This section describes sensor calibration processes and how to access additional information about specific calibration protocols, calibration targets, and associated reference data.
Calibration protocols have been defined by LemnaTec in cooperation with vendors and the TERRA-REF Sensor Steering Committee. Draft calibration protocols are currently in Google Drive and have been incorporated into the LemnaTec Scanalyzer Field sensor documentation.
A detailed calibration process is also provided for the Hyperspectral sensors, with further information below.
The following calibration targets are available:
- Aluminum 3D test object
The environmental sensor has been calibrated by LemnaTec. The output of the spectrometer is raw counts, users will need to use the calibration files to convert to units of µW m-2 s-1, taking into account the bandwidth of the chip (0.4nm) if converting to µmol m-2 s-1.
Calibration reference data is available via Globus
/sites/ua-mac/EnvironmentLogger/CalibrationDataor in Github Calibrations.zip
For the SWIR and VNIR sensors, factory calibration is repeated each year using the calibration lamp provided by Headwall. To convert the hyperspectral exposure image to reflectance requires the wavelength-dependent, factory calibrated reflectance of the spectralon at all VNIR and SWIR wavelengths and a good image of a spectralon panel from each camera. This includes periodic measurements of a white spectralon reflectance panel run with 20ms exposure to match panel calibration.
Dark reference measurement:
- Dark measurement for VNIR camera is taken at exposure times 20, 25, 30, 35, 40, 45, 50, 55ms.
- Data is in the same hypercube format with 180-200 lines, 955 bands, and 1600 pixel samples.
- Measurement was done using Headwall software, so there is no LemnaTec json file.
- The name of the folder is the exposure time. "current setting exposure" is showing the exposure time in ms.
- Custom workflow to process the calibration files.
- Dark counts handled internally, so no calibration files are necessary.
White reference measurement:
- White measurement for VNIR camera is taken at exposure times 20, 25, 30, 25, 40, 25, 50, 55 ms.
- The name of the folder is the exposure time. Data are 1600 sample, 955 bands and 268-298 lines. White reference is located in the lines between 60 to 100 and in the samples between 600 to 1000.
- Data is available via Google Drive.The white reference scans was done at around 1pm ( one hour after solar noon). I don’t see the saturation with 20ms and 25ms exposure time.
- For the calibration, this needs to be subtracted from the dark current in the same sample, band and exposure time.
- In the following file, I stored an extra file named "CorrectedWhite_raw". This file includes only a single white pixel( one line, one sample) in 955 bands for each exposure time. Data is stored in the similar format but it doesnot include any extra files like frameIndex, image, header ,..https:\/\/drive.google.com\/file\/d\/0ByXIACImwxA7dVNHa3pTYkFjdWc\/view?usp=sharingLet me know if you have issue with opening the files.
LemnaTec applied calibration matrix to the 3D scanners.
- There are calibrated reference panels and blackbody images taken with UAV sensors before and\/or after the each flight mission.
- There are also 4 white,grey and black panels laid on the ground during the flight. Knowing the proprieties of these targets would helps us radiometrically correct the UAV images.
- What are the reflectance properties of calibrated reference panels for multispectral camera?
- What are the thermal properties of reference target for thermal camera?
- What are the reflectance properties of the reference panels laid on the ground during the flight?
- Is there any other ground truth data collected during the flight for aerial data processing, such as surface reflectance, temperature and other environmental data? These type of data would be helpful for further atmospheric correction.
- There are two sets of reference reflectance panels: one that PDS uses, it is small, PDS will need to provide the specs; the second set consists of 4 8m x 8m canvas tarps, nominally 4%, 8%, 48% and 64% reflectance across vnir bands.
- We have data from an ASD spectrometer on many but not all flight days that can be used to give the most accurate actual reflectances for each. Kelly Thorp can provide the numbers. The tarps are old and the dark targets are more reflective than nominal and light targets darker than nominal.
- The thermal target is a passive black body, I dont know the surface emissivity, it is around 0.97. There are thermistors in the back of the metal plate to provide physical temperature of the body. The black body is stored in a wood box, insulated, to dampen thermal variations. Id guess it is accurate to 2C.
- There is a met station on farm for air temperature, humidity, wind speed, wind direction, solar radiation. we have a sun photometer that can be used for atmospheric water vapor content but currently dont deploy it routinely.
No per-wavelength analysis of light produced by the halogen lights is available from the vendor for Showtec 240V\/75W. Measurements are available for a similar halogen bulb Philips Twistline Halogen 230V 50W 18072 in Github: MeasurementPhilipsHalogenSpot.xlsx.
Relative spectral response data is available for the following sensors:
Where available, per device calibration certificates are included in the Device and Sensor information collections.