Specim Owl Thermal instrument

The Specim OWL is a multiband thermal instrument covering LWIR (specifically 7.6 to 12.3 μm) with 103 bands. In combination with SWIR measurements, this range enables better classification of minerals, particularly silicates. The sensitivity is sufficient to handle sub-zero temperatures and see differentiation (tested to -21C) and is capable of going up to fire temperatures (verified up to 900-1000C with a yellow-hot metal object, and plenty of headroom in the settings still).

As with the Sensors/Hawk, the detector technology is Mercury Cadmium Telluride, this time tuned for LWIR rather than SWIR. This means there may be a number of bad pixels that need to be masked out. The OWL has also been found to have "blinking" pixels (also called "naughty", as they're not reliably bad), which switch between two different levels during the capture - at the time of writing, these are masked out, though it is hoped that data can be recovered from them in future. The detector uses a full-frame readout method, so is not susceptible to straylight during readout (as is the case with the Eagle). Please see the attachments for further information on blinkers provided by Specim.

In order to calibrate the sensor, a pair of black bodies can be mechanically moved in front of the sensor lens one by one and a capture made. Under normal operational conditions, this is done automatically at the end of every line, after the autodark (closed shutter dark frames) capture. The black bodies should generally be set to bracket the temperature range of interest and can range from about 15C (lower is possible but risks condensation) to 120C. The sensor response has been rated as essentially linear.

The instrument and, more importantly, measurements are vulnerable to water absorption, so a desiccant cartridge is used to maintain conditions internally. The system (detector and optics) is temperature stabilised

The official Specim page can be found at http://www.specim.fi/index.php/products/airborne/aisaowl

Specim's optical calibration was carried out with a range of filters and a rotary stage. The bands used for calibration were: 32, 41, 76, 112 (out of 120). ARSF data quality and calibration/validation evaluation reports may be found in the Reports section.

The shutter is a "ball type", so forms an effective thermal barrier and may provide a third "emergency" black body (at roughly 15C, non-stabilised) to allow cross-calibration against other flightlines in the event of the other black bodies failing to be captured.

Instrument parameters

All numbers provided by Specim 15 May 2014:

Measurement Value
Measured FOV (total) 24.20 degrees
Measured FOV (from centre) -11.79 degrees, 12.41 degrees
Paraxial FOV -11.80 degrees, 12.39 degrees
Sensor IFOV 0.063 degrees or 1.103 mrad
Lens focal length 42.886
- Focal length evaluation wavelength 10014.19nm
Optical axes position (central pixel) 187.7
Max distortion 0.0755mm or 0.0210 degrees or 0.171%

Smile distortion is claimed to be neglible.

FWHM is as documented in the .hdr files.

Instrument type is "OWL3", serial number is 900013. The evaluation was made with an objective of type OLEL43, serial number 011402.

Internal hardware configuration

Details about the internal hardware (e.g. camera type, link protocol, optics, etc) are currently unknown, but will added here if they become generally available.

Raw file format

As with the other sensors, an ENVI BIL format is used. The data are stored in raw binary (14 bit) and the metadata in the accompanying .hdr file. Filenames are of the form NAME_YYYY-mm-DD_HH-MM-SS.{raw|hdr}, where NAME is assigned by the operator.

Black body captures are stored exactly as normal camera data are. Filenames are as above, but prepended with T1_ or T2_ depending on which black body was viewed. Details on the black body set temperatures are stored in the .hdr files.

Additional temperatures are also included in the header files. Under the temperature keyword are:
1) FPA temperature in Kelvin.
2) Optics (approximate) temperature in Celsius. This is close to temperature of the shutter that is the target in autodark.
3) Detector rear part cold plate temperature in Celsius.
4) Power unit temperature in Celsius.
5) Optics temperature stabilization controller temperature in Celsius. This is close to temperature 2 (shutter) but sensor is further away from shutter.

See Sensors/SpecimNavSync.

Last modified 10 months ago Last modified on Oct 3, 2016 4:18:24 PM

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