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  • Properties and Calibration of MEMS andPiezoactuated Fabry-Perot Spectral Imagers

    EUFAR Joint Expert Working Group Meeting: Imaging SpectroscopySensors, Calibration/ Validation, and Data Processing, Edinburgh,UK, 14th April 2011.

    Heikki SaariVTT Technical Research Centre of Finland

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting 2

    Outline

    Introduction and classification of imaging spectrometersOverview of the VTT Fabry-Perot Interferometer (FPI) technologiesFabry-Perot Interferometer principle and spectroscopy applicationsFPI hyperspectral imager major characteristics and operation modesThe Piezo-actuated FPI component and its controllingVTTs Surface Micro-Machined MEMS FPI StructureWorlds first MEMS FPI Spectral ImagerVTT UAS FPI VIS-VNIR Spectral Camera for forest and agriculture applicationsFPI based spectral camera F-number and throughputMathematical description of the FPI Spectral Imager conceptCalibration of the FPI Hyperspectral ImagerConclusions and future plansAdditional slides on the-state-of-the-art of the hyper and multispectral imagingtechnology and on planned UAV applications of FPI spectral imager

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 3

    Classification of imaging spectrometers for remote sensingapplications

    Ref. R. Sellar&G.Boreman, Classificationof imagingspectrometers forremote sensingapplications, Opt.Eng.44(1), Jan 2005.

    Ref. X. Prieto-Blanco, C.Montero-Orille, B.Couce, R. de la Fuente,Optical configurationsfor ImagingSpectrometers, Comput.Intel. For RemoteSensing, SCI 133, pp. 1-25, 2008 Springer-Verlag, Heidelberg 2008.

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 4

    Overview of the VTT Fabry-Perot Interferometer technologies

    Piezo-actuatedFabry-Perotinterferometermodules

    Detector modules including a Fabry-Perot interferometer detector andoptionally cooling.

    Microelectromechanical components (MEMS)- Development started 1990 / 1991, CARBOCAP -production at Vaisala / VTT 1997- Novel technologies: ALD, Si3N4, etc.- Small, robust, cheap, high volumes- Wavelength ranges 350 nm 14000 nm

    Piezo-components- More transmitted light, large range of wavelengths- Small and medium series with apertures from 3 mm to 50 mm.- Fits well to imaging systems

    VTT owns IPR of several Fabry-Perot inventionsFilter chips, 3x3 mm2

    Tunable band-pass filters (350 14000 nm)

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 5

    Fabry-Perot Interferometer and Spectroscopy

    Fingerprint of Ethanol

    Interference created by tworeflective surfacesUsage: Tunable band-pass filter

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 6

    Fabry-Perot Interferometer (FPI) and principle ofhyperspectral imaging based on it

    400 500 600 700 800 900 10000

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    Wavelength/[nm]

    Spec

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    Fabry-Perot Mirrors

    Air gapOrder sorting

    filter

    Object of thehyperspectral

    imager

    Image of thehyperspectral

    imager

    Front opticsfor collimation Focusing optics

    for imaging

    The Fabry-Perot Interferometer basedhyperspectral imager concept.

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 7

    FPI multispectral image sensor hyperspectral imager principle - Matchingthree Fabry-Perot Interferometer orders to a R-, G-, and B-pixels

    400 500 600 7000

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    Air gap = 1100 nm

    Wavelength/[nm]

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    B pixels at the Air gap = 1100 nmG pixels at the Air gap = 1100 nmR pixels at the Air gap = 1100 nm

    Wavelength/[nm]

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    Blue/B pixelsGreen/G pixlesRed/R pixels

    Wavelength/[nm]

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  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 8

    FPI hyperspectral imager major characteristics

    < 3 W< 400 g

    < 65 x 65 x 130 mm< 1.0 s

    2592 x 1944 pixels

    640 x 480 pixels80 %

    25 %

    < 1.0< 2.8

    < 1 nm< 1 nm

    3 - 11 nm @ FWHM400 1000 nm

    Typical value for Vis-VNIR

    < 25 WPower consumption< 2.0 kgWeight

    < 150 x 100 x 200 mmDimensions< 1.0 sRecording time for 100 layer data cube

    512 x 512 pixelsMaximum spectral image size

    384 x 288 pixelsDefault spectral image dimensions80 %Average FPI transmission with dielectric mirrors

    25 %Average FPI transmission with metallic mirrors

    < 1.0F-number @ 10 max FPI ray angle< 2.8F-number @ 5 max FPI ray angle

    < 1 nmSpectral instability< 1 nmSpectral step

    8 - 30 nm @ FWHMSpectral resolution900 2500 nmVisible-VNIR

    Typical value for SWIRParameter

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 9

    FPI hyperspectral imager operation modes

    Tunable FPI filter camera utilizing onlyone FPI order at a time for exampleVNIR camera for 700 1000 nm with airgap range 520 880 nm, res. 9..11 @FWHM or Vis-VNIR camera for 550 770 nm air gap range 360 560 nm, res.12..15 @ FWHMFalse color imager with relatively narrowGreen, Red and VNIR pass bands with+5 Mpix resolution (see next slide)The FPI camera can be used as atunable multispectral imager with +5Mpix resolution by taking images insequence at the selected spectral bands.Three 5 Mpix images can be recorded intypically in less than 0.3 s.

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    Air gap/[nm]

    Wav

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    gths

    of s

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    eaks

    at s

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    al F

    PI o

    rder

    s/[n

    m]

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 10

    False color imager with relatively narrow Green, Red and VNIR pass bands The FPI spectral camera operates like a

    normal false color camera except thegreen (G), red (R) and VNIR spectralbands are narrower.

    An example of the measured spectraltransmissions of the FPI module at airgap values of 1000 nm and 1500 nm areshown on the right.

    The spectral bands used in the falsecolor imaging at the air gap value 1500nm are:

    G: 560 nm @ 10 nm FWHM R: 670 nm @ 10 nm FWHM VNIR: 845 nm @ 10 nm FWHM

    400 450 500 550 600 650 700 750 800 850 900 950 10000

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    Measured transmission at air 1000 nm gapMeasured transmission at air 1500 nm gap

    Wavelength/[nm]

    Tran

    smiss

    ion

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 11

    The Piezo-actuated Fabry-Perot Interferometer component and its controlling

    Piezo-FPI

    Commercial piezo-actuators

    UV-curable glue

    Ti-Ag-SiO2 coating

    Three electrodes,split capacitances

    Contact pads only onlower substrate

    Fused silica substrate

    => FPI cavity stays parallel=> FPI cavity stays parallelHermetically packagedPiezo actuated FPI

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 12

    Piezo Actuated Fabry-Perot Interferometer control electronics

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 13

    VTTs Surface Micro-Machined MEMS FPI Structure

    Upper mirror acts as a tensioned membrane enables:Large optical apertures up to millimeters in diameterSuperior mirror flatnessVery high nominal frequencies, ~ 1 MHz (insensitive to vibrations)No gravitational effectsLow operation voltages

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 14

    Worlds first MEMS FPI Spectral Imager

    VTT has built worlds first MEMS FPIbased spectral imager device fordemonstration to Photonics West 2011fair

    The device consist of optics, apackaged MEMS Fabry-PerotInterferometer (aperture diameter 2mm), and a color imager, together withelectronics and mechanics.

    Device can be used for surfaceinspection (it has built-in white LEDs aslight source) or in remote sensingmode, focused to infinity

    The wavelength range is 460-585 nmand resolution ca. 5 nm

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 15

    FPI Spectral Imager trials with the UAV helicopter

    VTT has developed in previous VTT andTekes funded projects proprietaryhyperspectral imaging technology forlight weight UAS planes (Patents:FI119345B , GB 2445956, Pat. Appl.FI20095356).A proof of concept prototype has beenbuilt and used in VITO (Flemish Institutefor Technological Research) DraganflyX6 UAS helicopter.The results of the first trial flights hasbeen published in SPIE conferences onSeptember 20091 and April 20102.

    VTT proof-of-concept prototype integrationto the Draganfly X6 US helicopter

    VTT proof-of-concept prototype used for testtargets onboard Draganfly X6 US helicopter

    1Saari, H., Aallos, V., Akujrvi, A., Antila, T., Holmlund C., Kantojrvi, U.,Mkynen, J. and Ollila J., Novel Miniaturized Hyperspectral Sensor forUAV and Space Applications, Proc. SPIE 7474 (2009).

    2Saari, H., Aallos, V., Holmlund, C., Mkynen, J., Delaur, B., Nackaerts,K., and Michiels, B., Novel Hyperspectral Imager for Lightweight UAVs,Proc. SPIE 7668 (2010) to be published.

  • 4/27/2011 EUFAR Joint Expert Working Group Meeting: 16

    VTT UAS FPI VIS-VNIR Spectral Camerafor forest and agriculture applications

    Major specifications of the spectral cameraSpectral range: 500 900 nmSpectral Resolution: 7..10 nm @ FWHMFocal leng

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