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Results of second outdoor comparison between 

Absolute Cavity Pyrgeometer (ACP) and Infrared 

Integrating Sphere (IRIS) Radiometer at PMOD

Atmospheric System Research

Science Team Meeting (March 10-13, 2014)

by

Ibrahim Reda*, Julian Gröbner**, Stefan Wacker** 

* NREL , ** PMOD/WRC

NREL/PR-3B10-61147
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Outline

The Absolute Cavity Pyrgeometer (ACP) and InfraRed Integrating Sphere 

radiometer (IRIS) are developed to establish a world reference for 

calibrating pyrgeometers with traceability to SI units. The two 

radiometers are un-windowed with negligible spectral dependence, and 

traceable to SI units through the temperature scale (ITS-90).

The second outdoor comparison between the two designs was held from 

September 30 to October 11, 2013 at the Physikalisch-Metorologisches 

Observatorium Davos (PMOD). The difference between the irradiance 

measured by ACP and that of the IRIS was within 1 W/m2 (3 IRISs: PMOD 

+ Australia + Germany).

From the first and second comparisons, a difference of 4-6 W/m2 was 

observed between the irradiance measured by ACP&IRIS and that of the 

interim World Infrared Standard Group (WISG).

This presentation includes results from the first and second comparison in 

an effort to establish the world reference for pyrgeometer calibrations, a 

key deliverable for the World Meteorological Organization (WMO), and 

the DOE-ASR. 

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First ACP&IRIS Comparison Feb. 5, 2013

http://asr.science.energy.gov/meetings/stm/2013/presentations/ACP-ASR-presentation-58121-2013.pdf
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Second ACP&IRIS Comparison-Oct. 2&3, 2013

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18:57 19:21 19:46 20:19 20:43 21:18 21:42 22:06 22:43 23:07 23:42 0:06 0:33 1:05 1:29

W/m2 

UTC

Night-Time IR irradiance measured at PMOD by ACP & IRIS on October 2&3, 2013

(~15 mm H2O vapor column)

W(ACP-6) W(IRIS-2) W(IRIS-3) W(IRIS-5)

Scattered thin clouds 

ACP calibrations 
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First ACP&IRIS Comparison Feb. 5, 2013

http://asr.science.energy.gov/meetings/stm/2013/presentations/ACP-ASR-presentation-58121-2013.pdf

(~8 mm H2O vapor column)
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Second ACP&IRIS Comparison-Oct. 2&3, 2013

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18:57 19:21 19:46 20:19 20:43 21:18 21:42 22:06 22:43 23:07 23:42 0:06 0:33 1:05 1:29

W/m2 

UTC

Night-Time IR irradiance measured at PMOD by ACP, IRIS, & WISG on October 2&3, 2013

(~15 mm H2O vapor column)

W(ACP-6) W(IRIS-2) W(IRIS-3) W(IRIS-5) W(WISG)
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Irradiance difference (WISG minus IRIS) at PMOD

Results

Average Offset (IWV>10) -4.1 ± 1.5 Wm-2 

Gradient (IWV<10)-0.45 ± 0.1 Wm-2mm-1

IWV

WISG 1 WISG 2 WISG 3

WISG 4

Night averages

IRIS u95

From Julian’s presentation, IRS2012-Germany (Data from 180 nights)

1st and 2nd ACP&IRIS Comparisons Consistent with PMOD Outdoor Data
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1st and 2nd ACP&IRIS Comparisons Consistent with IPASRC I&IPASRC II 

difference

Calculated IR incoming irradiance measured by 11 pyrgeometers using their 

two sets of coefficients, derived during IPASRC I & IPASRC II
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1st and 2nd ACP&IRIS Comparisons Consistent with Poster in ARM-STM-2008

Reda, I.; Gröbner, J.; Stoffel, T.; Myers, D.; Forgan, B.: http://www.arm.gov/publications/proceedings/conf18/poster/P00028.pdf?id=1
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Transmission Curves of Pyreometer Domes

From: John Hickey & Tom Kirk, Eppley Laboratory, Inc.

The (4 to 6)W/m2 difference between the irradiance measured by

ACP&IRIS and that of the Interim WISG might be attributed to the variation

in dome transmittance when there is a spectral difference.
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Transmission Curves of Pyreometer Domes

From: Jörgen Konings, Hukseflux Thermal Sensors

The (4 to 6)W/m2 difference between the irradiance measured by

ACP&IRIS and that of the Interim WISG might be attributed to the variation

in dome transmittance when there is a spectral difference.
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Transmission Curves of Pyreometer Domes

From: Joop Mes, Kipp&Zonen; he noticed that the curve changes from batch to batch
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Preliminary Conclusions & Recommendations

1. Outdoor agreement between ACP & IRIS to within 1 W/m2

at water vapor of 8 mm of H2O & 15 mm of H2O 

2. Irradiance measured by WISG is 4 W/m2 to 6 W/m2 lower 

than that measured by ACP&IRIS, for 8 to 15 mm of H2O 

3. Future comparison with lower water vapor might resolve 

observed spectral effect on outdoor pyrgeometer 

calibrations (research!!)

4. Other designs might increase confidence in establishing a 

consensus reference with traceability to SI units

5. Once a consensus reference is established, a group of 

pyrgeometers with minimum dome-transmittance 

variation would be used as Transfer-Reference-Group for 

the pyrgeometers outdoor calibration.