Advantages of Fabry-Pérot Interferometer (FPI Detectors)

  • High flexibility in terms of the measurement substances
  • Application chemometric methods for multi-component analysis
  • Measurement of known compositions with overlapping bands
  • Identification of unknown substances
  • Cost-effective, robust and miniaturized solution

Areas of Application

The method of infrared spectroscopy, based on a physical sensor principle, is an established method in the field to determine concentrations and compositions very selectively and with long-term stability. The applications are so diverse that only a few important examples can be listed. While it plays a particularly important role in gas analysis, this method is also used for liquid and solid analysis.

Process Measurement – Analysis of Liquids in Chemical Processes

Fluid analysis in chemical manufacturing processes is an essential component for the optimization of product quality and safety. FPI detectors have the advantage, besides their high flexibility in terms of substances to be measured, that in their design, they are significantly cheaper, more robust and more compact than other large and very expensive process spectrometers or other complex measuring techniques such as ATR probes in combination with NDIR sensors.

Energy Technology – Determination of the Energy Content of Natural Gas or Biogas

The determination of the energy content, for billing purposes or for improving the energy efficiency of combustion processes, is very time-consuming and costly with previous instruments such as gas chromatography. These processes were also hardly economical when used in smaller plants. The resulting increase in demand for cheaper, faster and more precise sensors has been met with the FPI detector due to its design and measurement properties.

Medical Technology – Monitoring/Dosage of Anesthetic Gases

Medical Technology – Monitoring/Dosage of Anesthetic Gases In order to monitor and ensure the safe dosage of anesthetics, up to five gases, mainly in the wavelength range 8 ... 11 µm, in addition to nitrous oxide and carbon dioxide must be measured simultaneously. Due to the number of gas components to be measured, spectrometric measurement and evaluation is the best method. Due to the very compact design, there is a significant miniaturization advantage when compared to conventional methods based on fixed filters.

Further Applications for the Use of FPI-Detectors

  • In safety technology for the detection of toxic or hazardous material or explosive gases
  • In environmental and climate research, for the measurement of emissions, for the identification of plastics in waste sorting, leak detection in pipes, the composition of the atmosphere, etc.
  • For the measurement on surfaces to detect contaminants, to ensure the complete coating of reflective material, to test the thickness of coatings or ensure the complete wetting of lubricants

Product Range FPI Detectors

Different types with first order filters and wide tuning ranges are currently available. They span nearly the whole mid and long wave infrared range of about 3 ... 11 μm. Other ranges or customized designs may be possible on request.

Devices with integrated ASIC can be operated in a control loop – in this type designation they are characterized by the letter "C". Position feedback of the reflector plates is given by capacitance measurement. This means the filter can be stabilized against external acceleration forces. Temperature drifts are automatically compensated by closed loop operation as well.

Product overview FPI detectors

Tunable Detectors

Type LIE, LIM: Detectors with standard design
LME, LMM: Detectors with reduced microphonic effect
LRE, LRM: Detectors with micromechanical frame
LFP: Detectors with tunable Fabry‐Pérot filter
Housing TO18, TO46: 5.4 mm in diameter
TO5, TO39: 9.2 mm in diameter
TO8: 15.2 mm in diameter
The number following the hyphen indicates the number of pins.
Channel Number of measuring channels
Aperture (mm) Size of the circular or rectangular aperture through which radiation hits the sensitive elements.
Operating modes Size of the circular or rectangular aperture through which radiation hits the sensitive elements.
JFET/OpAmp Integrated signal condition like integrated junction field effect transistors (JFET) for detectors in voltage mode or integrated operational amplifiers (OpAmp) for detectors in current mode.
Thermal Compensation A pyroelectric detector is sensitive to changes in temperature. Changes in ambient temperature have an influence on the measurement signal and shifts the operating point. This effect reduces a thermal compensation. Therefore, each pyroelectric element gets an additional optically inactive element, which compensates for unwanted currents.
Low Micro Patented micromechanical chip attachment (LowMicro) by InfraTec to reduce the effects of interfering impact sounds.
Detectivity* As a key parameter, the specific detectivity D* characterizes the signal-to-noise ratio of infrared detectors.
InfraTec Detector Filter Tunable Detectors
Aperture (mm)
Operating modes
Thermal Compensation
Low Micro
Download further information
LFP-3144CTO8tunableø 1.9Current ModeOpAmpYesYes0.036LFP-3144C
LFP-3850CTO8tunableø 1.9Current ModeOpAmpYesYes0.03LFP-3850C
LFP-5580CTO8tunableø 2.0Current ModeOpAmpYesYes0.048LFP-5580C
LFP-80105CTO8tunableø 2.0Current ModeOpAmpYesYes0.04LFP-80105C

*(400 ºC, 10 Hz, 1 Hz, 25 ºC) [10E+8cmHz1/2/W]

Ein mikromechanisches, durchstimmbares Fabry-Pérot-Filter für die Nichtdispersive Gasanalytik im Spektralbereich 3 ... 5 µm (German)

Set-up of FPI detectors

Setup of FPI Detectors

The Fabry-Pérot filters are integrated into TO8 housings together with a LiTaO3-based pyroelectric detector. The latter is a state of the art thermally compensated current-mode type with an integrated op-amp. The element size of 2 × 2 mm2 matches the size of the filter aperture. The order sorting filter is integrated in the cap.

Calibration data are stored in an EEPROM. An integrated ASIC offers feedback on the position of the moving mirror plates. The closed loop control greatly improves center wavelength (CWL) stability under the influence of changing gravity, e. g. by rotating the detectors inside a portable device.

Different construction phases of the FPI detectors from InfraTec

FPI Brochure

Get further information about pyroelectric detectors with spectrometer functionality from InfraTec.

Tunable FPI detectors from InfraTec

Operation Principle of FPI Detectors

InfraTec’s tunable filter technology is based on a micromachined Fabry-Pérot interferometer (FPI), which is comprised of two flat and parallel reflector plates (Fig. 2). In simple terms, it acts as a half wave resonator, producing a series of transmittance peaks of successive interference orders (Fig. 3). By changing the plate separation the peak positions and thus the center wavelength (CWL) can be tuned. An additional, in series with the Fabry-Perot filter (FPF) placed bandpass filter (fine filter) allows an unique narrowband filtering.

Basic set-up and operation principle of a Fabry-Pérot filter
Transmittance spectra and key parameters of a FPF

Features and Advantages of Spectrometers

The basic functional parameters of spectrometers are wavelength range, spectral resolution and throughput (signalto- noise-ratio). In any case, the miniaturization of a spectrometer requires a certain compromise between these parameters that has to be found. In the infrared range this is especially true for resolution and throughput. Compared to grating spectrometers, linear variable filters or miniaturized Fourier-transform spectrometers, tunable FPF offer a very attractive solution in this regard:

  • No entrance slit is limiting the throughput
  • The scanning principle with a large single point detector has a throughput advantage over array detectors
  • Infrared array detectors are very expensive
  • No additional precision optics is needed
  • The parallel plate arrangement of the FPI is ideally suited for miniaturization

Despite the big advantage of the spectrometric sensing principle many applications first of all require a high signal-tonoise-ratio rather than a high spectral resolution. InfraTec’s tunable filters are designed for these specific needs:

  • First order operation (widest tuning range)
  • Large aperture of about 2 mm (high throughput)
  • Moderate resolution (high throughput)
Schematic cross section of the Fabry-Pérot filter

InfraTec’s FPF are comprised of two bonded silicon substrates, which are structured by means of bulk micromachining technology. They form the two plates of the interferometer as well as the electrodes for electrostatic actuation. This approach has some great advantages over competitive solutions:

  • Thick reflector plates, thereby no static or dynamic deformations
  • Reflector and electrode plates separated, thereby tuning range is not limited by electrostatic instability
  • Thin film material system for reflectors and AR coatings from NIR to LWIR

Detector Search

InfraTec offers five different product groups including approximately 50 standard pyroelectric detectors. Choose your suitable infrared detectors with the help of our detailed detector search.

Overview of all standard infrared detectors
Sensor division InfraTec detectors overview