Applying Results of Both International Pyrgeometer and Absolute 

Sky-scanning Radiometer Comparisons (IPASRC) 

Improvements in the Blackbody Calibration of Pyrgeometers 

By 

Ibrahim Reda1 , Julian Gröbner2, Tom Stoffel1, Daryl Myers1, and Bruce Forgan3 

1. National Renewable Energy Laboratory (NREL) 2. Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center (PMOD/WRC) 3. Bureau of Meteorology, Australia 

March 10, 2008 

NREL/PO-560-45867 Abstract
 

Pyrgeometers are used to measure the atmospheric longwave irradiance through out the ARM program sites. Previous 

calibrations of pyrgeometers using ARM/Eppley/NREL blackbody were consistent, but introduced a difference in the historical 

clear sky measured irradiance. This difference was believed to be in the order of 12 W/m2

. In this poster we show the 

improvements to the blackbody and calibration methodology by comparing our results to the results of a group of pyrgeometers 

that were recently calibrated against the World Infrared Standard Group, in the World Radiation Center, Davos/Switzerland. 

Improvements in the Blackbody	 Pyrgeometer Blackbody Calibration System

•		 Lower viscosity oil in Blackbody (BB); temperature gradient reduced from 3 C to 0.8 C @ - Simplified Diagram 30 C temperature plateau; clear sky bias reduced from 12 W/m2 to 6 W/m2 

•		 Added Wolfgang’s thermal mass to increase BB thermal capacity; outdoor data scatter 

Blackbody Temperature Probe

reduced from 6 W/m2 to 2 W/m2 

(RTD, in oil) 

•		 Proper polishing and gold plating for BB completion hemisphere. Nickel layer inserted 

Pyrgeometer

Dome 

RH = 0% to 10% 

RH = 0% to 10% 

Blackbody Completion 

Hemisphere

(Polished Gold plated) 

120 ° 

(Horizontally) 

RH = 0% to 10% 

Wolfgang Thermal Mass 

Blackbody Temperature Probes 

between copper and gold to prevent copper/gold diffusion (Thermistors, outside the Blackbody Hemisphere and in oil) (Chang, C., 1986, J. Appl.Phys.,
 

Vol. 60, No. 3, 1); improved BB emissivity
 

•		 Attached thermistor to gold completion hemisphere, added its emitted irradiance to BB 

irradiance, eG = 0.02 (NIST Data); BB irradiance increased by 4 W/m2 ( 1 W/m2 vs Tplateaus). Circulating oil 

(-80 °C to 50 °C) 

Evaluation Method Copper Plate to increase thermal mass 

and improve circulation/uniformity 

•		 Five pyrgeometers calibrated at PMOD, established NREL Reference Group (NRG): 3-PIRs 

Blackbody Hemisphere 

and 2-CG4s (Blackened on the inside) 

•		 Calibrated NRG using PMOD Blackbody (BB) 

•		 NRG deployed outdoors at PMOD vs World Infrared Standard Group (WISG) > 4 months 

Thermistor to measure temperature 

of the Completion Hemisphere 

•		 Adjusted PMOD-BB coefficient (C) to match the WISG irradiance 

•		 Calibrated NRG using NREL Blackbody (BB) 

•		 Using the same outdoor data at Davos, adjusted NREL-BB coefficients (K1and K2) to match Aluminum plate connected to circulating silicon oil 

(-40 °C to 50 °C) the WISG irradiance, and K3 to reduce scatter 

Completion Plate 

(Polished Gold plated) 

•		 Compared the results from PMOD’s and NREL’s BB and outdoor calibrations 

Difference between the reference irradiance and the irradiance 

measured by 30557F3 using NREL-BB-Coefficients
 

(Old vs New oil)
 

Clear Sky	 Cloudy Sky 

Comments/Conclusions 

1. NREL-BB improvements reduced ~12 W/m2 bias to (-1 to 3) W/m2 w.r.t. 

WISG 

2. What is the absolute value? 

Can IT BE FROM: 

- IPASRC-I; Present WISG?
 

- IPASRC-II; 2.5 W/m2 to 3 W/m2 lower than IPASRC-I?
 

- Outdoor calibration independent from reference irradiance (Reda et al., 2006, J. Atmospheric and Solar Terrestrial Physics, 68, 1416-1424); 3 

W/m2 from WISG?
 

- Improved NREL-BB; -1 W/m2 to 3 W/m2 from WISG?
 

Coefficient Source 

CG4-010547 CG4-060881 PIR-30557F3 PIR-31197F3 PIR-31233F3 

PMOD-BB NREL-BB PMOD-BB NREL-BB PMOD-BB NREL-BB PMOD-BB NREL-BB PMOD-BB NREL-BB 

K0 BB -0.29 -11.2 0.0589 -5.38 -1.77 -11.61 0.45 -11.4 0.117 -17.534 

K1 

BB 0.0854 0.0892 0.1174 0.12207 0.2205 0.22556 0.2422 0.24941 0.2735 0.27188 

Adjusted 0.076 0.0756 0.1195 0.1186 0.2174 0.222 0.2398 0.245 0.2537 0.2571 

Cadj =1/K1 13.158 13.228 8.368 8.432 4.600 4.505 4.170 4.082 3.942 3.890 

K2 

BB 0.9996 1.03485 0.9975 1.01891 1.0039 1.03738 1.0011 1.03854 1.0006 1.05145 

Adjusted 1.03 1.013 1.038 1.04 1.051 

K3 

BB N/A N/A 0.00 1.03 2.70 2.09 2.61 2.08 3.65 2.30 

Adjusted N/A N/A 0.00 0.00 2.70 2.80 3.30 

U95 (W/m2) 1.5 2.0 2.0 1.8 1.7 2.0 1.8 2.3 1.9 1.9 

3. The BB calibration coefficients must be adjusted outdoors to an 

Internationally accepted reference (i.e. WISG) for global uniformity, and

to account for the spectral response of pyrgeometers and the mismatch

between the BB (indoors) and the outdoors 

PMOD and NREL BB calibration coefficients vs adjusted coefficients to match the WISG irradiance, using NREL equation 

4. Other pyrgeometer calibration systems are needed to evaluate the WISG

PMOD Equation to establish a consensus reference with traceability to SI units. V	 3 Win

= * (1+ k1*σ *Tc ) + k2*Wc − k3 * (Wd − Wc ) C

NREL Equation Acknowledgements

Win

= K + K1*V + K *Wr − K3 * (Wd − W ) 0 2 r We would like to thank the ARM/NREL/PV/SRRL/Metrology Laboratory tasks for supporting this measurement research. We also thank the PMOD/WRC staff for their help in calibrating our 

reference pyrgeometers. Finally, we extend our gratitude to NREL staff: Afshin Andreas, Bev Kay, and Peter Gotseff for their continuous support in radiometry, data acquisition, and logistics.
 

PMOD-BB NREL-BB PMOD-BB NREL-BB PMOD-BB NREL-BB PMOD-BB NREL-BB PMOD-BB NREL-BB 

BB 11.562 9.542 8.547 7.560 4.322 3.628 4.193 3.488 3.615 2.955 

Adjusted 12.921 11.500 8.395 7.950 4.379 3.850 4.233 3.600 3.881 3.250 

k1 BB 0.0211 -0.0728 0.0267 -0.0203 0.0065 -0.0804 0.0557 -0.0552 0.0376 -0.0901 

BB 0.99875 1.00947 0.99769 1.00866 0.99871 1.01128 1.00247 1.01328 1.00080 1.01181 

Adjusted 0.99800 0.99900 1.00500 1.00850 1.00500 

k3 BB N/A N/A 0.00 0.00 2.70 3.31 2.61 3.16 3.66 4.14 

1.9 1.9 1.7 1.9 2.0 1.5 1.8 1.8 1.4 1.4 

PMOD and NREL BB calibration coefficients vs adjusted coefficients to match the WISG irradiance, using PMOD equation 

C 

U95 (W/m2) 

PIR-31233F3 

k2 

Coefficient Source 

CG4-010547 CG4-060881 PIR-30557F3 PIR-31197F3