Pyroelectric detectors

Pyroelectric Infrared Detectors (PIR)convert the changes in incoming infrared light to electric signals. Pyroelectric materials are characterized by having spontaneous electric polarization, which is altered by temperature changes as infrared light illuminates the elements. Since our sensor series uses this effect they can be used at ambient temperature even in the presence of thermal noise. By choosing appropriate IR receiving electrodes, they serve a wide range of application.

* Characteristics | High Sensitivity | | Versatile selection of IR wavelength filters | | Room temperature operation | | Low cost | | Roust under severe environmental conditions | | Stable against ambient temperature and atmospheric changes | | Stable against electromagnetic interference | * Applications | Light control | | Temperature measurement | | Flame detector | | Automatic door switch | | Visitor detector | | Home security | | | | | * IR Filters | AR coated silicon (low cost) | | 7µm cut-on, long pass, filtered silicon (body detection) | | 4.3 µm bandpass filtered quartz (flame detection) | | 4.5 µm AR coated silicon (body detection) | | 5µm cut-on silicon (body detection) |

Relative transmittance of window. | | | | |

Pyroelectric detectors are infrared sensitive optoelectronic components which are specifically used for detecting electromagnetic radiation in a wavelength range from 2 µm to 14 µm.

A receiver chip of pyroelectric detectors manufactured by InfraTec consists of single-crystalline lithium tantalate. Because of its very high curie temperature of 620 °C lithium tantalate guarantees an extremely low temperature coefficient with an excellent long-term stability of the signal voltage.

Unlike semiconductor detectors, pyroelectric detectors are thermal detectors working with a thermally isolated chip which is covered by a black absorption coating. This coating converts the infrared radiation falling on the chip to heat. The chip changes its temperature in the order of magnitude of µK … mK and, as a result of the pyroelectric effect, produces the electrical signal desired. Thermopiles, too, belong to the group of thermal detectors, however, the measuring effect is less significant. While pyroelectric detectors show a good signal/noise ratio up to modulation frequencies of 4 kHz, e.g. in FTIR spectrometers, thermopiles produce good results up to modulation frequencies of specific Hertz only.
Pyroelectric detectors with integrated beamsplittieogy InfraTec
In addition to the pyroelectric crystal, pyroelectric detectors by InfraTec contain optical and micro-mechanical components. Two-channel and four-channel pyroelectric detectors with integrated beamsplitter and integrated CMOS amplifier are micro-systemsconsisting of components which function thermally, electronically and optically.

As pyroelectricity is a characteristic of a subgroup of piezoelectric crystals, pyroelectric detectors react to airborne and solid-borne sound. This effect is often called microphony. However, a patented fastening of the pyroelectric chip by InfraTec reduces these negative ffects for pyroelectric detectors dramatically so that in many cases these negative effects are in the order of magnitude of other interference voltage or of the inherent noise of the detector only
How does a pyroelectric detector work?

When light radiation (UV, VIS, IR, THz) is applied to a thin pyroelectric crystal (<40µm) its temperature increases by fractions of a degree centigrade. Turning on the radiation an electrical charge is generated by heating, turning off the light the crystal cools down and an opposite charge is generated. These very small electrical charges are generally converted within the detector housing to convenient signal voltages by use of extremely low noise and low leakage Field Effect Transistors (JFET) or CMOS operational amplifiers (OpAmp). It is very important to remember that only modulated radiation creates a signal, therefore either pulsed or mechanically chopped IR sources are used and unmodulated disturbing background radiation is filtered out.

In what wavelength range do pyroelectric detectors operate?

As the thermal effect of the incoming radiation is used to produce the electrical detector response, electromagnetic radiation from deep UV (100nm) over the visible range to the far infrared up to the THz range (1000µm) can be detected, as long as the pyroelectric crystal is covered with a suitable absorption layer. InfraTec use two different coating technologies for black absorbing layers. The polymer black coating is used for most detectors and offers extremely stable long term absorption from UV up to 100µm IR even for high modulation frequencies (max 4kHz). A special metal black coating is used especially for spectrometer detectors and is characterized by an extremely flat and high absorbance but is sensitive for operating temperatures >60°C, high radiation power as well as strong vibrations.

What are pyroelectric detectors used for?

A pyroelectric detector can be used very exactly and with long term stability to measure IR radiation. As the pyroelectric element only reacts to a change of the IR radiation the detector must always be used with a modulated (mechanically chopped or electrically pulsed) radiation source. Since pyroelectric detectors operate on a thermal phenomenon they have a very broad spectral response - between 100 nm to over 1000 µm without any cooling like semiconductor detectors. Most common applications are motion detection, NDIR gas analysis, flame detection with spectroscopy and radiometry also possible. Even if pyroelectric detectors are thermal detectors they are able to measure signals up to some kHz with high performance. Short pulses can be detected down to some µs (microseconds) but with considerable loss of signal-to-noise.

Is it possible to use pyroelectric detectors which contain neither a JFET nor an OpAmp thus only the pyroelectric element in gas analysis and flame detection?

The charges created in the pyroelectric crystal are very small and need to be amplified by preamplifiers with very high input impedances (up to some 10 GOhm). At ambient conditions (for example 60% relative humidity and 23°C) such circuitry cannot be operated without disturbances. At least the high impedance part of the circuitry should be within the hermetically sealed detector housing. We therefore recommend using detectors with integrated JFET or OpAmp for gas analysis and flame detection.

Is it useful to cool down pyroelectric detectors for example by using a Peltier device to improve the signal-to-noise-ratio?

No. Pyroelectric detectors unlike PbS or PbSe detectors do not need any cooling even for the detection of longwave radiation in the range of 8-14 µm. But operating temperatures of over 50° C do however increase the detector noise, as the integrated amplifier components exhibit larger leakage currents at higher temperatures. It is noteworthy that CMOS OpAmps react much less as JFET's, as the gate leakage current of a JFET increases exponentially with temperature. For applications with operating temperatures above 60°C always consider the use of detectors with CMOS operational amplifiers.

Is it useful to heat pyroelectric detectors to improve the signal-to-noise-ratio?

No! Pyroelectric detectors for FTIR spectrometers are sometimes operated at higher temperatures of about 50°C because here a special pyroelectric crystal (DTGS) is used instead of LiTaO3 used by InfraTec. This DTGS crystal material has a Curie temperature of about 59°C, LiTaO3 shows 620°C. Near to the Curie temperature the pyroelectric coefficient and resulting signal voltages are growing remarkably but with the side effect of a very high temperature coefficient. A visible signal increase by warming the LiTaO3 detector is not possible due to the very high Curie temperature but otherwise the temperature coefficient of LiTaO3 is extremely low. Heating of LiTaO3 detectors (40 ... 60°C) is only common for gas analyzers to avoid a condensation of wet gases or to reduce the optical filter drift by temperature stabilizing.

In which temperature range is it possible to use pyroelectric detectors?

InfraTec uses single crystalline LiTaO3 polished on both sides as pyroelectric material. This permanently polarized material has a Curie temperature of 620° C and therefore does not limit the usable temperature range. The maximum operating temperature of the detector is therefore mainly limited by the parameters of the integrated preamplifiers. The mechanical properties of the built-in IR filters or windows and its assembling technology limit the minimum and maximum storage temperature.

Is it possible to work at operation temperatures over 100° C?

Principally this is no problem for the pyroelectric element itself, however care has to be taken when selecting the electronic components like resistor, JFET an OpAmp as these have to meet the increased specification. Additionally the built-in IR filter or window has to be designed for and its mounting technology must be able to guarantee a long term hermetical sealing by buffering mechanical tensions. A combination of both high (>85°C) and very low temperatures (<-25°C) is difficult. For applications with operating temperatures above 60°C always consider the use of detectors with CMOS operational amplifiers instead of JFET because their leakage current is lower.

What is the purpose of "thermal compensation?"

The DC output voltage (operating point) of the pyroelectric detector (JFET or OpAmp) can be stabilized in temperature ramps by about a factor 20 with the use of a blind, golden-mirrored antiparallel connected pyroelectric element. This helps to shorten the warm up phase or to increase the accuracy of a handheld IR system. Thermal compensation is very common for gas analyzers, less common for flame detection, not common for spectrometers.

What is the maximum distance an InfraTec detector can detect an IR radiation?

This depends on the detectivity of the detector and eventually additional optics in front of it. Distances of up to 100 meters are common in flame detection. To cover large distances current mode detectors with large pyroelectric elements are useful.

Excelitas offers a wide range of Pyroelectric Detectors in Single Element, Dual Element and Quad Element designs. To satisfy a broad range of applications, we provide various element geometries, filters and housings.
With the addition of the new DigiPyro® Family, setting the standard in digital motion and presence detection, we have successfully begun helping our customers convert analog detectors to digital. Our latest DigiPyro innovations include the “Smart” DigiPyro, for integrated motion electronic circuitry. With Excelitas, you are not limited to off-the-shelf designs; customized designs are also possible.

For a standard Dual Element Detector, the signals are in the range of 3000 V/W. That means the infrared radiation of a person moving at a speed of 1 m/s with a distance of 10 m from the detector generates a signal of some mV. Output signals can be analog as well as digital.
Pyroelectric Detectors utilize the “Pyroelectric Effect” for non-contact radiation measurement. Several types of crystals create charges on the surface, induced by temperature changes in the material that cause a change in the polarization of the sensor material. Whenever a change occurs in the radiation that is absorbed on the top surface of the sensitive material, the temperature of the material changes and the Pyroelectric Detector generates a signal. Using an IR filter allows one to select the sensitive wavelength range. If the radiation does not change, the temperature of the crystal will reach equilibrium and the Pyroelectric Element will discharge.

Pyroelectric detector has advantages for terahertz sensing

The Golay cell is a device traditionally used to measure the intensity of terahertz radiation, but an alternative device--the lithium tantalate (LiTaO3) pyroelectric detector--has some major advantages, according to Don Dooley, president of Spectrum Detector (Lake Oswego, OR). An application note written by Dooley describes and compares the two types of detectors.1
Dooley describes the two devices as follows.
The Golay cell is a "photo-acoustic" device that is sensitive, works at ambient temperatures and has broad spectral response. The basic elements that make up a Golay cell are the 6 mm HDPE (high-density polyethylene) input window, and a small fragile gas chamber that includes a thin, partially absorbing film and what is called an "optical microphone section." When infrared or terahertz radiation is absorbed by the thin film in the gas cell, the gas is heated, and expands and distorts the mirrored back wall of the cell. This distortion (or movement) is monitored and measured by the combination of an LED, optics, grating, and photodiode. The output of the photodiode is proportional to the displacement of the mirrored wall of the gas cell. Its output is calibrated against a source of known power output in volts/watt.
The LiTaO3 pyroelectric detector is an AC thermal detector that is sensitive, typically used at room temperature, and has broad, flat spectral response across most of the electromagnetic spectrum. The detector is based on a thin permanently poled, ferroelectric crystal (here, LiTaO3) that exhibits a pronounced thermal effect (the pyroelectric effect) where its instantaneous polarization is a function of the rate of temperature change of the crystal. By applying conductive electrodes to the top and bottom surfaces of the crystal, the resulting charge can be coupled out of the device and calibrated in terms of microamps per watt. The pyroelectric detectors normally include a voltage- or current-mode circuit for optimum performance and are ultimately calibrated in volts/watt or volts/joule.
Dooley notes that the Golay cell can take only one form at present, whereas the pyroelectric detector can take many forms--for example, the hybrid detector/amp, and the analog or digital radiometer (that is, a detector, discrete electronics, and microprocessor).
Next, Dooley compares the performance specifications for the two devices, choosing the pyroelectric hybrid detector/amp for the comparison. While the Golay cell has a 6 x 6 mm detector size and requires a HDPE window, the pyroelectric detector can range from 2 x 2 to 20 x 20 mm in size, requires no window, and includes a black absorbing coating. The Golay cell has a responsivity (in volts/watt) of 150 K at 15 Hz, a noise-equivalent power (NEP, in W/(Hz)0.5) of 1.2 x 10-10, a detectivity (in cm(Hz)0.5/W) of 7 x 109, and an optimum chopping frequency of 20 Hz. The pyroelectric detector has a responsivity of 150 K at 5 Hz, a NEP of 4.0 x 10-10, a detectivity of 4 x 108, and an optimum chopping frequency of 5 to 10 Hz.
Further, the Golay cell handles a maximum power of 10 microwatts, a wavelength range of 7 to 2000 microns, and an operating temperature of 5 to 40 ºC, while the pyroelectric detector handles a max power of 50 mW per square centimeter of detector area, a wavelength range of 0.1 to 3000 microns, and an operating temperature of -5 to 120 ºC. The Golay cell is bulky (126x45x87 mm) and slow (response time of 25 msec), while the pyroelectric detector is only 8 mm in diameter by 19 mm long and responds in microseconds to milliseconds.
From the comparison, Dooley draws the conclusions that the Golay cell is slightly more sensitive, has a larger sensing area (which may be important when measuring a point source), a fixed window (which will effect its spectral response), a slow response time, and is physically large; he notes that a Golay cell also requires AC voltage for operation.
The pyroelectric detector, on the other hand, is almost as sensitive, can be used windowless or with windows, includes a black absorber for flat spectral response, is inherently fast, has a large operating temperature, is small, and can be operated off either batteries or an AC supply.

Most-important advantages and disadvantages The Golay cell is very sensitive (subnanowatt), has a broad and well-characterized spectral response, and has been used as a standard in astronomy and infrared sensing for years. However, it is very fragile (thin membrane) and quite slow in response, has a large two-piece housing, is very sensitive to mechanical vibration, has a transient response (varies with room pressure), handles low power only (10 microwatts max), requires a HDPE window for operation, comes in one model only, has a long lead time, and is expensive (~$12K to $15K).
The LiTaO3 pyroelectric detector is sensitive (nanowatts), has a broad, flat spectral response, a small housing, a large operating-temperature range, and a fast response time, can operate without a window, can handle relatively high power (50 mW), comes in multiple models and detector sizes, is relatively inexpensive (~$450 to $1950), and is quite rugged and readily available. However, it also has a lower detectivity, its spectral response is not well established, and it can have a microphonic response.
Dooley notes that, while the pyroelectric detector has not been used until recently in the terahertz field (though the technology itself is mature), its small size, low NEP, broad spectral response, and low cost make it a good choice. As the sensor science develops to better match the radiation range from 20 to 0.1 THz, Dooley believes that the pyroelectric-detector choice will only look better.