Frequently Asked Ques­tions about Sensor Technology

Pyroelectric crystals have a rare asymmetry due to their single polar axis. This causes their polarization to change with temperature. This so-called pyroelectric effect is used in sensor technology. For this, a thin pyroelectric crystal is coated perpendicular to the polar axis with electrodes. On the upper electrode of the crystal, an absorbing layer (black layer) is applied. When this layer interacts with infrared radiation, the pyroelectric layer heats up and surface charge arises. If the radiation is switched off, a charge of the opposite polarity originates. However, the charge is very low. Before the finite internal resistance of the crystal can equalize the charges, extremely low-noise and low leakage current field-effect transistors (JFET) or operational amplifier (OpAmp) convert the charges into a signal voltage.

No, because with the pyroelectric effect only changes in radiation are recognized. Consequently, the static background is not operative even without additional signal processing algorithms.

As a pyroelectric element responds only to changes in infrared radiation, a pyroelectric detector must always be operated with a modulated source. An exception are measurement objects that give off rapidly changing radiation, such as flames. Examples of modulated sources could be miniature incandescent lamps in conjunction with mechanical choppers or electrically modulated infrared sources.

Pyroelectric detectors can convert infrared radiation into relatively high signal voltages very accurately and with long-term stability. Important areas of application of pyroelectric detectors are motion detection, gas analysis, flame detection, spectroscopy and pyrometry. In contrast to the thermopile, pyroelectric detectors can measure with high-resolution up to modulation frequencies of several kilohertz, which for example allows for their use in Fourier transform infrared spectrometers. Short laser pulses of the order of microseconds can still be detected. High accuracy energy measurements of pulsed infrared laser radiation are also possible, even with pulses in the nanosecond range.

Unlike quantum detectors such as photodiodes, thermal detectors have a very broad spectral response. Pyroelectric detectors can detect radiation from the ultraviolet range (100 nm) through the visible and infrared wavelength range to terahertz waves (1000 µm). This is possible as long as the pyroelectric crystal has a suitable absorption coating. InfraTec uses two different technologies for these so-called black films. For most applications, the polymer black layer is used. Made in a wafer process, it provides a temperature- and long-term stable high absorption of radiation from the ultraviolet range to the far infrared and up to wavelengths of about 100 µm, or for modulation frequencies up to a few kilohertz. For special applications, the metal black film was developed. Its spectral, highly homogeneous absorption makes it suitable for demanding spectrometer applications. However, it requires restrictions on the maximum operating temperature to 60 °C and is not suitable for high radiation power densities and strong vibrations.

The molecular vibrations of many chemical compounds can be excited by energies that lie in the infrared wavelength range. Therefore, these substances absorb infrared radiation. The absorption spectrum depends mainly on the structure of the molecule and thus the degrees of freedom for the movement of the molecular components, their mass, their compositions, their spacing and their binding forces. Therefore, each substance has a characteristic absorption spectrum. For example, in carbon dioxide molecules can be excited bending and stretching vibrations. Additionally rotational movements around different molecular axes are possible, which superpose the vibrational spectrum and generate their fine structures.

An IR spectrometer measures the absorption spectrums of gases. The comparison with the spectrums that are stored in a database allows a qualitative and quantitative reference for the substance.

For this form of gas analysis, the spectral sensitivity of a broadband thermal detector is limited by an optical bandpass filter. This is done for the range in which the absorption bands are found for the gas to be determined. With the thermal detector, the transmission of the measured gas mixture is determined in a defined arrangement. If the gas being searched for is not present, most of the infrared radiation will reach the detector and the signal will be at its maximum. If the concentration of the gas increases, absorption will also increase according to Lambert-Beer's law, and the signal will reduce accordingly.

Compared to infrared spectrometers, the NDIR gas analysis is significantly cheaper. However, their use is only possible if the gases to be measured are known, and their number is low.

Noble gases consist only of individual atoms. In order to vibrate, however, at least one bond is required. Noble gases can therefore not be detected with this method. In diatomic elemental gases such as oxygen (O2) or nitrogen (N2), only a few vibrational modes can be excited by infrared radiation, so that even here the method fails.

An NDIR gas analyzer consists of an electrically or mechanically modulated infrared source, a gas cell and usually a pyroelectric detector. An electronic device calculates the gas concentration based on signal voltage. InfraTec offers a large number of standard IR narrow bandpass filters (NBP), which are optimally matched to the absorption properties of the gases to be measured. The filters are mounted in the cap of the detector, which is welded to the detector base body. With multiple gases to be measured, the use of multi-channel detectors is recommended.

Yes, for NDIR gas analyzers, it is advisable to use a reference channel and normalize the signal of the gas duct via the quotient process on this reference. The optical, mechanical and electronic drift of the overall system is reduced considerably, and the interval between calibrations can be significantly extended. The spectral position of the optical reference should be located as close as possible to the spectral lines of the gases to be measured. A reference channel can be shared for multiple gases, if all absorption bands lie within a spectral window from 3 to 5 µm or from 8 to 12 µm. Without an optical reference channel, a reference can still be done by periodically introducing a reference gas into the channel, such as nitrogen.

The pyroelectric detector of the flame sensor detects the typical spectral radiance of burning organic materials such as wood, natural gas, oil or plastic. In order to prevent a false alarm due to sunlight or other intense light sources, such as light from arc welding, two independent criteria of a flame are analyzed: First a typical flame is characterized by a flicker frequency of 1 to 5 Hz. Secondly, a hydrocarbon flame contains the combustion gases carbon monoxide (CO) and carbon dioxide (CO2). Their emission bands lie in the infrared spectral range of 4.0 to 4.8 µm. In order to obtain a high signal, one uses wide bandpass filters for the detector window, which include both the radiation emission of CO and of CO2. Optionally, a further channel can be used to recognize a further combustion by product, water.

Yes, especially InfraTec’s pyroelectric detectors in current mode with a chip size of (2 x 2) mm2 or (3 x 3) mm2 have a high signal-to-noise ratio and a very small popcorn noise. This allows for reliable detection.

The charges generated in the pyroelectric crystal are very low, requiring a preamplifier with extremely high input impedances of up to some 10 GΩ. Even at normal humidity of about 60 % at an ambient temperature of 22 °C, such preamplifier circuits no longer work without interference. Therefore, it must be inside the sealed detector housing. Due to the necessary reinforcement, detectors without integrated junction field effect transistors (JFET) or operational amplifier (OpAmp) are hardly suitable for gas analysis or flame sensors.

No, pyroelectric detectors do not need cooling of their pyroelectric chips even to detect long-wave infrared radiation of, for example, 14 µm. 

InfraTec uses on both sides polished monocrystalline lithium tantalate (LiTaO3) as a pyroelectric material. This permanently polarized material has a Curie point of about 620 °C and thus does not limit the possible temperature range for use and storage of the detectors. Other factors may have influence, however. The upper operating temperature is limited by the parameters of the integrated preamplifier. Operating temperatures above 60 °C increase the detector noise, since the integrated amplifier components exhibit larger leakage currents at higher temperatures. The thermo-mechanical properties of the detector window and its mounting technology restrict the lower and upper storage temperature.

Generally, yes. The pyroelectric element made of lithium tantalate itself has practically no upper temperature limit. However, temperature-dependent leakage currents of the integrated active components result in an increase of the noise content in the signal and place significant restrictions on the dimensioning of the impedance converter. The reverse current of the junction field-effect transistors increases exponentially with temperature. Therefore detectors with CMOS operational amplifiers should be used for applications with operating temperatures above 60 °C because their residual currents are lower. Since the infrared window must guarantee a tight seal of the detector housing over the entire lifetime, there are significant engineering and technological requirements in terms of mechanical stresses in the sensitive infrared window or the interference filter layer system to be managed. Detectors with sensitive detector window materials should generally not be exposed to extreme temperature differences. The data sheets contain information on the respective permissible temperature ranges.

DTGS detectors are sometimes operated at an elevated temperature of about 50 °C. Just below the Curie point (about 60 °C for DTGS), the pyroelectric coefficient and thus the signal voltage is increased significantly. This advantage is, however, accompanied by a very high temperature coefficient. InfraTec uses LiTaO3 as its pyroelectric, whose extremely high Curie point of 620 °C ensures a minimum temperature coefficient in the operating temperature range. A significant signal increase can not be achieved, therefore, by heating. In gas analyzers, however, it is common practice to heat the whole detector block to about 40 to 60 °C and operate it at a stable temperature. This reduces the formation of condensate and the temperature-induced wavelength drift of the infrared filters.

All pyroelectric materials are also piezoelectric for physical reasons. This is why the pyroelectric chip of a detector responds to physical and sound waves like a microphone or an accelerometer. This phenomenon is called microphonic effect or acceleration sensitivity.

The dense detector housing already reduces the impact of airborne sound waves. InfraTec reduces the effects of interfering impact sounds through a patented micromechanical chip attachment (LowMicro). As a result, the interference voltage is compensated up to a few percent in all three spatial directions. The type designation of these detectors begins with LME (single-channel) or LMM (multi-channel) instead of LIE or LIM.

Since the pyroelectric detector is sensitive to changes in temperature, changes in ambient temperature also have an influence on the measurement signal and shifts the operating point. This effect reduces a thermal compensation by a factor of 20. This is why additional optically inactive pyroelectric elements are connected to the opposite phase of active pyroelectric elements. The result is a significantly more stable operating point. In addition, this shortens the warm-up time of the device. This is especially important for the accuracy and stability of handheld instruments. The thermal compensation is often used in gas analyzers.

The thermal compensation does not reduce the temperature drift of the built-in infrared filters and the temperature coefficients of signal voltage and gas concentration measurements.

The pyroelectric detector element first delivers a charge which charges the element electrically acting also as a capacitor. The resulting voltage is amplified and results in the signal voltage of the detector. The amplifier, with its high-impedance input resistance, concurrently acts as an impedance converter. Serving as a base is usually a JFET, providing a gate resistor and external source resistance. In conventional modulation frequencies of 1 Hz to 10 Hz, the pyroelectric detector is already working at the 1/f - range above the electric time constant. Typical signal voltages are a few millivolts.

The charge supplied by the pyroelectric detector element flows within a certain time through the input of an amplifier and provides an input resistance dependent current. The amplifier operates as a current-to-voltage converter and supplies the signal voltage of the detector. To this end, a transfer impedance amplifier composed of an operational amplifier with feedback components is typically used. Depending on the dimensioning of the feedback components, pyroelectric detectors generate a nearly constant output voltage in a wide frequency range, a constant radiation of the source assumed. The detector operates between the thermal and the electrical time constant. This typically causes signal voltages of several 100 mV.

For the performance of a pyroelectric detector, the frequency response of the signal voltage is essential. This is determined by two time constants: the thermal time constant (typically 150 ms) arises from the thermal coupling of the pyroelectric element to the environment. It is not influenced by the operating mode and is therefore identical for current and voltage operation. The electrical time constant (typically a few s) in voltage mode is the product of the capacitance of the pyroelectric element and the gate resistance. Since both can not be chosen freely, a shortening of the electrical time constant is limited. In current mode, the electrical time constant is the product of the feedback resistor and feedback capacitance. This can largely be selected independently of the pyroelectric element (typically < 20ms). Frequency response and signal voltage can therefore be adjusted in a wide range when using current mode detectors. With typical dimensioning results, there is a more than 100 times greater signal voltage than with voltage mode detectors. The desired signal level is selectable by the feedback resistor in the range of about 1:20. The significantly shorter electrical time constant (typically 1 % compared with the voltage mode) enables short settling times of the amplifier electronics after signal jumps. Since low-noise low-power operational amplifiers are available, a comparatively high signal-to-noise ratio can be reached like using detectors in voltage mode.

The reduction in the gate resistance of an uncompensated pyroelectric detector in voltage mode is a simple method for stabilizing the operating point for temperature changes. It is a cost effective alternative to the use of the thermally compensated detector. However, the low-frequency detector noise, which is inversely proportional to the square root of the gate resistance, increases relative to the thermal compensation significantly.

An example shows this clearly. For a similar stability of an uncompensated detector, its gate resistance must be reduced to about 1/16, for example 82 GΩ would be reduced to 5 GΩ. While the thermal compensation of the signal-to-noise ratio is reduced by about 70%, this ratio sinks with decreasing the gate resistance to root (1/16) = 25%. A thermal compensation is thus, at the same operating point stabilization, about three times better than the reduction of the gate resistance based on the noise.

All detectors are hermetically welded. InfraTec uses nitrogen in the process, which has a dew point of below -50 °C. The IR-permeable window or filter is mounted with a specially qualified adhesive in the housing cap for this purpose.

Electrically modulation capable thermal infrared sources in TO5 or TO8 housing are state of the art. They are now available as micromechanical components with a long service lifetime. They have no moving mechanical parts, unlike a chopper.

But turning the radiant heating area of the infrared sources on and off causes a modulation of the spectral characteristics in addition to the desired modulation of the radiation power on the basis of Planck's radiation law. As thermal radiators, modulated infrared sources have a thermal time constant through which, depending on type, the modulation depth of the miniature heating area is reduced from 3 to 30 Hz. For modulation frequencies of about 100 Hz or for a precisely rectangular radiation modulation, InfraTec still recommends using a mechanical chopper.

Almost all pyroelectric detectors by InfraTec contain a pre-amplifier, which consists of a JFET source follower or a CMOS operational amplifier. In both cases, test the function of the detector by controlling the DC output voltage. The voltage must be in the thermally stable state corresponding to the value in the measurement report. For a source follower, a deviation of ± 10 % is typical. The value of the operational amplifier can differ by typically 1 mV. In addition, the voltage should only show a slow drift.

Beam splitter detectors are multi-channel detectors with only a single aperture, in which an array of a plurality of micro mirror surfaces works as an internal beam splitter. The division of the beam into two or four spectral channels is done only within the detector. The aperture with a diameter of 2.5 mm allows for the use of cells with very small dimensions for NDIR gas analysis. This result is very low dead volume of the measurement system. Mechanical expansion in the optical system of the non-dispersive infrared gas analyzer, aging effects or eventual smudging can effect detectors only before the beam splitting occurs and thus impact equally to all channels. So the signal ratio between the individual channels remains permanently stable.

Multi channel standard detectors by InfraTec capture up to four spectral channels simultaneously, usually three gas channels and a reference. If more spectral channels are required, by maintaining the simultaneous measurement in all spectral channels, the beam splitting for two multichannel detectors can be done via an external beam splitter. In the simplest case, this is by an uncoated silicon chip set in the beam path at 45°.

An alternative to this are classical mechanical filter wheels. Despite the filter wheel mechanism, they allow for very compact and reliable sensor assemblies in combination with a fast pyroelectric miniature detector (for example LIE-200 or LME-300). In multi-gas analysis, up to eight filter positions are common.

The latest variant of the multi-channel gas analyzer uses pyroelectric detectors with an integrated tunable infrared filter. InfraTec uses a micromechanical Fabry-Pérot filter. There are four types with different spectral tuning ranges from 3 to 10 µm in a TO8 casing. The detectors come standard with an EEPROM containing the calibration data.

The European Union has established a number of guidelines in order to protect its citizens and the environment. The most important of these for the electronics industry are as follows:

The EG-Directive 2011/65/EU – RoHS = Restriction of (the use of certain) Hazardous Substances.

RoHS restricts the use of six potentially environmentally hazardous substances and substance groups in electronics and electrical equipment. This includes lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB) and polybrominated diphenyl ether (PBDE). There are, however, a number of large product groups that are not covered by this directive. Additionally, there is a series of applications exempted from the restrictions. In 2015, further substances (phthalates) were added.

The EU-Directive 2002/96/EG – WEEE = Waste of Electrical and Electronic Equipment.

WEEE establishes extended responsibilities for manufacturers by return and recycling of end-of-life electronics and electrical equipment.

The EG-Regulation 1907/2006 – REACH = Registration, Evaluation, Authorisation and Restriction of Chemicals.

REACH is the result of a harmonization process of laws pertaining to the allowance, documentation and notification requirements for chemicals and chemical substances and the limitations of their use. A pre-stage for a necessary authorisation, or limitation of use, is the classification SVHC (Substance of Very High Concern) in products and manufacturing processes.

These regulations are binding for all EU member states and are to be enforced as national laws in each state.

All companies that place goods on the market in the European Union are required to, and must guarantee that they are in compliance with all guidelines and regulations pertaining to their products, including all national laws for that given country.

InfraTec detectors are not devices under regulation by the RoHS Directive. InfraTec, as a manufacturer of components, however, exclusively uses materials and constructions that enable our customers to produce devices that are in full compliance with the RoHS Directive.

WEEE regulates the return requirements for electronic devices, but not for their individual components. For this reason, there are no requirements under this directive that pertain directly to pyroelectric detectors by InfraTec.

Our detectors only contain substances, or substances in small enough quantities, that there are no restrictions on their use and no specific information requirements under REACH regulations.

Each delivery includes a declaration of compliance, taking into consideration the regulations of both RoHS and REACH.