion in Greek or lyncurium inLatin, that had the property of attracting straws and bits ofwood. Those attractions were no doubt the effects of elec-trostatic charges produced by temperature changes mostprobably in the mineral tourmaline.

Sidney Lang is an emeritus professor of chemical engineering atBen-Gurion University of the Negev in Israel.

© 2005 American Institute of Physics, S-0031-9228-0508-010-5 August 2005 Physics Today 31

Changes in the net dipole moment of certain materials formthe basis for a broad range of IR detectors.

Sidney B. Lang

Figure 1. If a pyroelectric crystal with an intrinsic dipolemoment (top) is fashioned into a circuit with electrodesattached on each surface (middle), an increase in temper-ature T prompts the spontaneous polarization PS to de-crease as the dipole moments, on average, diminish inmagnitude. The horizontal tilting of the dipoles, picturedat bottom, signifies the effect. A current flows to compen-sate for the change in bound charge that accumulates onthe crystal edges. (Adapted from ref. 13.)

Theophrastus and other writers of the two millenniathat followed were far more interested in the origin of thestone and its possible therapeutic properties than they werein physical explanations. Theophrastus proposed that lyn-gourion was formed from the urine of a wild animal, lateridentified by Pliny the Elder as a lynx (see figure 2).1,2

Two thousand years after Theophrastus, tourmaline’sunusual physical properties were reintroduced to Europethrough the publication in 1707 of a book entitled CuriöseSpeculationes bey Schlaflosen Nächten (Curious Specula-tions During Sleepless Nights). Its author, Johann GeorgSchmidt, using the pen name Immer Gern Speculirt (Al-ways Gladly Speculating), wrote a series of 48 dialogs, oneof which contained a section describing hard and glassybodies that were not magnetic. He described the experi-ences of Dutch gem cutters when they tested the durabil-ity of tourmaline in a fire:

The ingenious Dr. Daumius, chief physician tothe Polish and Saxon troops on the Rhine, toldme that, in the year 1703, the Dutch firstbrought from Ceylon in the East Indies a pre-cious stone called tourmaline, turmale, or trip,which had the property of not only attractingthe ashes from the warm or burning coals, asthe magnet does iron, but also repelling themagain.

In 1717 the physician and chemist Louis Lemery wrote thefirst scientific description of pyroelectricity in a journal.The naturalist Carl von Linné (Linnaeus) was the first torelate the pyroelectric property of tourmaline to electric-ity; he called the mineral lapis electricus—electric stone.Experiments on tourmaline during the 18th century byFranz Ulrich Theodor Aepinus, Johann Karl Wilcke, Ben-jamin Wilson, Joseph Priestley, John Canton, and TorbenBergman made major contributions to the rapidly devel-oping field of electrostatics.

A more quantitative understanding of pyroelectricityemerged during the 19th century as more sophisticated re-search techniques were developed. In 1824 David Brew-ster, famous for his work in optics, was the first author touse the term “pyroelectricity.” One of the materials hestudied was a “tartrate of soda and potash”—Rochellesalt—the same material in which Joseph Valasek discov-ered ferroelectricity almost exactly a century later.

Shortly after Antoine Becquerel and others developedthe electrometer, John Mothée Gaugain made the first pre-cise measurements of pyroelectric charges in 1859. Hereached some important conclusions: The total quantity ofelectricity produced by a crystal of tourmaline dependsuniquely upon the limits within which its temperature isvaried; within those limits, the amount of electricity pro-duced during heating is the same as that produced duringcooling, but with the signs of the charges reversed; and the

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The triangular diagram below illustrates the thermodynam-ically reversible interactions that may occur among the

thermal, mechanical, and electrical properties of a crystal;magnetic properties are ignored for simplicity. The lines join-ing pairs of circles signify that a small change in one of thevariables produces a corresponding change in the other. Thethree short bold lines that connect pairs of thermal, elastic,and electric variables define the physical properties of heatcapacity, elasticity, and electrical permittivity, respectively. Asan example, a small increase in temperature q produces anincrease in entropy s proportional to the heat capacity di-vided by temperature. The diagram also illustrates coupledeffects, denoted by lines joining pairs of circles at differentcorners of the diagram.

Pyroelectricity is a coupled effect that relates a change intemperature to a change in electrical displacement D (withunits of C/m2),

where p is the pyroelectric coefficient (with units of C/m2�K).Both displacement and the pyroelectric coefficient are vec-tors or first-rank tensors. The pyroelectric coefficient is de-fined by

where PS is the spontaneous polarization. The constraints areconstant electric field E, and constant elastic stress T. Con-stant stress means that the crystal is not clamped, but com-pletely free to expand or contract thermally.

The diagram’s colored lines indicate that the two contri-butions make up pyroelectric effect. In the first, the crystal isrigidly clamped under constant strain S, to prevent expansionor contraction. A change in temperature causes a change inelectric displacement as shown by the green line, which sig-nifies the primary pyroelectric effect. The second contribu-tion—the secondary pyroelectric effect—is a result of crystal

deformation: Thermal expansion causes a strain that altersthe electric displacement via a piezoelectric process, asshown by the dashed red lines.

Measuring the primary effect directly is extremely difficult.But the secondary effect can be readily calculated from thevalues of the thermal expansion coefficient, the elastic stiff-ness, and the piezoelectric strain constant. So experimentally,the pyroelectric effect under the constraint of constantstress—the so-called total effect, the sum of red and greenlines—is what is usually measured.

The table on page 33 lists the primary, secondary, andtotal pyroelectric coefficients of several materials for compar-ison.4 Moreover, it shows a division between ferroelectric andnonferroelectric materials. Ferroelectrics are pyroelectricswhose direction of polarization can be reversed by a suffi-ciently strong electric field. They are also characterized by

Box 1. Crystal Properties and the Pyroelectric Effect

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amount of charge produced is proportional to the cross-sectional area of the crystal and is independent of itslength. William Thomson (Lord Kelvin) published the firstmajor theoretical treatment of pyroelectricity in 1878; hispaper included a prediction of the electrocaloric effect.Jacques and Pierre Curie proposed that the electrical ef-fects due to nonuniform heating of quartz crystals mighthave been caused by pressure, a speculation that led totheir 1880 discovery of piezoelectricity (related to pyro-electricity as discussed in box 1). During the latter part ofthe 19th century and the early decades of the 20th century,seven Nobel laureates—Wilhelm Röntgen, Pierre Curie,Gabriel Lippman, Heike Kammerlingh Onnes, ErwinSchrödinger, Archer J. P. Martin, and Max Born—publishedpapers on pyroelectricity. These scientists are, of course,much better known for their research in other fields.

Joseph Valasek studied the properties of Rochelle saltand in 1920 discovered ferroelectricity. Then interest in py-roelectricity virtually vanished until 1938, when Yeou Tapublished a paper that initiated the great growth that con-tinues in the field today. Ta, a chemist at the Sorbonne inParis, proposed that tourmaline crystals could be used asIR sensors in spectroscopy. Some research was conductedon pyroelectric IR detectors during and immediately afterWorld War II in the UK, US, and Germany, but the resultsappeared only in classified documents. In 1962 J. Coopermade the first detailed analysis of the behavior of fast IR

detectors and conducted experiments using barium ti-tanate. That year, I proposed the use of pyroelectric de-vices for measuring temperature changes as small as 0.2mK. An explosive growth in theoretical studies, basic mea-surements, and applications had begun: More than 8500papers on pyroelectricity have been published since 1960.8

ApplicationsThe detection of long-wavelength IR radiation is of greatinterest in a wide range of applications.9⊗12 Two windowsin the atmosphere exist—one from 3 to 5 mm and one from8 to 14 mm—where the absorption due to water vapor andthe scattering due to dust are minimal. The latter windowis especially important because the peak in emission of theblack-body curve for objects at 300 K occurs around 10 mm.Consequently, IR detectors with high sensitivity in thiswavelength region can easily detect human beings andother warm-blooded animals.

There are two general classes of IR detectors: quantumor photon, and thermal. Quantum detectors are based onthe photoelectric effect and are produced from III–V or II–VIsemiconductors such as gallium arsenide and mercury cad-mium telluride. (The pyroelectric effect, based on differentphysics, does not depend on the size of the bandgap.) Thesematerials are difficult to grow and fabricate into devices,and they require cooling, usually to 77 K, for operation inthe long-wavelength IR spectrum. Astronomical quantumdetectors are extremely sensitive in the far IR, but they do

the Curie temperature TC, above which the material is para-electric (nonpolar). Below TC, ferroelectrics are polar and canexhibit pyroelectricity. These materials generally have largerpyroelectric coefficients than nonferroelectrics and are ofgreater interest for applications.

An effect called tertiary pyroelectricity is produced bynonuniform heating causing nonuniform stresses that can re-

sult in polarization through a piezoelectric effect.Tertiary pyroelectricity is of little interest now, al-though it was probably responsible for many ofthe unusual effects observed by early workers.The electrocaloric effect is an inverse pyroelectriceffect, in which a change in an applied electricfield produces a change in entropy and, conse-quently, a change in temperature.

The existence of the pyroelectric effect in anysolid material requires that three conditions be sat-isfied: The molecular structure must have anonzero dipole moment; the material must have nocenter of symmetry; and the material must have ei-ther no axis of rotational symmetry or a single axisof rotational symmetry that is not included in an in-version axis. Of the 32 crystal point-group symme-

tries, only 10 permit the existence of pyroelectricity.The lattice model above shows a projection of the unit cell

of barium titanate (BaTiO3) on the (100) plane at a tempera-ture of 291 K. The displacement of the atoms from their equi-librium positions on a cubic lattice gives rise to the sponta-neous polarization; its variation with temperature is thepyroelectric effect.

Primary, secondary, and total pyroelectric coefficients of various materi-als. (Units are mC/m2.K)

Material Primary coefficient

Secondary coefficient

Total coefficient

Ferroelectrics

Poled ceramicBaTiO3 ⊗260 ⊕60 ⊗200PbZr0.95Ti0.05O3 ⊗305.7 ⊕37.7 ⊗268

CrystalLiNbO3 ⊗95.8 ⊕12.8 ⊗83LiTaO3 ⊗175 ⊗1 ⊗176Pb5Ge3O11 ⊗110.5 ⊕15.5 ⊗95Ba2NaNb5O15 ⊗141.7 ⊕41.7 ⊗100Sr0.5Ba0.5Nb2O6 ⊗502 ⊗48 ⊗550(CH2CF2)n ⊗14 ⊗13 ⊗27

Triglycine sulfate ⊕60 ⊗330 ⊗270

Nonferroelectrics

Crystal

CdSe ⊗2.94 ⊗0.56 ⊗3.5

CdS ⊗3.0 ⊗1.0 ⊗4.0

ZnO ⊗6.9 ⊗2.5 ⊗9.4

Tourmaline ⊗0.48 ⊗3.52 –4.0

Li2SO4. 2H2O ⊕60.2 ⊕26.1 ⊕86.3

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require very low-temperature cooling (see the article byPaul Richards and Craig McCreight in PHYSICS TODAY, Feb-ruary 2005, page 41). Thermal detectors convert the energyof IR photons into heat and are usually operated at ambi-ent temperatures. They are not restricted to use in limitedwavebands as are quantum detectors. Thermal devices aremuch cheaper but less sensitive.

Pyroelectric thermal detectors have five main advantages:� Sensitivity over a very large spectral bandwidth—inprinciple, over the entire electromagnetic spectrum, de-pending on the absorption characteristics of the pyroelec-tric material and its electrodes.� Sensitivity over a very wide temperature range—froma few degrees kelvin to hundreds, depending on pyroelec-tric material.� Low power requirements—just enough to drive the fieldeffect transistor that amplifies the signal.� Fast response, over periods as short as picoseconds.� Low-cost manufacture from inexpensive materials.

Because pyroelectric devices only respond to changesin temperature, they are used to observe moving objects ina scene or stationary objects through a light chopper; runat video frequencies (usually 25 to 60 Hz), the light chop-per provides a continuous source of temperature variationthat allows the material to generate charge while a sceneis monitored.

Infrared detectors, such as the two types sketched infigure 3, can consist of single pyroelectric elements or one-or two-dimensional arrays of elements. Densely spaced 2Darrays generate a thermal image, similar to the visible-lightimage of a television. Some of the applications of IR pyro-electric detectors include intrusion detectors and burglaralarms, flame and fire detectors, IR spectrometers, laser de-

tectors, and pollution monitors. Other major areas of pyro-electric research and applications unrelated to IR detectioninclude electron-emission devices and the measurement ofthermal and optical properties of materials.

Device configurationsThe most important component of any pyroelectric deviceis the detector material. Triglycine sulfate (TGS) and itsisomorphs have very favorable properties, including highpyroelectric coefficients and relatively low permittivity.Despite their hygroscopic nature, these materials are a fa-vorite for high-sensitivity applications. Lithium tantalateis very stable because of its high Curie temperature andinsensitivity to humidity and vacuum, and is often usedfor space applications. Polyvinylidene fluoride polymer(PVDF) and its copolymers have low pyroelectric coeffi-cients but their low thermal conductivity and dielectricconstant make them useful for large-area detectors and ar-rays. Ceramics based on the lead zirconate titanate sys-tem are probably the most widely used materials; rela-tively cheap to manufacture, they are both mechanicallyand chemically robust. Variation of the Zr/Ti ratio and theaddition of dopants permit great variation in the physicalproperties of these ceramics.

Thermally isolated from its surroundings, the detectorelement is usually coated with a blackened thin film to in-crease the light absorption. Low-noise and high-impedanceamplifiers are necessary to detect the small amount ofcharge produced by heating, and typical circuits incorpo-rate a field effect transistor with an appropriate load re-sistor or an operational amplifier with a feedback resistorto boost the signal. Doping the detector with various ele-ments can control the detector’s resistance and therebyeliminate the load resistor.12

Pyroelectric elements can also be configured to atten-uate noise from ambient temperature changes and fromvibrations. In the configuration shown in figure 3a, for in-stance, two elements are connected in electrical oppositionand one is shielded from the radiation source. A germa-nium window is often chosen for devices because it isopaque to visible light but transparent at wavelengthsnear 10 mm. For details on the response and noise levelsof pyroelectric detectors, see box 2.

Some of the more unusual applications of single-elementdetectors have been in space missions. The earliest space-related application was the vertical temperature-profileradiometer, launched into Earth’s orbit in 1972. ThePioneer Venus Orbiter launched in 1978 was used to mapcloud temperatures on Venus. The sounder probe of thatmission was dropped into the planet’s atmosphere to mea-sure the net thermal flux. The Galileo mission launched in1989 included a photopolarimeter–radiometer to deter-mine thermal radiation on Jupiter and its moons. The

Figure 2. The mythological origin of pyroelectric crystals datesback to about 315 BC, when the Greek philosopher Theophras-tus wrote his treatise “On stones,” about lyncurium (tourmaline,most likely) and other minerals. Just 14 pages long, that text wasa valuable work on mineralogy for 18 centuries. The hand-colored woodcuts shown here illustrate Hortus Sanitatis Major(The Garden of Health), a book published in 1497 that de-scribes medicinal and therapeutic values of plants, animals, andminerals in terms of superstitious views held in the late MiddleAges. The hardness of lyncurium contrasts with the softness ofamber, whose formation is pictured in the top left. The sketchon the right depicts Theophrastus’s bizarre belief that pyroelec-tric lyncurium comes from the urine of a lynx.

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a b

Figure 3. Single- and multiple-element pyroelectric detectors. (a) In thissingle-element configuration intended to monitor radiation with a wave-length near 10 microns, two lead titanate pyroelectric elements are used.One detector element is exposed to incoming radiation, another shieldedunder a metal strip. Both crystals are electrically connected with oppositepolarities, to cancel out the effect of any drift in ambient temperature. (b) Aferroelectric-hybrid focal-plane device comprises a lens, usually made ofgermanium to block visible light, and an array of individual ferroelectric ele-ments that are each bonded by tiny balls of solder to elements from a siliconmultiplexer. The incident IR radiation must be periodically blocked, here bya chopper, to ensure that a temperature variation is measured. (Adaptedfrom ref. 14 and 15.)

Box 2. Characterization of Pyroelectric Elements

Awell-understood theory describes the response and noisesources of pyroelectric IR detectors.9⊗12,14 The pyroelectric

current I generated per watt of input power, W0, is called thecurrent responsivity,

where p is the pyroelectric coefficient amplitude, h is the de-tector electrode absorptivity, A is the cross-sectional area ofdetector, w is the radiation modulation frequency, G is thethermal conductance to surroundings, and tT is the thermaltime constant (to account for heat losses to surroundings).Multiplying the current responsivity by the impedance of thecircuit produces the voltage responsivity,

where tE is the electrical time constant (to account for RClosses in the circuit) and RT is the total resistance of the detec-tor element and circuitry. RV has a maximum at a frequencybetween the reciprocals of the thermal and electrical timeconstants.

There are three major noise sources in a simple pyroelectricdetector: thermal noise, dielectric (Johnson) noise, and ampli-fier noise (input-voltage, input-current, and shunt-resistornoise). Johnson noise is usually dominant above frequenciesof 20 Hz. And environmental noise can arise due to ambienttemperature fluctuations, mechanical vibration, and electro-magnetic fluctuations. A signal-to-noise ratio is given by thenoise-equivalent power,

where VN is the root mean square noise voltage and the NEPis normalized by the square root of the device bandwidth Df.Both thermal and Johnson noise are proportional to the squareroot of the area of the detector. This gives rise to one of themost frequently used parameters, the area-normalized (or spe-cific) detectivity,

To appreciate typical magnitudes of the responsivity anddetectivity of a pyroelectric, consider a 0.01 mm2 lanthanum-modified lead titanate (PLT) element supported on a thin ce-ramic membrane.11 The maximum voltage responsivity wascalculated to be about 40 000 V/W at a modulation frequencyof 1 Hz. The maximum current responsivity was 50 mA/W andthe maximum D* was 6.4 × 108 cm Hz1/2/W at 5 Hz. The the-oretical maximum value of D* in any pyroelectric detector oc-curs if all noise sources other than thermal noise are elimi-nated; the effect of thermal noise cannot be eliminated.1 Thedetectivity corresponding to thermal noise alone at 300 K is1.8 × 1010, one to two orders of magnitude higher than thebest contemporary values. That disparity leaves room for im-provement as new types of materials and improved circuitryare developed in coming years.

In practice, a detector element should be thin to maximizethe temperature rise and mounted so as to minimize heat lossto the substrate. Low thermal-conductivity insulators orbridge-like constructions are often used in devices. A large py-roelectric coefficient and small values of dielectric permittiv-ity are also required.

36 August 2005 Physics Today http://www.physicstoday.org

thermal flux data collected by the Galileo net-flux radiometer released toward Jupiter’s surface helped re-searchers analyze the chemical composition and structureof the planet’s atmosphere. The Mars Exploration Rovermission launched in 2003 carried a miniature thermal-emission spectrometer to identify minerals by their IRspectrum and to collect temperature data. The detectormaterials used in the space missions have usually beendeuterated TGS or LiTaO3.

The earliest version of a device for two-dimensionalpyroelectric imaging was developed in the late 1970s. Itwas a modified television vidicon tube with a TGS target,electron beam readout, a germanium lens, and a lightchopper. The problem was that lateral thermal diffusionin the target in the time between electron-beam sweeps re-sulted in low resolution. Techniques for reticulating thetarget into an array of 19-mm islands by ion-beam millingwere then developed to minimize the thermal diffusion.For this device the noise-equivalent temperature differ-ence (NETD)—that is, the smallest detectable tempera-ture difference on a target in the focal plane—varied from0.1 K at low frequencies to 0.5 K at 100 Hz.

The ferroelectric-hybrid focal-plane array, as picturedin figure 3b, was developed in the 1980s. Detector ele-ments made of lead strontium titanate (PST) or bariumstrontium titanate (BST) are commonly used. These ma-terials have Curie temperatures close to room tempera-ture. The devices operate at temperatures above the Curielevel (in the so-called paraelectric phase) and an appliedelectric field induces the pyroelectric effect. Laser-assisted chemical etching produces an array with a largenumber of detector pixels. The elements on that array aresoldered (or “bump bonded”) to a silicon multiplexer. Con-

temporary versions have as many as 384 x 288 pix-els and an NETD as low as 75 mK. Micromachinedthin-film elements with even lower NETDs areunder development.

Applications exist in areas as diverse as firefighting, law enforcement and border patrol, landmine detection, building surveillance, process con-trol, vision testing, facial recognition, and trafficcontrol. Figure 4 shows two examples of the imageresolution pyroelectric devices can reach.

Toward future technologyPhysicists and engineers are likely to develop in-creasingly sensitive pyroelectric devices. Currentresearch has probably reached the sensitivity limitof pure materials, but devices composed of multiplelayers having different compositions and materialsoffer new possibilities. Incorporating thin-film py-roelectrics directly on semiconductor substrates isemerging as an important research area and IR de-tection and imaging using pyroelectrics is likely tobecome much more widespread as costs go down.Important areas to watch for might include night-time visual aids for automobile drivers, devices that

count and guide customers in department stores, devicesincorporated into home appliances, and security aids forthe elderly. Pyroelectricity has been known for 24 cen-turies. There is no reason to think the effect will outliveits usefulness anytime soon.

References1. S. B. Lang, Sourcebook of Pyroelectricity, Gordon & Breach

Science, London (1974).2. S. B. Lang, Br. Ceram. Trans. 103, 65 (2004).3. M. E. Lines, A. M. Glass, Principles and Applications of Fer-

roelectrics and Related Materials, Clarendon Press, Oxford(1977).

4. R. E. Newnham, Properties of Materials: Anisotropy, Symme-try, Structure, Oxford U. Press, New York (2005).

5. J. F. Nye, Physical Properties of Crystals, Clarendon Press,Oxford (1957).

6. S. Bauer, S. B. Lang, IEEE Trans. Dielectr. Electr. Insul. 3,647 (1996).

7. E. R. Caley, J. F. C. Richards, Theophrastus On Stones, TheOhio State U., Columbus (1956).

8. For a list of relevant papers in the literature, see S. B. Lang,Ferroelectrics 308, 193 (2004).

9. S. B. Lang, D. K. Das-Gupta, in Handbook of Advanced Elec-tronic and Photonic Materials and Devices, vol. 4, H. S.Nalwa, ed., Academic Press, San Diego, CA (2001), p. 1.

10. M. H. Lee, R. Guo, A. S. Bhalla, J. Electroceram. 2, 229(1998).

11. P. Muralt, Rep. Prog. Phys. 64, 1339 (2001).12. R. W. Whatmore, Rep. Prog. Phys. 49, 1335 (1986).13. S. B. Lang, Modern Bioelectricity, A. A. Marino, ed., Marcel

Dekker Inc, New York (1988), p. 243.14. W. Wersing, in Ferroelectric Ceram., N. Setter, E. L. Colla,

eds., Birkhauser Verlag, Basel, Switzerland (1993), p. 299.15. R. Watton, Ferroelectrics 91, 113 (1989). �

Figure 4. Pyroelectric imaging. These images were takenusing a ferroelectric-hybrid focal-plane array (top) having256 × 128 pixels (courtesy of DERA Malvern, Crown© 1989 and 1998) and a micromachined thin-film array(bottom) with 320 × 240 pixels (courtesy of RaytheonCommercial Electronics). Lighter colors correspond towarmer temperatures.