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Ground-based FTIR Update
CAVIAR MeetingNPL, 29th September 2010
Tom Gardiner, Marc Coleman
Monday 10 April 2023
2
Summary
• Aged mirror reflectivity measurements• ‘Smoothed’ calibration function• CAVIAR blackbody – uncertainty
assessment and paper preparation
Monday 10 April 2023
3
Aged mirror reflectivity
• Absolute reflectivity measurements of gold mirror used in solar tracker during Jungfrau campaign now underway.
• Measurements under ambient conditions just completed.
• Measurements under different humidity conditions underway.
Monday 10 April 2023
4
Updated reflectivity results
94
95
96
97
98
99
100
3500 4000 4500 5000 5500 6000 6500
Wavenumber / cm -1
Ref
lect
ance
/ %
Unused low Unused high Cam low Cam high Jungf low
Monday 10 April 2023
5
Updated calibration function
• Latest mirror reflectivity measurements have confirmed transmission properties for both field campaigns and related calibration exercises.
• Extrapolation outside of reflectivity measurement region has been revisited, and a (hopefully) improved estimate derived.
• Updated calibrations functions now available that include these revisions, and integrate solar limb darkening effects.
Monday 10 April 2023
6
Raw Calibration Function (Phase 1 – broadband InSb measurement)
Monday 10 April 2023
7
Issues with raw function
• Calibration function should be a smoothly varying function across the the spectral range.
• Raw function contains lots of high resolution information relating to laboratory water vapour and instrumental noise. This high frequency noise will feed into field results analysis.
• Particularly the case in the interesting high frequency region
• Raw function also not suited to re-sampling at different resolutions.
• Smooth underlying function is visually obvious, but need an analytical method to extract it.
Monday 10 April 2023
8
Smoothing / fitting procedure
• Filter raw spectrum using repeated application of a sharp gradient exclusion filter.
• This removes the effects of the water lines and some high frequency noise.
• Fit a high order polynominal to the remaining points.
• This gives an analytical expression for the shape of the calibration curve, that can be applied to any frequency scale within the frequency limits of the original data.
• Should be particularly useful when ratioing two calibration curves to determine scaling functions for different gains / apertures.
Monday 10 April 2023
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Polynominal fit to smoothed calibration curve
Monday 10 April 2023
10
Residual between smoothed calibration curve and original function
Monday 10 April 2023
11
CAVIAR Blackbody
• Now have a write up of the blackbody uncertainties.• Covers the various sources of uncertainty including :
– Nextel coating reflectance– Cavity emissivity– Ambient reflection– Uniformity– Themometry– Cavity thermal gradient– Thermal gradient across coating– Formal traceability to the SI through AMBER facility
• This will form the basis for calibration paper.
Monday 10 April 2023
12
CAVIAR Blackbody Uncertainty Table
Source Comment Condition Uncert. Units divisor dT (K)Nextal reflectance Accounted for in following section 0.5 % REmissivity and ambient reflection Accounts for uncertainty in emissivity modelling Blackbody @ 303K 0.005 K 1.73 0.003
(Use highest value) Blackbody @ 293K 0 KBlackbody @ 283K 0.002 KBlackbody @ 278K 0.002 K
Uniformity Blackbody @ 303K 0.01 K 1.73 0.006PRT Calibration 0.012 K 2 0.006PRT measurement by DMM 0.014 % 2 0.018Conductive heat loss along PRT Accounted for in following sectionDifference between PRTs Typically <20mK @ 303K, and <10 mK between
278K and 293K 0.01 K 1 0.01Thermal gradient across coating Assume coeff = 25 Blackbody @ 303K 0.0045 K 1.73 0.003Thermal gradient along cylinder 0.05% per K gradient; assume 0.5K gradient 0.015 K 1.73 0.009
Combined Standard Uncertainty (k=1) 0.024
Monday 10 April 2023
13
International Intercomparison• CAVIAR blackbody used in first part of CEOS comparison of IR
brightness temperature measurements in support of satellite validation : Laboratory and Ocean surface temperature comparison of radiation thermometers.
• Took part in two stages– Stage 1 took place at NPL and involved laboratory
measurements of participants’ blackbodies calibrated using the NPL reference transfer radiometer (AMBER), while participants’ radiometers were calibrated using the CAVIAR blackbody.
– Stage 2 took place at Rosenstiel School of Marine and Atmospheric Science (RSMAS) and involved laboratory measurements of participants’ blackbodies calibrated using the NIST Thermal-Infrared Transfer radiometer (TXR), while participants’ radiometers were calibrated using the RSMAS and NIST water bath blackbodies.