Keywords: sky, modtran, mirror
Summary
This scene contains a mirrored hemisphere that reflects what a MODTRAN-driven sky irradiance field looks like at 8:10 AM local time.
Details
The mirrored hemisphere employs a unique material description to make a surface that is a perfect mirror and can efficiently modeled by DIRSIG. This mirrored hemisphere is then used to visualize the sky irradiance pattern predicted by a MODTRAN-driven Classic Atmospheric model. The sensor uses the Raw capture method to create a spectral radiance image cube for the Vis/NIR/SWIR region of the spectrum.
Important Files
This section highlights key files important to the simulation.
The Scene
The scene geometry is contained in the geometry folder. The ground,
arrows and letters are described by polygon geometry in various GDB
files that are instanced in geometry/demo.odb. This ODB file
also contains as SPHERE geometry primitive that is half buried
in the ground. The simple materials for the ground, arrows and
letters are not worthy of discussion. However, the mirror material
on the hemisphere employs a material pair. The sphere geometry is
attributed with material ID #100, which uses the Mirror radiometry
solver, which is a specialized radiometry solver that requires no optical
properties. It models the surface as perfect specular reflector with a
spectrally constant, unity reflectance.
MATERIAL_ENTRY {
NAME = Mirror
ID = 100
DOUBLE_SIDED = TRUE
RAD_SOLVER_NAME = Mirror
RAD_SOLVER {
}
}The Atmosphere
The atmosphere uses the Classic radiative transfer model to access
MODTRAN for the atmospheric path radiances and transmissions. The
MODTRAN model is driven by the mls_rural_23km.tp5 tape5 file,
which uses the mid-latitude summer (MLS) atmospheric profile and
rural aerosol model with a 23 km visibility. The atmospheric
database (ADB) is stored in the file classic_mls_rural_23km.adb.
The Platform
The demo.platform file describes a simple 320 x 240 (QVGA) camera
that is configured with the Raw capture method to create a spectral
radiance cube from 0.400 to 2.500 microns at 0.010 micron spectral
sampling. The platform was placed at a low view angle (high
declination angle) and just North of East in order to see the
reflection of the sun rise at this early morning time.
Results
A RGB visualization of the spectral image cube (see demo.img)
generated by the simulation was created using the 0.65, 0.55 and
0.45 micron channels. At the 8:10 AM local time, the sun is just
rising in the East, which can be seen in the sky reflected in the
mirrored hemisphere. The structure within the mirrored hemisphere
is an artifact of the course sky sampling employed by the Classic
atmosphere model. Use of the Threshold atmosphere model to
interface with MODTRAN would result in improved sampling of the sky
at the cost of additional run-time.
A select set of pixel radiance profiles are shown in the plot below. The spectral profiles show the relative magnitude differences across the scene and the impact of atmospheric absorption (for example, the water absorption bands in the Near-IR and Short-wave IR near channel #55, #75, #100 and #150).
