Ground-based FTIR Update

Ground-based FTIR Update

CAVIAR MeetingNPL, 29th September 2010

Tom Gardiner, Marc Coleman

Wednesday 19 April 2023

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Summary

• Aged mirror reflectivity measurements• ‘Smoothed’ calibration function• CAVIAR blackbody – uncertainty

assessment and paper preparation


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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.


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Updated reflectivity results

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3500 4000 4500 5000 5500 6000 6500

Wavenumber / cm -1

Ref

lect

ance

/ %

Unused low Unused high Cam low Cam high Jungf low


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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.


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Raw Calibration Function (Phase 1 – broadband InSb measurement)


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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.


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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.


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Polynominal fit to smoothed calibration curve


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Residual between smoothed calibration curve and original function


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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.


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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


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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.

Ground-based FTIR Update

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