54 CIRCUIT CELLAR® • www.circuitcellar.com

Mar

ch 2

012

– Iss

ue 2

60

I

by Richard Wotiz (USA)

remember being fascinated by infrared(IR) noncontact thermometers years ago

when they first became popular in the con-sumer marketplace. The concept of measuringtemperature at a distance almost seemed likemagic at the time. I wanted to try one out formyself, but they were too expensive backthen. Now that advances in sensor technologyhave brought costs down considerably, I wasable to pick up a few different models to study(see Photo 1). This month I’ll discuss varioustypes of sensors used for measuring thermalradiation. I’ll also describe how some productsuse them.

Any object with a temperature aboveabsolute zero will radiateenergy. The hotter anobject, the more energy itemits and the shorter thewavelength of the energy.The total radiant energy isproportional to the fourthpower of temperature. Aradiometer is any devicethat measures this energy.If it’s calibrated with atemperature scale, it’sknown as a radiationthermometer. If it’s specif-ically designed to detectIR energy, it’s commonlycalled an IR thermometer.The energy detected froman object also depends on

its emissivity. That indicates how much of theenergy is radiated from the object itself, ratherthan reflected off its surface from nearbysources. Many inexpensive IR thermometersassume a fixed emissivity of 0.95, which isreasonably accurate for most nonreflectivematerials.

THERMAL RADIATIONThe first radiation thermometer was devel-

oped in 1828. It used the decidedly low-techmethod of focusing thermal radiation onto thebulb of a thermometer using a concave mirror.In 1892, Henry Louis Le Chatelier created thefirst comparison-type optical pyrometer, which

Infrared Thermal Detectors

Thermal sensing technology has come a long way since the first radiationthermometer was developed in the early 19th century. Review this surveyof thermal sensing technologies—from thermopile sensors to pyroelectricdetectors—before starting your next temperature-sensing design.

Embedded Unveiled

Photo 1—This is an inside view of an ear thermometer. The thermopile sensor fitssnugly inside the cone-shaped tip, which you place in your ear to take your tem-perature. The dark blob next to the battery is the microcontroller. It’s mountedchip-on-board, which is less expensive in high production volumes than using apackaged part. There’s also a serial EEPROM on the back of the board to storecalibration data and recent measurements.

www.circuitcellar.com • CIRCUIT CELLAR® 55

Mar

ch 2

012

– Iss

ue 2

60

available sensors. Thermal detectorsfunction by heating up as a result ofabsorbing thermal energy, which cantake from milliseconds to seconds,depending on detector size. Quantumdetectors measure incoming photonsdirectly. This gives them greaterspeed, typically in the nanosecond-to-millisecond range. They are alsomore sensitive than thermal detec-tors, but operate over a narrowerrange of wavelengths that’s depend-ent on the detector material. Theyoften need external cooling to reducenoise when measuring temperaturesat or below room temperature. We’lltake a closer look at thermopile andpyroelectric detectors, since theseare the most common noncontact IRsensors found in low-cost consumer-grade products.

THERMOPILE SENSORSA thermopile consists of a number

of series-connected thermocouples.To understand the operation of a ther-mocouple, we need to go back to1821, when T. J. Seebeck discoveredthat a current flowed through twowires of dissimilar metals that werejoined at both ends if one junctionwas at a different temperature thanthe other. This is known as the ther-moelectric, or Seebeck, effect, andresults from electrons diffusing froma hotter to a cooler section of wire. Ifyou heat up part of a single wire, elec-trons will diffuse equally in bothdirections along the wire, causing avoltage gradient within the wire butno net voltage difference at its ends.But the diffusion rate varies for differ-ent materials. So, if you heat the junc-tion of two dissimilar wires, the gradi-ents will be different for each wire,and you’ll see a voltage developedacross the opposite ends of the wires.Figure 1 shows this phenomenon in

was useful for measuring objects thatwere hot enough to glow. The userwould adjust an iris diaphragm untilthe brightness of the object, seenthrough a viewing telescope, matchedthe flame of an oil lamp. The iris set-ting would then indicate the tempera-ture. This technique was made morepractical when an electric bulbreplaced the oil lamp in the early1900s. This instrument, known as adisappearing-filament optical pyrome-ter, still provides highly accurate tem-perature measurements for the steeland petrochemical industries.[1] Bothof these devices required manual oper-ation, a limitation that Charles Féryovercame in 1901 with the first radia-tion thermometer using a lens tofocus thermal energy onto a thermo-couple. The temperature could thenbe read directly from a galvanometerdriven by the thermocouple.[2] By the1950s this led to a more sensitiveinstrument using a photomultipliertube as a detector, which evolved tothe noncontact IR thermometers wehave today.

Table 1 shows various commonly

Table 1—Noncontact radiant energy sensors

Type Sensor Output

Thermal detector Thermopile Voltage

Bolometer Resistance

Pyroelectric detector Charge displacement

Quantum (photon)

detector

Photovoltaic detector Voltage

Photoelectromagnetic detector Voltage

Photoconductive detector Resistance

Figure 1—The thermoelectric effect inaction. Heating two identical wires joined atone end has no net effect (top), but heatingtwo dissimilar wires results in a measurablevoltage (bottom).

Hot Cold

0 V

–

–

–

+

+ V

– V

–

–

––

– –

+

+ +

+ –

action. The voltage is typically inthe microvolt range for small tem-perature differences (below 100°C),so multiple thermocouples can beassembled into a thermopile to get amore manageable output signal. Youcan learn more about thermopilesensors in Brian Millier’s article“Noncontact Infrared Thermometry”(Circuit Cellar 175, 2005).

Photo 1 shows the inside of an earthermometer. It uses a thermopilesensor with a built-in reference ther-mistor. The thermopile provides avoltage related to the difference intemperatures between its hot andcold junctions, and the thermistorreports the temperature of the coldjunction. A microcontroller combinesthese two values using the appropri-ate mathematical formulas to producean accurate temperature reading ofthe hot junction. When you insert thethermometer properly into your ear,this reading will represent your inter-nal body temperature. I wanted to seehow easy it was to use the sensor’soutput directly, so I measured its out-put while varying the input tempera-ture over the thermometer’s statedoperating range of 93.2°F to 109.4°F.I’ve read about certain Buddhistmonks who can vary their body tem-perature while deep in meditation,but that wasn’t very practical for mytests. Instead, I found I could aim thesensor at my desk lamp from a fewinches away and cover the entirerange by slightly varying the distance.The results of the experiment areshown in Figure 2. The curve has aslope of approximately 50 µV per °F.

Figure 2—Sensor output of the ear thermometer.The nonlinearity in the curve isn’t from thesensor. It came from slight variations inambient temperature during the experimentas I moved the thermometer closer or fartheraway from the desk lamp.

110

106

102

98

94

0.4 0.5 0.6 0.7 0.8

mV

0.9 1.0 1.1

°F

Photo 3 shows the business end of anear thermometer with a pyroelectricdetector. This particular model is afew years old, but it serves as a usefulexample of detector operation. Nor-mally, the detector is hidden behind ashutter that’s mounted inside themetal block located to the left of thebutton. A thermistor mounted withinthe block tracks the temperature ofthe block and shutter. When you pressthe button, a spring-loaded mecha-nism snaps the shutter out from infront of the detector, giving it a clearview of whatever the probe is aimedat. The detector will output a signalthat’s a function of the difference intemperatures.Figure 3 shows the thermometer’s

analog front end. It uses a dual-slopeanalog-to-digital converter (ADC)made up of a multiplexer IC and anop-amp integrator. You can follow themeasurement process in Photo 4.Normally, the system repeatedlymeasures the temperature of the ther-mistor (SEL2:0 = 4,5) and the pyro-electric detector (SEL2:0 = 1,2). Thecursors show when I pressed andreleased the Measure button. Whenpressed, the system immediatelyresets the integrator for 100 ms(SEL2:0 = 0) and then integrates thedetector input for 200 ms (SEL2:0 = 1).Then it integrates a reference voltageuntil the comparator output goes high(SEL2:0 = 2). The input voltage isrelated to the integration times andmultiplexer input resistors as follows:

This formula applies to a steadyinput voltage, but you’ll notice thatthe input isn’t constant during theintegration period. Rather, it isdecaying with an RC time constantdetermined by the sensor capacitanceand the sensor input op-amp stage’sinput resistance. Since this time con-stant is fixed, and the ADC’s integra-tion time always starts 100 ms afterthe shutter is opened and lasts for200 ms, it’s possible for the micro-controller to compute the value ofthe initial peak. It uses this valuealong with the thermistor value tocompute the temperature.

V VIN REF

= R

R

t

t

IN

REF

REF

IN

− × ×

56 CIRCUIT CELLAR® • www.circuitcellar.com

Mar

ch 2

012

– Iss

ue 2

60

where relative amounts of differentgases can be measured by looking atthe variations in IR absorption at dif-ferent wavelengths.

PYROELECTRIC DETECTORSA pyroelectric detector is a capacitive

sensor that changes its polarization inresponse to a change in temperature.This results in a change in surfacecharge ∆Q = A × p × ∆T, where A isthe sensor area, p is the pyroelectriccoefficient of the material, and ∆T isthe temperature change. It’s inherent-ly an AC-coupled device, so it needssome external process to cause thesensed temperature to change in orderto produce any output. This makes itperfect for motion sensors, but it’salso used for thermometers that use amechanical shutter to produce a con-trolled change in input temperature.[3]

The liquid crystal display (LCD)shows the temperature to a tenth of adegree, which would be a resolutionof 5 µV. I find that rather impres-sive—there must be quite a bit ofaveraging being done to reduce noise.If you extrapolate the curve to theleft, it would hit 0 mV at about 88°F.That’s a reasonable value for ambienttemperature, since the thermometerhad been sitting under the desk lampfor a while by the time I started tak-ing measurements.You can also find thermopile sen-

sors in general-purpose IR thermome-ters. Photo 2 shows one such device.The sensor is hidden underneath theplastic lens, but it’s similar to the oneused in the ear thermometer. Thismodel has a laser pointer to locate thecenter of the sensing area. Ther-mopiles are also used in gas analyzers,

Photo 2—A general-purpose noncontactthermometer. TheFresnel lens focusesincoming IR energyonto the sensormounted underneathit. The copper-coloredlaser diode module(far right) points outthe center of the areabeing measured.

Photo 3—The sensing mechanism of an ear thermometer. It uses a pyroelectric sensor witha pushbutton-activated mechanical shutter. The metal block stabilizes the temperature ofthe entire mechanism.

www.circuitcellar.com • CIRCUIT CELLAR® 57

Mar

ch 2

012

– Iss

ue 2

60

Finally, let’s look at a motion sensor, which is a morenatural application for a pyroelectric detector since herethe AC-coupled output is an advantage. The detector isslightly more complex, as it contains two pyroelectricelements connected in series back-to-back inside thepackage, separated by a fixed distance. Some detectors,including this one, also include an FET, which can beconnected as a source follower to lower the detector’s

output impedance. In a motion sensor, the detector ismounted behind a Fresnel lens whose multiple segmentsalternately focus any incoming IR energy on one elementthen the other as a person walks by. Photo 5 shows thisprocess as I waved my arm in front of a sensor. TheSENSE signal normally sits at 1.04 V, and you can see itoscillating around this value as the lens alternatelyfocuses the thermal energy from my arm onto each of

Figure 3—The pyroelectric ear thermometer’s analog input circuitry and dual-slope ADC. Calibration is done using several adjustment pots.

RIN

MUX

SEL

CINT

MCU

RREF

0

1

2

4

5Thermistor

Measure

Shutter

Pyroelectricdetector

IR –

+

– –+

800.975.4743 USA • 1 630.245.1445

gridconnect.com/gridarm.html

TH E N ETWO RK I NG E XPE RTS

Leaders in the embedded and networking marketplace providing network hardware, high quality software and services

The gridARM™ System on a Chip (SOC) is a high performance, low cost, low power, highly integrated single chip with 10 / 100 / 1000 Mbps Ethernet, USB, CAN, Serial, SRAM Memory, SPI, I2C, RTC and internal peripherals designed to provide a complete solution for embedded applications.

$10.00

(Qty 100)each

58 CIRCUIT CELLAR® • www.circuitcellar.com

Mar

ch 2

012

– Iss

ue 2

60

REFERENCES[1] D. DeWitt and G. Nutter, “Theory and Practice ofRadiation Thermometry,” Wiley-Interscience, 1988.

[2] C. Féry, “Thermoelektrisches Pyrometer,” GermanPatent 135064, 1901.

[3] J. Fraden, “Infrared Electronic Thermometer andMethod for Measuring Temperature,” U.S. Patent4797840, 1989.

Richard Wotiz has been taking products apart ever since hewas old enough to pick up a soldering iron. He’s been helpingothers put them back together since 1991, when he startedhis design consulting business. Richard specializes in hard-ware and software for consumer products and children’s toys.He can be reached at rw601@spiraltap.com.

the detector’s two sensing elements. As the energy hitsthe first element, the detector’s output goes positivemomentarily. Then, as it leaves that element and hits thesecond one, the voltage across the first element goes neg-ative while the second goes positive. The inverse seriesconnection means these voltages will add, effectivelydoubling the output voltage. It’s still not very large, onlyaround 1 mV to produce the 100-mV peak shown in thephoto.

COOLING DOWNI hope I have helped take some of the mystery out of

thermal IR sensors and their everyday applications. I’mready to do more exploring now that I have a handful ofsensors to work with. Maybe you are now inspired totake a closer look at some firsthand. I

Photo 5—The motion sensor in action. SENSE is the pyroelectricdetector output driving a 47-kΩ resistor and amplified by 100. TRIGshows when the output first exceeds the motion detection threshold.

Photo 4—The ADC signals during a temperature measurement.You can see the multiplexer input select, comparator output, andintegrating capacitor voltage. VSNS shows the pyroelectric sensorinput signal shifting as I press and release the Measure button,indicated by the cursors. NEED-TO-KNOW INFO

Knowledge is power. In the computer applicationsindustry, informed engineers and programmersdon’t just survive, they thrive and excel. For moreneed-to-know information about some of the topicscovered in this article, the Circuit Cellar editorialstaff recommends the following content:

—Plastic Extruder Thermal Analysisby Ed NisleyCircuit Cellar 251, 2011Measuring and understanding the thermal per-

formance of the Thing-O-Matic 3-D printer’sextruder head provides some interesting results.Topics: Thermal Analysis, Extruder, Thermocouple,Calibration, Insulation, Thermal Resistance

—Thermal Performanceby Ed NisleyCircuit Cellar 249, 2011Understanding the principles of heat transfer and

having the ability to work out heat transfer problemsare essential skills for any serious electronics engi-neer. The techniques detailed in this article willenable you to figure out the origin of a project’s heatand how to measure its progress. Topics: Heatsink,Heat Transfer, Dissipation, Energy Management,Thermal Resistance, Cooling, Derating

—Noncontact Infrared Thermometryby Brian MillierCircuit Cellar 175, 2005Noncontact infrared thermometry has enabled

doctors to get rid of mercury thermometers. Thistechnology also enables distributors to monitor thetemperature of food products without contaminatingthem with sensors. Topics: Temperature, Infrared,Thermometry, Radiation, Thermopile

—Go to Circuit Cellar’s webshop to find these articlesand more: www.cc-webshop.com