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The
radiation plants
reflect may vary due to factors that are completely unrelated to
the plant itself. The incident radiation that lights
the scene, for example, can vary
due to cloud cover,
humidity, time of day, the season, pollution, and dozens of other
factors.
The camera that views the scene can have its view altered by its
own settings. Cameras in auto exposure mode, for
example, will vary their exposure in order to obtain the best
results for the overall picture. This might indicate a
problem where none exist or mask a problem where one does.
Tetracam's patented
Micro-MCA electronic Incident Light Sensor (e-ILSTM) is
designed to overcome precisely these types of remote sensing
setbacks. The e-ILSTM is an optional
accessory that is available with all Micro-MCA and Micro-MCA Snap systems.
The device consists of an up-looking imaging sensor that is equipped with
a standard 16 GB micro SD image memory, a fast USB 2.0 interface,
a single Mini-MCA 1280 x 1024 CMOS sensor and 8.0 mm filters,
each with
the same center wavelength and bandwidth values as as those present in the down-looking Micro-MCA. The
filters are contained beneath a translucent lens that diffuses
incoming radiation. The e-ILSTM is attached to the Micro-MCA
by means of an electrical umbilical cable in a position that
affords a clear un-obscured view of the sky. The umbilical
carries power, ground and a serial trigger signal between the e-ILSTM
and the Micro-MCA
system. Extension cables are available for mounting
the e-ILSTM
up to seven meters away from the Micro-MCA system.
The
job of the
e-ILSTM
is to gather down-welling radiation at the same wavelengths
as the up-welling reflected radiation captured by the Micro-MCA.
The
e-ILSTM
saves a calibration image of the sky at the same time
that the Micro-MCA master camera signals all of the other cameras
in the array to capture images. Once the mission is
completed, the calibration images can be extracted from the
e-ILSTM
over the device's USB 2.0 connection or by physically removing the
16 GB micro SD image memory from the e-ILSTM
camera. When processed by Pixelwrench2, every pixel in
every image in the array will have two values to process; the reflected value from
the objects in the scene and the e-ILS calibration value. Comparing the two enables
the software to represent each pixel by the exact fraction of
radiation that it reflects back to the camera.
The ratio of upwelling reflected
radiation to down-welling incident light for each pixel in the
image is expressed as some fraction of
either 255 for an 8-bit image or 1023 for a 10-bit image. The
resulting image accurately represents the reflectance value of the
radiation at each pixel in the image. Images that represent
reflectance values typically appear darker than those that are
optimized to display best overall image exposure by the user (see
example images below).
Reflectance values
do not change with changes in incident light
because a change in the light that lights the scene changes the
scene as well. Lowering incident light proportionally lowers
reflected light and the reflectance value stays the same. In
addition, since the Micro-MCA system's master camera controls
exposure of all cameras in the array as well as the
e-ILSTM,
despite varying exposure settings reflectance values will remain constant.
Reflectance
values more accurately reflect the physical and chemical characteristics of the
subject being viewed and are less affected by lighting
inconsistencies. Even though incident light may vary from
one day to another, if there is no change in the subject then the
ratio of reflected light to incident light will stay the
same. Capturing reflectance values provides a better
day-to-day comparison between monitored areas of interest because variations in
reflectance values provide a more consistently accurate indication
of physical or chemical changes in the
reflecting bodies themselves.
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Example MCA Image without ILS |
Example MCA Image with ILS |
The electronic-Incident Light Sensor may be ordered with new Micro-MCA systems
or existing Micro-MCA system owners may contact Tetracam, obtain an RMA,
ship their Micro-MCA systems to our Chatsworth facility and have an
e-ILSTM
added to their system. Contact us for current pricing, availability, and further details. |
