Detection of vegetation fluorescence gives information about plant functioning, stress and vitality. During the past decades several ground based laser fluorosensors have been developed to investigate these processes and to demonstrate the value of this technique.
FLEX (= FLuorescense EXplorer) is a space mission to measure the fluorescence of vegetation on earth over large areas from space. Such a mission would greatly improve the understanding and enhance the capability to quantify e.g. the role of terrestrial vegetation in global carbon sequestration. Because the fluorescence signal, which is excited by solar irradiation is low with respect to the reflected sunlight the signal from a satellite is proposed to be measured in the solar Fraunhofer lines, where the reflection signal is much reduced.
The heart of FLEX is a high resolution imaging spectrometer with 2 channels: channel 1 around the Fraunhofer lines at ‡ = 397 nm, ‡= 423 nm and/or ‡ = 434 nm and channel 2 around the Fraunhofer line at ‡ = 656 nm. The required spectral resolution will depend on the linewidth (0.02-0.3 nm).
A first definition of the field of view is 8.4 degrees, leading from an 800 km satellite altitude to a swath of about 120 km. For detection a 1024x1024 pixel frame transfer CCD detector is proposed, with a pixel dimension of 13 x 13 ‡ mm2. The maximum footprint is about 500x500m2. The optical configuration contains a scan mirror for solar calibration, for pointing the FOV in swath direction and for freezing the observed ground scene up to a few seconds to increase the signal to noise performance.
At this moment the concept of FLEX is elaborated in a feasibility study. Both the scientific and instrument requirements are updated and the concept is studied in detail. Besides a development plan for FLEX is made.
In this paper the idea and the headlines of FLEX are described.
At present there is an increasing interest in remote sensing of aerosols from space because of the large impact of aerosols on climate, earth observation and health. TNO has performed a study aimed at improving aerosol characterisation using a space based instrument and state-of-the-art aerosol retrieval algorithms, based on requirements for up-to-date regional and global aerosol transport models. The study has resulted in instrument specifications and a concept design for aerosol detection from space. Based on the study the main requirements for a dedicated aerosol spectrometer are: a spectral range from 330-1000 nm with a spectral resolution from 2 nm (UV) to > 30 nm (NIR), observation in at least 3 polarisation directions (Stokes parameters) over a field of view (FOV) in swath directions of > 114 degrees and observation in at least 3 viewing directions (backwards, nadir, forward). The spectrometer design is a prism imaging spectrometer using a single detector array to measure the complete spectra for 2 polarisation directions. In this way the requirements for each viewing direction can be met with only 2 detector arrays. The system has a modular set-up, which makes the implementation of, for example, a change in the number of observation directions very simple. The basic requirements to discriminate between aerosol types are currently only met POLDER, that combines multiple view angles with polarization. The DARE concept shows an attractive potential for the development of next generation aerosol sensors.
For the calibration of OMI a slit function measurement stimulus has been developed. The slit function is the monochromatic image of the entrance slit of the spectrometer on the detector. Accurate knowledge of this slit function for all wavelengths and field angles is very important both for the spectral calibration and for the DOAS retrieval algorithm. Determination of this function with a spectral line source is inaccurate, because of the detector resolution and incomplete, because of the limited number of discrete spectral lines, that are available. For accurate and complete measurement of the slit function an echelle monochromator has been developed, that offers a number of spectral lines, that can be scanned with a small wavelength step over the entire spectral range of OMI. The spectral bandwidth of these lines is about 0.1 x the spectral resolution of OMI and the wavelength step during scanning is even smaller. The wavelength scanning is performed by accurate rotation of the echelle. In this paper the scientific background of the slit function measurement, the stimulus and first OMI slit function calibration measurements are described.
The fluorescence signal emitted from vegetation is directly linked to the photosynthesis and as such may be used as an indicator for plant functioning, stress and vitality. Observation of solar induced fluorescence from space is proposed by measuring the weak signal contribution in the Fraunhofer line wavelengths. In an ESA funded study various aspects of measuring the fluorescence signal from space have been analysed for it's feasibility. Both scientific and instrumental aspects have been considered in the analysis. The scientific requirements have been studied in detail, looking to aspects such as the selection of Fraunhofer lines, the solar induced fluorescence radiance, measurement accuracy, spatial resolution, atmosphere influence, etc. This has resulted in instrument requirements, that are the basis for a trade off study of optical observation techniques. The main choice was between applying a grating spectrometer or a filter spectrometer, each having advantages and disadvantages for Fraunhofer line detection (FLD). From both spectrometer types a preliminary optical design has been made. Besides a model has been developed to evaluate the different configurations for S/N, integration time, radiance level etc. For these calculations it appeared, that the information about solar excited fluorescence intensity of vegetation is minimal. In the study of feasibility of Fraunhofer line detection from space is demonstrated, albeit, that the observation strategy will depend on the real amount of the solar excited fluorescence intensity. The results of the study are a good basis for further development of a spaceborn Fraunhofer line detector.
Specular and diffuse reflectance (BRDF) of black absorbing coatings, meant to be used for the HIFI instrument aboard the FIRST satellite, has been studied in the sub-millimeter region (0.1 < (lambda) < 0.9 mm). These coatings have to meet space qualification requirements and must be usable for at least the overall wavelength band (0.1 - 0.6 mm) covered by the HIFI spectrometer. Existing materials, coatings obtained from other research groups and home made samples have been studied. Optical characterization of these coatings has been performed at wavelengths of 96.5 micrometers , 118.8 micrometers , 184.3 micrometers , 496 micrometers and 889 micrometers , for a large range of directions of incident and reflected light and for different polarization directions. A limited number of reflectance measurements at cryogenic temperatures have been carried out too. A simple experimental set up to study the effect of double scattering has been constructed to investigate the accuracy of numerical simulations based on experimental BRDF values. Data show that the best samples (home made) have BRDF values below about 2.10-2 Sr-1 throughout the wavelength range of interest, quite independent of directions of incidence, reflection and polarization. The Total Hemispherical Reflection of such a coating will then be 0.06.
FLEX is a scientifically driven space mission to provide demonstration/validation of the instrumentation and technique for measuring the natural fluorescence of vegetation in the Fraunhofer lines. The payload consists of high spectral resolution (0.1 - 0.3 nm) CCD imaging grating spectrometer with two channels: one in the red (648 - 664 nm) and one in the blue (391 - 438 nm) for working with several Fraunhofer lines. The across track FOV is 8.4 degrees; ground spatial resolution is better than 0.5 X 0.5 km2. To increase the S/N ratio a steering mirror will be used, if necessary, to 'freeze' the image and also to provide plus or minus 4 degrees across track depointing. Calibration is made by viewing the sun via a diffuser plate switched into the telescope field of view. A separate CCD camera will allow cloud detection and scene identification. A TIR radiometer will provide simultaneous surface temperature measurements. The spacecraft, overall mass estimated at 200 kg, is derived from the ASI-MITA bus which provides all the necessary subsystems and stabilized platform. By use of on-board storage, ground requirements for satellite control and data link are minimized; the possibility of local stations for real time reception/distribution is also envisaged. Provisional orbit characteristics are: LEO sun synchronous, 500 - 900 km altitude. Priority will be given to highest revisit frequency on a sufficient number of selected test sites.
The Ozone Monitoring Instrument (OMI) is a Dutch-Finnish contribution to NASA's EOS-Chemistry satellite, which is due to be launched in December 2002. The aim of OMI is to contribute to climate monitoring and atmospheric chemistry research by providing daily global measurements of the total ozone column, ozone profile, NO2 column, other trace gases like SO2 and BrO2, aerosols, cloud fraction, cloud to pressure, and surface UV irradiance.
Light scattering measurements are important tools for characterizing optical surfaces can basically be divided into two main groups: total scatter measurements (TS) and Angle Resolved Scattering (ARS). Since TS measurements are fairly straight forward and widely used, international standardization has formulated an international draft standard on it, ISO/DIS 13696. ARS is a more complex method and not as common as TS measurements. However ARS data in form of the Bi-directional Reflectance Distribution FUnction (BRDF) can be used to predict stray light in Laser and Industrial Optical Systems. Because of increasing importance of this topic the EC is sponsoring a project regarding 'Standard procedures for stray light specification, measurement and testing - SLIOS'. During the project two of the activities are: performing a round robin experiment of measuring BRDF data at 5 different sites including some complementary techniques; compiling of an open access data base of measured BRDF data, measured according to procedures agreed upon between the SLIOS partners and proposed for 'Standard Procedures'. Results of these two activities will be presented.
In education in optics the explanation and demonstration of aberrations are an essential part. Of the third order monochromatic aberrations usually most attention is paid to spherical aberration, astigmatism, field curvature and distortion. Coma is more complex to explain and more difficult to demonstrate in its pure form. In 1933 Van Heel has designed a 'lens with pure coma' for demonstration purposes. With this lens a number of photographs have been made to show the development of the coma circle from the pupil plane (single circle) to the image plane (double circle) and to demonstrate the coma images around the focal plane. In 1948 a second lens for demonstration of coma was made by Van Heel. Both lenses are described in this paper and their performance is determined by computer calculations. Comparison with original coma photographs is made.
The Fabry-Perot Velocity Interferometer System (F-PVIS) is designed and built for measuring the Doppler shift of light by recording positional changes in the interferometric pattern behind the Fabry-Perot interferometer. The velocity of a surface can be deduced from the Doppler shift which is caused by reflection on the moving surface. The finesse of the Fabry-Perot interferometer is found to be about 50. The F-PVIS is designed for measuring velocities of up to 20 km/s. The sensitivity of this system can be tuned by changing the distance between the Fabry-Perot mirrors. In the most sensitive state of operation, the accuracy is found to be better than 100 m/s while the time resolution is typically a few ns. In addition to the velocity measurement of the moving surface in the electric gun experiments, the fiber optic F-PVIS can be used for other measurements. By embedding the optical fiber into the target material, information about the shock wave inside the target can be achieved.
The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is selected by ESA for the ENVISAT-mission, scheduled for launch in 1999. The instrument will measure the concentration of atmospheric trace gases in the earth atmosphere in a spectral region from 4.15 - 14.6 micrometers . MIPAS consists of scan mirrors, a telescope, a Michelson Interferometer, an optical reducer and a focal plane assembly. The optical reducer consists of a 2 concave and 1 convex mirror system. The focal plane assembly consists of a configuration of mirrors and dichroics, with which the spectral range is divided in 4 spectral bands. TNO Institute of Applied Physics has designed the optical/mechanical system and after manufacturing of the components has aligned the system with high accuracy. The design and alignment of this system is described.
For earth observation instruments accurate on ground calibration is necessary to characterize sensor parameters that are used for the in orbit calibration. TNO Institute of Applied Physics has developed an absolute radiometric calibration facility for the calibration of both complete instruments and diffusers. The facility operates in the wavelength range of 240 - 2400 nm. It incorporates a specially designed spectral irradiance source and a so called Brewster polarization. For accurate diffuser calibration a detector assembly is developed. In this paper the calibration facility with its features is described in some detail and a few calibration results are given.
An elegant measuring setup for contouring strong aspherical surfaces is introduced. Moire deflectometry is chosen as the measuring method because the configuration is simple, robust, and variable in sensitivity. The instrument is capable of measuring height deviations between an aspherical surface and its best fitting sphere ranging from minimally 1 micrometers to maximally 30 micrometers with a relative accuracy of 10%, which is useful for the production of surfaces in infrared optics. It is possible to measure transparent as well as reflecting surfaces, both convex and concave. A CCD-camera and a PC make part of the setup to automate the measurements. The short measurement time of less than 60 seconds makes the instrument useful in the manual production of aspherical surfaces.
SCIAMACHY (SCanning Imaging Absorbtion spectroMeter for Atmospheric CHartographY) is a spectrometer in the UV, VIS and near IR region, with which the concentration of atmospheric trace gases both in the troposphere and in the stratosphere can be determined. It has been selected for ENVISAT of which the launch is scheduled in 1999. SCIAMACHY is a common German-Dutch project. The optical configuration consists of 2 scan mirrors (a nadir/elevation mirror and an azimuth mirror), a telescope and a complex spectrometer, with which a spectral range from 0.24 - 2.4 micrometers is covered in 8 channels with array detectors. Axillary optics consist of two on-board calibration light sources, a sun/moon follower and a polarization measurement device (PMD). The sun/moon follower enables SCIAMACHY to track the sun or moon during occultation and sun/moon calibration measurements. The PMD is a simple seven channel spectrometer with basically sensitivity for one polarization direction only. The PMD is used to correct for the difference in sensitivity for different polarization states of the regular spectrometer channels. The optical system is described in detail elsewhere. The operational success of an instrument like SCIAMACHY depends strongly on the accuracy of the calibration. Proper calibration enables comparison of measurements with those of other instruments, that measure atmospheric trace gases. Besides it gives information about degradation of the spectrometer during its lifetime, so correction of the measurements can be performed to make the real trends in detection of ozone (and other gases) visible. For on-ground calibration of the diffuser and of the spectral channels at room temperature a special facility has been developed at our institute. In the following chapters some essential aspects of the calibration of SCIAMACHY and of the calibration facility are described.
The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) has been selected by ESA for the ENVISAT-Mission, scheduled for launch in 1998. The instrument will measure the concentration of a number of atmospheric trace gases in the earth atmosphere in a spectral region from 4.15 - 14.6 micrometers . Within this region measurements are performed with high spectral resolution. The MIPAS optical system consists of scan mirrors, a telescope, a Michelson interferometer, an afocal reducer and a focal plane assembly. TNO Institute of Applied Physics is involved in the design and development of the afocal reducer and the focal plane assembly. The beam reducing optics of the afocal reducer consist of 2 concave and one convex mirror. Both the housing and the mirrors are made of aluminum to ensure temperature invariance. The optics of the focal plane assembly consist of aluminum mirrors, dichroic beamsplitters and Ge lenses in front of the detectors. The optical/mechanical design is developed to the level that phase C2/D activities can start.
The Short-Wavelength Spectrometer (SWS) for ISO operates in the wavelength range from 2.4 to 45 micrometers. It consists of two, almost identical, grating spectrometers that provide resolving powers varying between 1000 to 2000. In the wavelength region from 12 to 45 micrometers a much larger (>20.000) resolution can be obtained with a pair of Fabry-Perot interferometers. This paper describes the design of the SWS.