Uula Kantojärvi

Novel miniaturized hyperspectral sensor for UAV and space applications

In many hyperspectral applications it is beneficial to produce 2D spatial images with a single exposure at a few selected wavelength bands instead of 1D spatial and all spectral band images like in push-broom instruments. VTT has developed a new concept based on the Piezo actuated Fabry-Perot Interferometer to enable recording of 2D spatial images at the selected wavelength bands simultaneously. The sensor size is compatible with light weight UAV platforms. In our spectrometer the multiple orders of the Fabry-Perot Interferometer are used at the same time matched to the sensitivities of a multispectral RGB-type image sensor channels. We have built prototypes of the new spectrograph fitting inside of a 40 mm x 40 mm x 20 mm envelope and with a mass less than 50 g. The operational wavelength range of built prototypes can be tuned in the range 400 - 1100 nm and the spectral resolution is in the range 5 - 10 nm @ FWHM. Presently the spatial resolution is 480 x 750 pixels but it can be increased simply by changing the image sensor. The hyperspectral imager records simultaneously a 2D image of the scenery at three narrow wavelength bands determined by the selected three orders of the Fabry-Perot Interferometer which depend on the air gap between the mirrors of the Fabry-Perot Cavity. The new sensor can be applied on UAV, aircraft, and other platforms requiring small volume, mass and power consumption. The new low cost hyperspectral imager can be used also in many industrial and medical applications.

Spectral imaging device based on a tuneable MEMS Fabry-Perot interferometer

The trend in the development of single-point spectrometric sensors is miniaturization, cost reduction and increase of functionality and versatility. MEMS Fabry-Perot interferometers (FPI) have been proven to meet many of these requirements in the form of miniaturized spectrometer modules and tuneable light sources. Recent development of MEMS FPI devices based on ALD thin film structures potentially addresses all of these main trends. In this paper we present a device and first measurement results of a small imaging spectrometer utilizing a 1.5 mm tuneable MEMS FPI filter working in the visible range of 430-580 nm. The construction of the instrument and the properties of the tuneable filter are explained especially from imaging requirements point of view.

Compact large-aperture Fabry-Perot interferometer modules for gas spectroscopy at mid-IR

VTT has developed Fabry-Pérot Interferometers (FPI) for visible and infrared wavelengths since 90’s. Here we present two new platforms for mid-infrared gas spectroscopy having a large optical aperture to provide high optical throughput but still enabling miniaturized instrument size. First platform is a tunable filter that replaces a traditional filter wheel, which operates between wavelengths of 4-5 um. Second platform is for correlation spectroscopy where the interferometer provides a comb-like transmission pattern mimicking absorption of diatomic molecules at the wavelength range of 4.7-4.8 um. The Bragg mirrors have 2-4 thin layers of polysilicon and silicon oxide.

Intelligent MEMS spectral sensor for NIR applications (Conference Presentation)

Near Infrared (NIR) spectrometers have been widely used in many material inspection applications, but mainly in central laboratories. The role of miniaturization, robustness of spectrometer and portability are really crucial when field inspection tools should be developed. We present an advanced spectral sensor based on a tunable Microelectromechanical (MEMS) Fabry-Perot Interferometer which will meet these requirements. We describe the wireless device design, operation principle and easy-to-use algorithms to adapt the sensor to number of applications. Multiple devices can be operated simultaneously and seamlessly through cloud connectivity. We also present some practical NIR applications carried out with truly portable NIR device.

Advanced MEMS spectral sensor for the NIR

Near Infrared (NIR) spectrometers are widely used in many fields to measure material content, such as moisture, fat and protein in grains, foodstuffs and pharmaceutical powders. These fields include applications where only highly miniaturized and robust NIR sensors can be used due to small usable space, weight requirements and/or hostile working environment. Handheld devices for material inspection, online process automation and automotive industry introduce requirements for size, robustness and cost, which is currently difficult to meet. In this paper we present an advanced spectral sensor based on a tunable Microelectromechanical (MEMS) Fabry-Perot Interferometer. The sensor is fibercoupled, weighs 125 grams and fits to an envelope of 25x55x55 mm3. Three types of sensors cover the wavelength ranges from 1.35-1.7 μm, 1.55-2.0 μm and 1.7-2.2 μm, utilizing only a single pixel extended InGaAs detector, avoiding the expensive linear array detectors. We describe the design, principle of operation and calibration methods together with the control schemes. Some environmental tests are described and their results and finally application measurement results are presented along with discussion and conclusions.

Performance of the Imaging Spectral Signature Instrument (ISSI) breadboard

With hyperspectral pushbroom imaging spectrometers it is possible to identify ground pixels by their spectral signature. The Imaging Spectral Signature Instrument (ISSI) concept performs optical on-board processing of the hyperspectral data to identify pixels with a pre-defined and programmable spectral signature. An aircraft compatible breadboard of ISSI has been developed and manufactured. It consists of an objective, which images an object line on the input slit of a first imaging spectrograph, which disperses each pixel of the object line into its spectral content on a liquid-crystal spatial light modulator. This component is programmed with a spatial transmission behaviour, which is constant along the spatial pixels and equal to the spectral filter vector of the searched specific signature along the spectral pixels. A second inverted spectrograph re-images the transmitted flux into a line of pixels on a CCD detector. ISSI operates at wavelengths between 470 nm and 900 nm. The spectral filter vector can be selected for 800 spatial pixels with a spectral resolution of better than 4 nm and almost 8 bit modulation capability. ISSI effectively performs the spectral angle mapping (SAM) operation used in hyperspectral data processing. The signatures are acquired with ISSI in a spectrometer operating mode. The breadboard has undergone a test program consisting of calibration and verification of the spectral, spatial and radiometric performance and object identification capability.

Imaging spectral signature satellite instrument for the real-time identification of ground scenes with a dedicated spectral signature

With hyperspectral pushbroom imaging spectrometers on Earth observation satellites it is possible to detect and identify dedicated ground pixels by their spectral signature. Conventional time consuming on-ground processing performs this selection by processing the measured hyperspectral data cube of the image. The Imaging Spectral Signature Instrument (ISSI) concept combines an optical on-board processing of the hyperspectral data cube with a thresholding algorithm, to identify pixels with a pre-defined and programmable spectral signature, such as water, forest and minerals, in the ground swath. The Imaging Spectral Signature Instrument consists of an imaging telescope, which images an object line on the entrance slit of a first imaging spectrometer, which disperses each pixel of the object line into its spectral content and images the hyperspectral image on the spatial light modulator. This spatial light modulator will be programmed with a spatial transmission or reflection behavior, which is constant along the spatial pixels and along the spectral pixels identical to a filter vector that corresponds to the spectral signature of the searched specific feature. A second inverted spectrometer reimages the by the first spectrometer dispersed and by the spatial light modulator transmitted or reflected flux into a line of pixels. In case the spectral content of the ground scene is identical to the searched signature, the flux traversing or reflecting the spatial light modulator will be maximum. The related pixel can be identified in the final image as a high signal by a threshold discriminator. A component test setup consists of an imaging lens, two Imspector™ spectrographs, a spatial light modulator, which is a programmable transmissible liquid crystal display and a CCD sensor as a detector. A mathematical model was developed for the instrument and its performance was evaluated in order to compare different concept variations. All components were measured and characterized individually, and the results were used in the simulations. Performance was then analyzed by means of radiometric throughput and spatial and spectral resolutions. The simulations were performed at wavelengths of 450 nm to 900 nm. The throughput was found to be between 1% and 4.5%.