Tunable detectors (FPI detectors)

Tunable detectors (FPI) from InfraTec

Here you can find all tunable detectors from InfraTec

This unique new infrared detector design from the R&D lab at InfraTec was developed to allow scanning over a range of wavelengths by simply adjusting the control voltage. Instead of mounting filters to inspect specific areas of interest, the Fabry-Pérot filter control can be used to monitor a broad spectral region continuously.

  • Electrostatic drive
  • > 1 µm tuning range
  • Resolving power λ/ Δλ up to 60

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

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
Product overview FPI detectors

Overview tunable detectors

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Aperture/Window Size (mm)

Current/Voltage Mode (CM/VM)


Temperature Compensated

Low Micro

Downloads and further information
LFP-3144C NewTO8-12tunableø1.9CMOpAmpyesyes 0.036PDF LFP-3144CLFP-3144C
LFP-3850C NewTO8-12tunableø1.9CMOpAmpyesyes 0.03PDF LFP-3850CLFP-3850C
LFP-5580C NewTO8-12tunableø2.0CMOpAmpyesyes 0.048PDF LFP-5580CLFP-5580C
LFP-80105C NewTO8-12tunableø2.0CMOpAmpyesyes 0.04PDF LFP-80105CLFP-80105C
XFP-3137 NewTO8-12tunableø1.8CMnonenono 0.034
Info XFP-3137FP Filter in fourth order of interference with PbSe photoresistor
*(500K, 10Hz, 1Hz, 25ºC, without window) [10E+8cmHz1/2/W]
2(400ºC, 10Hz, 1Hz, 25ºC) [10E+8cmHz1/2/W]

Further information on the benefits of the integrated ASIC can be found here.

Setup of FPI Detectors

Setup of FPI Detectors

The Fabry-Pérot filters (FPF) are integrated into TO8 housings together with a LiTaO3-based pyroelectric detector. The detector 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. A version with an integrated ASIC for position feedback of the moving mirror plates, is optionally available. 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.

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
Basic set-up and operation principle of a Fabry-Pérot filter
Transmittance spectra and key parameters of a FPF
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 (Fig. 4). 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

Evaluation Kit

Evaluation Kit from InfraTec

The kit supports customer needs for an initial test of FPI detectors without having to develop test circuitry or software themselves. It allows easy control of detector and IR source with customized software to analyze the detector signals. With the help of this kit a quick and easy configuration of a simple Fabry-Pérot spectrometer is possible.

Advantages Evaluation Kit

  • Activation of FPI detector and analysis of signals „
  • Activation of IR source
  • „„Extensive configuration options including mappings of complete spectra by use of the "Fabry-Pérot evaluation workbench" software

The Evaluation kit includes a basic board and a detector board with USB interface, USB cable, ribbon cable, CD-ROM with test and measurement software, USB driver, and a manual. The basic board is compatible to all FPI detectors and FPI detectors with ASIC control. A version of this kit with an IR source is also available upon request.

Technical Data


  • „„Power supply 12 V
  • „„Current consumption 150 mA (without IR source)
  • „„Integrated detector supply ± 5 V
  • „„Integrated Fabry-Pérot control (e. g. max. 35 V for LFP-3144-337)
  • Control of IR source up to 800 mA,
  • DC to 500 Hz (square, (0 ... 100) % dury cycle)
  • Signal acquisition with 12 Bit, 2 kHz, analysis with FFT-technique


  • „„PC with Windows 2000, XP, Vista, 7 or higher with USB 2.0 interface
  • Power supply 12 V DC (300 mA minimum)

Further technical information can be found here:
Datasheet Evaluation kit for Fabry-Pérot detectors