A unique, hyperspectral imaging plane "on-a-chip" developed for deployment as a High Performance Payload (HPP) on
a micro or small unmanned aerial vehicle is described. HPP employs nanophotonics technologies to create a focal plane
array with very high fill factor fabricated using standard integrated circuit techniques. The spectral response of each
pixel can be independently tuned and controlled over the entire spectral range of the camera. While the current HPP is
designed to operate in the visible, the underlying physical principles of the device are applicable and potentially
implementable from the UV through the long-wave infrared.
A standing wave spectrometer is turned into a wavelength tunable band-pass filter by the addition of a reflective coating.
It results in the standing wave filter (SWF), a miniaturized Fabry-Perot band-pass filter with a semi-transparent detector
that can be constructed into a pixel-tunable focal plane array, suitable for hyperspectral imaging applications. The
asymmetric Fabry-Perot cavity is formed between the reflective coating and a tunable mirror, originally part of the
spectrometer. The predicted performance of the SWF is optimized through modeling based on the matrix formalism used
in thin film optics and with FDTD simulations. The SWF concept is taken from an ideal device to a focal plane array
design that was fabricated with 40 micron pixels using semi-conductor processing technology. First-light spectra
measured from the 100 pixel Standing Wave Filter array agree with predictions and prove the concept.
The modeling of a novel filter for the visible spectrum, constructed of an array of micron sized Fabry-Perot band-pass
filters, is presented. An example filter array consists of a hyperspectral 5x5 sub-array, where each squared cavity pixel,
9 μm wide, is resonating at a different wavelength than the neighboring pixels. For such small cavities, traditional 1D
modeling of Fabry-Perot cavities of infinite extent is insufficient. To study pixel edge effects and pixel cross-talk, 2D
FDTD simulations were carried out. Extensive modeling was done for a cavity array with several pixels, and sloped
cavity edges were compared to vertical ones. Both the optical field inside each cavity, and the corresponding integrated
power over the whole cavity were calculated. Comparisons of the peak power and spectral bandwidth were made
between a finite pixel cavity and a cavity of infinite extent. Both normal and oblique incident light was included in the
simulations.
Fabry-Perot filter arrays have been fabricated comprised of six million individual filters using standard semiconductor
processing techniques. The current 3000x2000 array consists of 5x5 sub-arrays in which each of the nine micron wide
Fabry-Perot filters in the sub-array has a different color response. The 5x5 sub-array is replicated to create a 600x400
matrix of 5x5 micro Fabry-Perot filter sub-arrays. This Fabry-Perot matrix has been integrated with a commercially
available panchromatic 6 Megapixel CCD focal plane array to create a 25 color hyperspectral camera with 600x400
imaging pixels. Near-UV, visible and NIR filter arrays have been fabricated. The semiconductor processing technique
permits filter arrays of general filter size, shape, configuration and distribution to be implemented with ease.
Since shortly after launch the radiometric performance of band 6 of the ETM+ instrument on Landsat 7 has been evaluated using vicarious calibration techniques for both land and water targets. This evaluation indicates the radiometric performance of band 6 has been both highly stable and accurate. Over a range corresponding to a factor of two in radiance (5 to 55 C in kinetic temperature terms) the difference between the in-situ derived radiance and the image derived radiance is on average 0.5% or less. Water targets are the easiest to use but are limited to the temperature range from 0 to about 32 C. Land targets can reach 55 C or more but are far less spatially homogeneous than water targets with respect to both local surface temperature and spectral emissivity. The techniques used and the results are described.
Calibration of the five EOS ASTER instrument emission bands (90 m pixels at surface) is being checked during the operational life of the mission using field measurements simultaneous with the image acquisition. For water targets, radiometers, temperature measuring buoys and local radiosonde atmospheric profiles are used to determine the average water surface kinetic temperature over areas roughly 3 X 3 pixels in size. The in-band surface leaving radiance is then projected through the atmosphere using the MODTRAN radiation transfer code allowing an at sensor radiance comparison. The instrument at sensor radiance is also projected to the water surface allowing a comparison in terms of water surface kinetic temperature. Over the first year of operation, the field measurement derived at sensor radiance agrees with the image derived radiance to better than plus/minus 1% for all five bands indicating both stable and accurate operation.
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