NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Technical Report

NREL/TP-1900-72607

October 2018

NREL Pyrheliometer Comparisons:

September 24 – October 5, 2018 

(NPC-2018)

Ibrahim Reda, Mike Dooraghi, Afshin Andreas, 

and Aron Habte

National Renewable Energy Laboratory 
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Technical Report

NREL/TP-1900-72607

October 2018

NREL Pyrheliometer Comparisons:

September 24 – October 5, 2018 

(NPC-2018)

Ibrahim Reda, Mike Dooraghi, Afshin Andreas, 

and Aron Habte

National Renewable Energy Laboratory 

Suggested Citation 

Ibrahim, Mike Dooraghi, Afshin Andreas, and Aron Habte. 2018. NREL Pyrheliometer 

Comparisons: September 24 – October 5, 2018 (NPC-2018). Golden, CO: National 

Renewable Energy Laboratory. NREL/TP-1900-72607.

https://www.nrel.gov/docs/fy19osti/72607.pdf.
NOTICE

This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable 

Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding 

provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy’s Solar Energy 

Technologies Program and the Atmospheric Radiation Measurement (ARM) Research Facility. The views 

expressed herein do not necessarily represent the views of the DOE or the U.S. Government.

This report is available at no cost from the National Renewable 

Energy Laboratory (NREL) at www.nrel.gov/publications.

U.S. Department of Energy (DOE) reports produced after 1991 

and a growing number of pre-1991 documents are available 

free via www.OSTI.gov.

Cover Photos by Dennis Schroeder: (clockwise, left to right) NREL 51934, NREL 45897, NREL 42160, NREL 45891, NREL 48097, 

NREL 46526.

NREL prints on paper that contains recycled content.
iv

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Acknowledgments

We sincerely appreciate the support of Solar Radiance Research Laboratory (SRRL) staff and 

National Renewable Energy Laboratory (NREL) management, the U.S. Department of Energy 

(DOE) Office of Energy Efficiency and Renewable Energy/Solar Energy Technologies Program, 

Environmental Research/Atmospheric Radiation Measurement Program, and NREL’s

Environment, Safety, Health, & Quality center (ESH&Q). Our thanks also go to all the 

participants for their patience and cooperation during this weather-dependent exercise.

Figure 1. NPC-2018 participants

Photo by Werner Slocum
v

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List of Acronyms

AHF Automatic Hickey-Frieden

BMS Baseline Measurement System

BORCAL Broadband Outdoor Radiometer Calibration

DOE U.S. Department of Energy

IPC International Pyrheliometer Comparison

IPC-XII Twelfth International Pyrheliometer Comparisons

ISO International Organization for Standardization

MST Mountain Standard Time

NPC National Renewable Energy Laboratory Pyrheliometer Comparisons

NREL National Renewable Energy Laboratory

PMOD/WRC Physikalisch-Meteorologisches Observatorium Davos—World 

Radiation Center

SDp pooled standard deviation

SI International System of Units

SRRL Solar Radiation Research Laboratory

TSG Transfer Standard Group

WMO World Meteorological Organization

WRR World Radiometric Reference

WRR-TF World Radiometric Reference transfer factor

WSG World Standard Group

S/N serial number of radiometer

%uA Percentage Type-A standard uncertainty

NRdg number of readings

uC combined standard uncertainty

Eff DF effective degrees of freedom
vi

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Executive Summary

Accurate measurements of direct normal (beam) solar irradiance from pyrheliometers1 are 

important for developing and deploying solar energy conversion systems, for improving our 

understanding of the Earth’s energy budget for climate change studies, and for other science and 

technology applications involving solar flux. These measurements place many demands on the 

quality system used by operators of commercially available radiometers. Maintaining accurate 

radiometer calibrations that are traceable to an international standard is the first step in producing 

research-quality solar irradiance measurements.

In 1977, the World Meteorological Organization (WMO) established the World Radiometric 

Reference (WRR) as the international standard for the measurement of direct normal solar 

irradiance (Fröhlich 1991). The WRR is an internationally recognized, detector-based 

measurement standard determined by the collective performance of six electrically selfcalibrated absolute cavity radiometers comprising the World Standard Group (WSG). Various 

countries, including the United States,

2 have contributed these specialized radiometers to the 

Physikalisch-Meteorologisches Observatorium Davos—World Radiation Center (PMOD/WRC)

to establish the WSG. 

As with all measurement systems, Absolute Cavity Radiometers (ASR) are subject to 

performance changes over time. Therefore, PMOD/WRC in Davos, Switzerland, hosts an 

International Pyrheliometer Comparison (IPC) for transferring the WRR to participating 

radiometers quinquennially by invitation. The National Renewable Energy Laboratory (NREL)

has represented the U.S. Department of Energy (DOE) in each IPC since 1980. As a result, 

NREL has developed and maintained a select group of absolute cavity radiometers with direct 

calibration traceability to the WRR and uses these reference instruments to calibrate 

pyrheliometers and pyranometers using the International Organization for Standardization (ISO)

17025 accredited Broadband Outdoor Radiometer Calibration (BORCAL) process (Reda et al. 

2008).

To fill the gap between each IPC, NREL pyrheliometer comparisons (NPCs) are held annually at 

the Solar Radiation Research Laboratory (SRRL) in Golden, Colorado. Open to all ACR owners

and operators, each NPC provides an opportunity to determine the unique WRR transfer factor 

(WRR-TF) for each participating pyrheliometer. By adjusting all subsequent pyrheliometer 

measurements by the appropriate WRR-TF, the solar irradiance data are traceable to the WRR.

NPC-2018 was September 24 through October 5, 2018. Participants operated 43 ACRs to 

simultaneously measure clear-sky direct normal solar irradiance during this period. The Transfer 

Standard Group (TSG) of reference radiometers for NPC-2018 consisted of four NREL 

radiometers with direct traceability to the WRR, having participated in the Twelfth International 

Pyrheliometer Comparisons (IPC-XII) in the fall of 2015. As a result of NPC-2018, each 

participating absolute cavity radiometer was assigned a new WRR-TF, computed as the 

reference irradiance determined by the TSG divided by the observed irradiance from the 

participating radiometer. The performance of the TSG during NPC-2018 was consistent with 

previous comparisons, including IPC-XII. The measurement performance of the TSG allowed 

 1 Pyrheliometers are a type of radiometer used to measure solar irradiance (i.e., radiant flux in Watts per square 

meter) on a surface normal to the apparent solar disk within a 5.0° or 5.7° field of view, depending on the optical 

design of the instrument. A solar tracker is used to maintain proper alignment of the pyrheliometer with the sun 

during daylight periods.

2 The WSG includes radiometers on permanent loan from the Eppley Laboratory, Inc., and NREL.
vii

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the transfer of the WRR to each participating radiometer with an estimated uncertainty of ± 

0.36% with respect to the International System of Units.

The comparison protocol is based on data collection periods called “runs”. Each run consists of 

an electrical self-calibration requiring five minutes for the Automatic Hickey-Frieden (AHF)

cavities, a series of 49 solar irradiance measurements at 30-second intervals, and a postcalibration. More than 2,300 reference irradiance measurements were collected by the TSG 

during NPC-2018. Clear-sky daily maximum direct normal irradiance level was 1017 Wm-2.

Ancillary environmental conditions (e.g., broadband turbidity, ambient temperature, relative 

humidity, wind speed, precipitable water vapor, and spectral data) collected at SRRL during the 

comparison are presented in Appendix B to document the environmental test conditions.

NPCs are planned annually at the SRRL to ensure worldwide homogeneity of solar radiation 

measurements traceable to the WRR.
viii

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Table of Contents

List of Figures ............................................................................................................................................ ix

List of Tables .............................................................................................................................................. ix

1 Introduction........................................................................................................................................... 1

2 Reference Instruments......................................................................................................................... 2

3 Measurement Protocol......................................................................................................................... 1

4 Transferring the World Radiometric Reference ................................................................................ 2

4.1 Calibration Requirements.............................................................................................................. 2

4.2 Determining the Reference Irradiance .......................................................................................... 2

4.3 Data Analysis Criteria ................................................................................................................... 2

4.4 Measurements................................................................................................................................ 3

4.5 Results........................................................................................................................................... 3

4.6 Recommendations....................................................................................................................... 25

5 Ancillary Data...................................................................................................................................... 27

References................................................................................................................................................. 28

Appendix A: List of Participants and Pyrheliometers........................................................................... 29

Appendix B: Ancillary Data Summaries ................................................................................................. 32
ix

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List of Figures

Figure 1. NPC-2018 participants ................................................................................................................. iv

Figure 2. History of WRR reduction factors for NREL reference cavities.................................................. 4

Figure 3. WRR-Transfer Factor vs. Mountain Standard Time (MST) NPC-2018 for AHF 0000................ 7

Figure 4. WRR-Transfer Factor vs. MST NPC-2018 for AHF 14915.......................................................... 7

Figure 5. WRR-Transfer Factor vs. MST NPC-2018 for AHF 17142.......................................................... 7

Figure 6. WRR-Transfer Factor vs. MST NPC-2018 for AHF23734........................................................... 8

Figure 7. WRR-Transfer Factor vs. MST NPC-2018 for AHF 28486.......................................................... 8

Figure 8. WRR-Transfer Factor vs. MST NPC-2018 for AHF28553........................................................... 9

Figure 9. WRR-Transfer Factor vs. MST NPC-2018 for AHF 28556.......................................................... 9

Figure 10. WRR-Transfer Factor vs. MST NPC-2018 for AHF 28560...................................................... 10

Figure 11. WRR-Transfer Factor vs. MST NPC-2018 for AHF 29219 - Windowed................................. 10

Figure 12. WRR-Transfer Factor vs. MST NPC-2018 for AHF 29222 – Windowed ................................ 11

Figure 13. WRR-Transfer Factor vs. MST NPC-2018 for AHF 29223...................................................... 11

Figure 14. WRR-Transfer Factor vs. MST NPC-2018 for AHF 30110...................................................... 12

Figure 15. WRR-Transfer Factor vs. MST NPC-2018 for AHF 310495-Window..................................... 12

Figure 16. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31041...................................................... 13

Figure 17. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31104-Window....................................... 13

Figure 18. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31105...................................................... 14

Figure 19. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31107...................................................... 14

Figure 20. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31108...................................................... 15

Figure 21. WRR-Transfer Factor vs. MST NPC-2018 for AHF 32452AWX-Window............................. 15

Figure 22. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31113AWX ............................................ 16

Figure 23. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31114AWX ............................................ 16

Figure 24. WRR-Transfer Factor vs. MST NPC-2018 for AHF 32448AWX ............................................ 17

Figure 25. WRR-Transfer Factor vs. MST NPC-2018 for AHF 32455...................................................... 17

Figure 26. WRR-Transfer Factor vs. MST NPC-2018 for AHF 33392...................................................... 18

Figure 27. WRR-Transfer Factor vs. MST NPC-2018 for AHF 34926AWX ............................................ 18

Figure 28. WRR-Transfer Factor vs. MST NPC-2018 for AHF 37816...................................................... 19

Figure 29. WRR-Transfer Factor vs. MST NPC-2018 for PMO6 0816..................................................... 19

Figure 30. WRR-Transfer Factor vs. MST NPC-2018 for PMO6 1601..................................................... 20

Figure 31. WRR-Transfer Factor vs. MST NPC-2018 for PMO6 81109................................................... 20

Figure 32. WRR-Transfer Factor vs. MST NPC-2018 for PMO6 911204................................................. 21

Figure 33. WRR-Transfer Factor vs. MST NPC-2018 for PMO6cc 0103 ................................................. 21

Figure 34. WRR-Transfer Factor vs. MST NPC-2018 for PMO6cc 0401 ................................................. 22

Figure 35. WRR-Transfer Factor vs. MST NPC-2018 for PMO6cc 0803 ................................................ 22

Figure 36. WRR-Transfer Factor vs. MST NPC-2018 for PMO6 1611-Linard-00.................................... 23

Figure 37. WRR-Transfer Factor vs. MST NPC-2018 for TMI 67603 ...................................................... 23

Figure 38. WRR-Transfer Factor vs. MST NPC-2018 for TMI 67811 ...................................................... 24

Figure 39. WRR-Transfer Factor vs. MST NPC-2018 for TMI 68020 ...................................................... 24

Figure 40. WRR-Transfer Factor vs. MST NPC-2018 for TMI 68022 ...................................................... 25

Figure 41. WRR-Transfer Factor vs. MST NPC-2018 for TMI 68835 ...................................................... 25

List of Tables

Table 1. IPC-XII Results Summary for the NPC-2018 TSG........................................................................ 2

Table 2. Summary Results for Proficiency Test During NPC-2018............................................................. 4

Table 3. Results for Radiometers Participating in NPC-2018 ...................................................................... 5
1

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1 Introduction

Accurate measurements of broadband solar irradiance require radiometers with proper design and 

performance characteristics, correct installation, and documented operation and maintenance procedures, 

including regular calibration. Calibrations of any measuring device must be traceable to a recognized 

reference standard. The World Radiometric Reference (WRR) is the internationally recognized 

measurement standard for direct normal irradiance measurements of broadband solar radiation (Fröhlich 

1991).

The WRR was established by the World Meteorological Organization (WMO) in 1977 and has been 

maintained by the Physikalisch-Meteorologisches Observatorium Davos—World Radiation Center

(PMOD/WRC) in Switzerland (www.pmodwrc.ch). This reference is maintained for broadband solar 

irradiance with an absolute uncertainty of better than ± 0.3% with respect to the International System of 

Units (SI) (Romero et al. 1996). This standard is widely used to calibrate pyrheliometers and 

pyranometers with a wavelength response range that is compatible with the solar spectrum wavelengths of 

280–3,000 nm. Every five years, the WRR is transferred to WMO regional centers and other participants 

at International Pyrheliometer Comparisons (IPC) held at the PMOD/WRC. The Twelfth IPC (IPC-XII) 

was completed in 2015 (Finsterle 2018). At each IPC, instantaneous measurements from the World 

Standard Group (WSG) are compared at 90-second intervals with the data from participating radiometers 

recorded under clear-sky conditions. A new WRR transfer factor (WRR-TF) is calculated for each 

participating radiometer based on the mean WRR of the WSG radiometers for each IPC. Multiplying the 

irradiance reading of each radiometer by its assigned WRR-TF will result in measurements that are 

traceable to SI units through WRR and therefore consistent with the international reference of solar 

radiation measurement.

In compliance with Organization for Standardization (ISO) 17025 accreditation requirements for 

demonstrating interlaboratory proficiency, the National Renewable Energy Laboratory (NREL) hosts 

annual pyrheliometer comparisons at the Solar Radiation Research Laboratory (SRRL) in Golden, 

Colorado, during non-IPC years. The NREL Pyrheliometer Comparisons in 2018 (NPC-2018) was 

September 24 through October 5, 2018. Participants operated 43 absolute cavity radiometers during the 

comparisons. See Appendix A for the list of participants and affiliations.

The results presented in this report are based on clear-sky, direct normal solar irradiance data collected 

during the NPC. See Appendix B for environmental conditions.
2

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2 Reference Instruments

NREL developed the transfer standard group (TSG) of four absolute cavity radiometers to serve as the 

transfer reference for each NPC. The radiometers comprising the TSG were included in the most recent 

IPC and maintain the WRR for NREL (see Table 1). Using the method described by Reda (1996), the 

mean of the TSG measurements was maintained for establishing the reference irradiance data for NPC2018 data reduction. Table 1 provides a list of the TSG absolute cavity radiometers with their WRR-TFs

and pooled standard deviation (SDp) as determined from the latest IPC in 2015 (Finsterle 2016).

Table 1. IPC-XII Results Summary for the NPC-2018 TSG

Serial Number WRR Factor

(IPC-XI)

Standard Deviation

(%)

Number of

Readings

AHF 28968 0.99763 0.063 519

AHF 29220 0.99749 0.0621 523

AHF 30713 0.99723 0.0639 525

ATMI 68018 0.99660 0.0669 522

Mean WRR for the TSG 0.99724 SDp for the TSG: 0.06%

The pooled standard deviation, SDp for the TSG was computed from the following equation:

SSSSpp = �∑ nnii mm

ii=1 ∗SSii

2

∑ nnii mm

ii=1

1

where,

i = i

th cavity

m = number of reference cavities

Si = standard deviation of the ith cavity from IPC-XII

ni = number of readings of the ith cavity from IPC-XII
1

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3 Measurement Protocol

The decision to deploy instruments for a comparison was made daily. Data were collected only during 

clear-sky conditions, which were determined visually and from the stability of pyrheliometer readings.

Simultaneous direct normal solar irradiance measurements were taken by most cavity radiometers in 

groups of 49 observations at 30-second intervals (PMO6 used 90, 80, or 30-second open/closed-shutter 

cycles). Each group of observations is called a “run.” An electrical self-calibration of each Automatic 

Hickey-Frieden (AHF) absolute cavity was performed prior to each run. Previous WRR-TFs determined 

from results of IPCs or NPCs were not applied to the observations. The original manufacturer’s

calibration factor was used according to the standard operating procedure provided by the manufacturer 

for each radiometer. A timekeeper announced the beginning of each calibration period and gave a fiveminute countdown prior to the start of each run to facilitate the AHF cavity self-calibrations and the 

simultaneous start for each participant.

By consensus, at least 300 observations from each radiometer were required to determine the WRR-TF

for an NPC. A statistically significant dataset was required to derive the WRR-TF for each pyrheliometer. 

Data from each pyrheliometer/operator system were emailed at the end of the day.
2

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4 Transferring the World Radiometric Reference

The primary purpose of an NREL pyrheliometer comparison is to transfer the current WRR from the 

NPC-TSG to each participating absolute cavity pyrheliometer. This requires that the participating 

pyrheliometers and the TSG collect simultaneous measurements of clear-sky direct normal (beam) solar 

irradiance. Because the NPC data analysis is intended for absolute cavity pyrheliometers only, users of 

pyrheliometers other than absolute cavity pyrheliometers might interpret their NPC results differently.

4.1 Calibration Requirements

Using WMO guidelines (Romero 1995), the following conditions were required before data collection 

was accomplished during NPC-2018:

• The radiation source was the sun, with irradiance levels > 700 Wm-2.

• A Digital Multimeter with uncertainty > 0.05% reading or better was used to measure the 

thermopile signals from each radiometer.

• Solar trackers were aligned within ± 0.25° slope angle.

• Wind speed was low (< 5 m/s) from the direction of the solar azimuth ± 30°.

• Cloud cover was < 1/8 of the sky dome, with an angular distance > 15° from the sun.

4.2 Determining the Reference Irradiance

Four absolute cavity radiometers maintained by NREL and were part of IPC-XII were used as the TSG to 

transfer the WRR in the comparison. The WRR-TF for each TSG is presented in Table 1 above. The 

reference irradiance at each reading was calculated using the following steps, as described by Reda 

(1996):

1. Each irradiance reading of the TSG is divided by the irradiance measured by AHF28968, for its 

participation in many IPCs.

2. By maintaining the mean of WRR for the TSG, a new WRR-TF for NPC-2018 is recalculated for 

each of the TSG cavities (see Figure 2).

3. The reference irradiance for each 30-second observation in a run is computed as the mean of the 

simultaneous reference irradiances measured by the TSG. The reference irradiance reading for 

each cavity in the TSG is the irradiance reading of the cavity multiplied by its new WRR-TF

calculated in Step 2.

4.3 Data Analysis Criteria

AHF28968 was used to check irradiance stability at the time of each comparison reading during a run.

Stable irradiance readings are defined to within 1.0 Wm-2 during an interval of two seconds centered on

each reading time (i.e. one second before and one second after the recorded reading). Unstable irradiance 

readings are marked in the data record and automatically rejected from the data analysis. Historically, this 

has affected fewer than 10% of the data collected during an NPC.

Additionally, all calculated ratios of the test instrument irradiance divided by AHF28968 irradiance that 

deviated from their mean by 0.3% were rejected (Reda 1996). Typically, data rejected from the analysis in 

this manner were the result of failed tracker alignment, problems with the pre-calibration, or a similar 

cause for bias greater than expected from a properly functioning absolute cavity radiometer.
3

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4.4 Measurements

NPC-2018 was completed for most participants on October 4, after more than 2,000 data points were 

collected by the reference cavities during the requisite clear-sky conditions. The actual number of 

readings for each participating radiometer compared with the reference irradiance varied according to the 

data analysis selection criteria described above. Additionally, some instruments experienced minor data 

loss because a variety of problems occurred with the measurement systems and operations.

4.5 Results

The historical results for the TSG are presented in Figure 2. To evaluate the performance of these 

instruments, the standard deviations of each radiometer were monitored during the comparisons. The 

results suggest successful performance of the TSG during this NPC:

• For the TSG, the NPC-2018 WRR-TF did not change by more than a fraction of the standard 

deviation derived during IPC-XII in 2015 (see Figure 2. History of WRR reduction factors for 

NREL reference cavities).

• For NPC-2018 Proficiency Test, the results of the participating cavities in IPC-XII and NPC-2018

were evaluated using the following equation:

EEnn = WWWWWWIIIIII−WWWWWWNN

�UU95IIIIII

2 +UU95NN2

2

where En must lie in the interval -1 to +1.

From Figure 2. History of WRR reduction factors for NREL reference cavities

Table 2, En for all cavities was well within the interval -1 to +1 (i.e., the WRR from NPC-2018 is 

consistent with the WRR from IPC-XII).
4

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Figure 2. History of WRR reduction factors for NREL reference cavities

Table 2. Summary Results for Proficiency Test During NPC-2018

0.996

0.997

0.998

0.999

1

WRR

DATE

WRR-RF for NREL Reference Cavities

AHF28968 AHF29220 AHF30713 ATM68018 MEAN WRR-RF

Spider web in TMI
5

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Table 3. Results for Radiometers Participating in NPC-2018

S/N WRR (NPC-2018) SD NRDG %U95

AHF 0000 1.00275 0.00073 1997 0.39

AHF 14915 0.99954 0.00063 1378 0.38

AHF 17142 0.99828 0.00061 1441 0.38

AHF 23734 0.99830 0.00038 1543 0.37

AHF 28486 0.99722 0.00064 1452 0.38

AHF 28553 0.99781 0.00049 1431 0.37

AHF 28556 0.99056 0.00103 1498 0.42

AHF 28560 1.00289 0.00089 1419 0.40

AHF 29219-Window 1.06176 0.00069 1495 0.38

AHF 29222-Window 1.05900 0.00068 1965 0.38

AHF 29223 0.99904 0.00072 1325 0.39

AHF 30110 1.06312 0.00087 968 0.40

AHF 30495-Window 1.05572 0.00085 1959 0.39

AHF 31041 0.99663 0.00067 1169 0.38

AHF 31104-Window 1.03877 0.00089 1258 0.40

AHF 31105 0.99861 0.00056 1183 0.38

AHF 31107 1.04821 0.00121 929 0.43

AHF 31108 0.99684 0.00074 1498 0.39

AHF 32452AWX-Window 1.03139 0.00091 1484 0.40

AHF 31113AWX-Window 1.05056 0.00075 1399 0.39

AHF 31114AWX 1.00093 0.00064 1407 0.38

AHF 32448AWX 1.00017 0.00065 1424 0.38

AHF 32455 1.00140 0.00072 2278 0.39

AHF 33392 0.99890 0.00058 1284 0.38

AHF 34926AWX 1.00117 0.00088 1408 0.40

AHF 37816 0.99933 0.00067 1406 0.38

PMO6 0816 1.00026 0.00155 335 0.47

PM06 1601 1.00310 0.00061 473 0.38

PMO6 81109 0.99823 0.00073 584 0.39

PMO6 911204 1.00013 0.00096 593 0.41

Pmo6cc 0103 0.99911 0.00073 360 0.39

PMO6cc 0401 1.02097 0.00074 374 0.39

PMO6cc 0803 1.00005 0.00078 382 0.39

PMO6 1611-Linard-00 1.00165 0.00075 1097 0.39

TMI 67603 0.99993 0.00070 1488 0.39

TMI 67811 0.99866 0.00104 1338 0.42

TMI 68020 0.99897 0.00096 1213 0.41

TMI 68022 0.99996 0.00144 1327 0.46

TMI 68835 0.99983 0.00099 2169 0.41
6

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The uncertainty of the WRR-TF associated with each participating radiometer with respect to SI was 

calculated using the following formula:

UU95 = ±1.96 ∗ �uuAA

2 + uuBB

2 3

where,

U95 = Uncertainty of the WRR-TF (in percent) determined at NPC-2018 with 95% confidence level

1.96 = Coverage factor

uA = Type A standard uncertainty = standard deviation of each participating radiometer (in %) 

determined at NPC-2018

uB = Type B standard uncertainty

uB = ± �(

0.3

√3 )2 + 0.062

where,

0.3 = Estimated expanded uncertainty of the WRR scale with respect to SI, in %

√3 = Coverage factor for rectangular distribution

0.06 = Pooled standard deviation of the four reference radiometers (TSG) that participated in IPC-XII 

(September/October 2015), in %.

The statistical analyses of WRR-TF for the participating pyrheliometers are presented in the following 

figures. These graphical summaries indicate the mean, standard deviation, and histograms of the WRR-TF

determined during NPC-2018.

1.0000

1.0005

1.0010

1.0015

1.0020

1.0025

1.0030

1.0035

1.0040

1.0045

1.0050

1.0055

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 0000

WRR-TF
7

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Figure 3. WRR-Transfer Factor vs. Mountain Standard Time (MST) NPC-2018 for AHF 0000

Figure 4. WRR-Transfer Factor vs. MST NPC-2018 for AHF 14915

Figure 5. WRR-Transfer Factor vs. MST NPC-2018 for AHF 17142

0.9970

0.9975

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

1.0020

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

14915

0.9960

0.9965

0.9970

0.9975

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

17142

WRR-TF WRR-TF
8

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 6. WRR-Transfer Factor vs. MST NPC-2018 for AHF23734

Figure 7. WRR-Transfer Factor vs. MST NPC-2018 for AHF 28486

0.9970

0.9975

0.9980

0.9985

0.9990

0.9995

1.0000

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

23734

0.9950

0.9955

0.9960

0.9965

0.9970

0.9975

0.9980

0.9985

0.9990

0.9995

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF

28486

WRR-TF WRR-TF
9

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 8. WRR-Transfer Factor vs. MST NPC-2018 for AHF28553

Figure 9. WRR-Transfer Factor vs. MST NPC-2018 for AHF 28556

0.9960

0.9965

0.9970

0.9975

0.9980

0.9985

0.9990

0.9995

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

28553

0.9870

0.9880

0.9890

0.9900

0.9910

0.9920

0.9930

0.9940

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF

28556

WRR-TF WRR-TF
10

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 10. WRR-Transfer Factor vs. MST NPC-2018 for AHF 28560

Figure 11. WRR-Transfer Factor vs. MST NPC-2018 for AHF 29219 - Windowed

0.9990

1.0000

1.0010

1.0020

1.0030

1.0040

1.0050

1.0060

6:28:48 7:40:48 8:52:48 10:04:48 11:16:48 12:28:48 13:40:48 14:52:48 16:04:48

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

28560

1.0590

1.0595

1.0600

1.0605

1.0610

1.0615

1.0620

1.0625

1.0630

1.0635

1.0640

1.0645

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

29219-

Window

WRR-TF WRR-TF
11

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 12. WRR-Transfer Factor vs. MST NPC-2018 for AHF 29222 – Windowed

Figure 13. WRR-Transfer Factor vs. MST NPC-2018 for AHF 29223

1.0565

1.0570

1.0575

1.0580

1.0585

1.0590

1.0595

1.0600

1.0605

1.0610

1.0615

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

29222-

Window

0.9970

0.9975

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

29223

WRR-TF WRR-TF
12

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 14. WRR-Transfer Factor vs. MST NPC-2018 for AHF 30110

Figure 15. WRR-Transfer Factor vs. MST NPC-2018 for AHF 310495-Window

1.0590

1.0600

1.0610

1.0620

1.0630

1.0640

1.0650

1.0660

1.0670

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

30110

1.0520

1.0530

1.0540

1.0550

1.0560

1.0570

1.0580

1.0590

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

30495-

Window

WRR-TF WRR-TF
13

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 16. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31041

Figure 17. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31104-Window

0.9940

0.9945

0.9950

0.9955

0.9960

0.9965

0.9970

0.9975

0.9980

0.9985

0.9990

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

31041

1.0350

1.0360

1.0370

1.0380

1.0390

1.0400

1.0410

1.0420

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

31104-

Window

WRR-TF WRR-TF
14

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 18. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31105

Figure 19. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31107

0.9965

0.9970

0.9975

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

31105

1.0430

1.0440

1.0450

1.0460

1.0470

1.0480

1.0490

1.0500

1.0510

1.0520

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

31107

WRR-TF WRR-TF
15

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 20. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31108

Figure 21. WRR-Transfer Factor vs. MST NPC-2018 for AHF 32452AWX-Window

0.9940

0.9945

0.9950

0.9955

0.9960

0.9965

0.9970

0.9975

0.9980

0.9985

0.9990

0.9995

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

31108

1.0470

1.0480

1.0490

1.0500

1.0510

1.0520

1.0530

1.0540

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

32452AW

XWindow

WRR-TF WRR-TF
16

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 22. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31113AWX-Window

Figure 23. WRR-Transfer Factor vs. MST NPC-2018 for AHF 31114AWX

1.0470

1.0480

1.0490

1.0500

1.0510

1.0520

1.0530

1.0540

6:43:127:55:129:07:1210:19:1211:31:1212:43:1213:55:1215:07:1216:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

31113AWX

-Window

0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

1.0020

1.0025

1.0030

1.0035

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

31114AWX

WRR-TF WRR-TF
17

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 24. WRR-Transfer Factor vs. MST NPC-2018 for AHF 32448AWX

Figure 25. WRR-Transfer Factor vs. MST NPC-2018 for AHF 32455

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

1.0020

1.0025

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

32448AWX

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

1.0020

1.0025

1.0030

1.0035

1.0040

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

32455

WRR-TF WRR-TF
18

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 26. WRR-Transfer Factor vs. MST NPC-2018 for AHF 33392

Figure 27. WRR-Transfer Factor vs. MST NPC-2018 for AHF 34926AWX

0.9970

0.9975

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

33392

0.9980

0.9990

1.0000

1.0010

1.0020

1.0030

1.0040

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

34926AWX

WRR-TF WRR-TF
19

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 28. WRR-Transfer Factor vs. MST NPC-2018 for AHF 37816

Figure 29. WRR-Transfer Factor vs. MST NPC-2018 for PMO6 0816

0.9970

0.9975

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

AHF 

37816

1.0010

1.0015

1.0020

1.0025

1.0030

1.0035

1.0040

1.0045

1.0050

1.0055

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

PMO6 

0816

WRR-TF WRR-TF
20

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 30. WRR-Transfer Factor vs. MST NPC-2018 for PMO6 1601

Figure 31. WRR-Transfer Factor vs. MST NPC-2018 for PMO6 1611-Linard-00

0.9950

0.9960

0.9970

0.9980

0.9990

1.0000

1.0010

1.0020

1.0030

1.0040

1.0050

1.0060

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

PM06 

1601

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

1.0020

1.0025

1.0030

1.0035

1.0040

1.0045

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

PMO6 

1611-

Linard-00

WRR-TF

WRR-TF
21

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 32. WRR-Transfer Factor vs. MST NPC-2018 for PMO6 81109

Figure 33. WRR-Transfer Factor vs. MST NPC-2018 for PMO6cc 0103

0.9955

0.9960

0.9965

0.9970

0.9975

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

PMO6 

81109

0.9970

0.9980

0.9990

1.0000

1.0010

1.0020

1.0030

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

Pmo6cc 

0103

WRR-TF WRR-TF
22

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 34. WRR-Transfer Factor vs. MST NPC-2018 for PMO6 911204

Figure 35. WRR-Transfer Factor vs. MST NPC-2018 for PMO6cc 0401

0.9970

0.9980

0.9990

1.0000

1.0010

1.0020

1.0030

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

PMO6 

911204

1.0180

1.0190

1.0200

1.0210

1.0220

1.0230

1.0240

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

PMO6cc 

0401

WRR-TF

WRR-TF
23

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 36. WRR-Transfer Factor vs. MST NPC-2018 for PMO6cc 0803

Figure 37. WRR-Transfer Factor vs. MST NPC-2018 for TMI 67603

0.9975

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

1.0020

1.0025

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

PMO6cc 

0803

0.9975

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

1.0020

1.0025

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

TMI 

67603

WRR-TF

WRR-TF
24

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 38. WRR-Transfer Factor vs. MST NPC-2018 for TMI 67811

Figure 39. WRR-Transfer Factor vs. MST NPC-2018 for TMI 68020

0.9950

0.9960

0.9970

0.9980

0.9990

1.0000

1.0010

1.0020

1.0030

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

TMI 67811

0.9950

0.9960

0.9970

0.9980

0.9990

1.0000

1.0010

1.0020

1.0030

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

TMI 68020

WRR-TF

WRR-TF
25

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 40. WRR-Transfer Factor vs. MST NPC-2018 for TMI 68022

Figure 41. WRR-Transfer Factor vs. MST NPC-2018 for TMI 68835

4.6 Recommendations

As a result of these comparisons, it is suggested that participants observe the following measurement 

practices:

0.9950

0.9960

0.9970

0.9980

0.9990

1.0000

1.0010

1.0020

1.0030

1.0040

1.0050

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

TMI 68022

0.9960

0.9970

0.9980

0.9990

1.0000

1.0010

1.0020

1.0030

1.0040

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

WRR-Transfer Factor vs MST NPC-2018

WRR-TF 2 * SD

Histogram

TMI 68835

WRR-TF WRR-TF
26

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

• For the purpose of pyrheliometer comparisons, such as NPC-2018, it is recommended that the user 

apply only the manufacturer’s calibration factor, not the WRR-TF or the new calibration factor, to 

report his or her absolute cavity radiometer’s irradiance readings. This eliminates the possibility of 

compounding WRR factors from previous comparisons.

• For data collection in the field, the manufacturer’s calibration factor should be used to calculate the 

cavity responsivity. Each irradiance reading should then be multiplied by the appropriate WRR-TF to 

provide homogeneity of solar radiation measurements that are traceable to the WRR. This approach is 

recommended to realize the benefits of participating in the NPC.
27

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

5 Ancillary Data

The environmental conditions (i.e., temperature, relative humidity, barometric pressure, wind speed, 

precipitable water vapor, and spectral data) were measured during the comparisons using the 

meteorological station at SRRL. Additional information, including data and graphical summaries, can be 

found at the Measurements and Instrumentation Data Center: www.nrel.gov/midc/srrl_bms.

Time-series plots and other graphical presentations of these data collected during the pyrheliometer 

comparisons are presented in Appendix B.
28

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

References

Finsterle, W. (2018). WMO International Pyrheliometer Comparison, IPC-XII, 28 September – 16

October 2015: Final Report. WMO IOM Report No. 124. Davos, Switzerland; 98 pp.

Fröhlich, C. (1991). “History of Solar Radiometry and the World Radiometric Reference.” Metrologia,

(28:3); pp. 111-115.

Reda, I. (1996). Calibration of a Solar Absolute Cavity Radiometer with Traceability to the World 

Radiometric Reference. NREL/TP-463-20619. Golden, CO: The National Renewable Energy Laboratory.

Accessed April 9, 2013: www.nrel.gov/docs/legosti/fy96/20619.pdf

Reda, I.; Myers, D.; Stoffel, T. (December 2008). “Uncertainty Estimate for the Outdoor Calibration of 

Solar Pyranometers: A Metrologist Perspective.” Measure (NCSLI Journal of Measurement Science) 

(3:4); pp. 58-66. NREL/JA-581-41370.

Romero, J. (1995). Direct Solar Irradiance Measurements with Pyrheliometers: Instruments and 

Calibrations. IPC-VIII. Davos, Switzerland. 16 pp.

Romero, J.; Fox, N.P.; Fröhlich, C. (May 1996). “Improved Comparison of the World Radiometric 

Reference and the SI Radiometric Scale.” Metrologia (32:6); pp. 523-524.

WRC/PMOD (1996). International Pyrheliometer Comparison, IPC VIII, 25 September – 13 October 

1995, Results and Symposium. Working Report No. 188. Davos Dorf, Switzerland: Swiss Meteorological 

Institute, Dorfstrasse 33, CH-7260; 115 pp.
29 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Appendix A: List of Participants and Pyrheliometers

S/N Operator 1 Operator 2 Affiliation

AHF 29223 Adriana Gonzalez UNAM

AHF 23734 Afshin Andreas Mike Dooraghi

National Renewable Energy 

Laboratory (NREL)

(Metrology, SePA, and Cell 

& Module Certification 

Group)

AHF 29219-Window Afshin Andreas Mike Dooraghi

NREL

(Metrology, SePA, and Cell 

& Module Certification 

Group)

AHF 31104-Window Afshin Andreas Mike Dooraghi

NREL

(Metrology, SePA, and Cell 

& Module Certification 

Group)

AHF 32452AWX-Window Afshin Andreas Mike Dooraghi

NREL

(Metrology, SePA, and Cell 

& Module Certification 

Group)

PM06 1601 Ajay Singh Matt Perry Campbell Scientific Inc.

PMO6 0816 Akiyama Akihito Will Beuttell EKO Instruments

AHF 33392 Anthony Bucholtz Naval Research Laboratory

AHF 31108 Bill Boyson Charles Robinson

Sandia National 

Laboratories

(Photovoltaic Systems 

Evaluation Lab)

 

TMI 67603 Bill Boyson Charles Robinson

Sandia National 

Laboratories

(Photovoltaic Systems 

Evaluation Lab)

 

AHF 32455 Christian Thomann PMOD/WRC

PMO6cc 0401 Christian Thomann PMOD/WRC

PMO6cc 0803 Christian Thomann PMOD/WRC

AHF 29222-Window Craig Webb

DOE Atmospheric

Radiation Measurement

(ARM) Program

AHF 30495-Window Craig Webb

DOE Atmospheric

Radiation Measurement

(ARM) Program

AHF 28553 Emiel Hall Jim Wendell NOAA/ESRL/GMD

S/N Operator 1 Operator 2 Affiliation

AHF 31113AWX-Window Emiel Hall Jim Wendell NOAA/ESRL/GMD
30 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

AHF 31114AWX Emiel Hall Jim Wendell NOAA/ESRL/GMD

AHF 32448AWX Emiel Hall Jim Wendell NOAA/ESRL/GMD

AHF 28560 Erik Naranen ISO-CAL North America, 

LLC

AHF 37816 Erik Naranen ISO-CAL North America, 

LLC

AHF 31041 Fred Denn Science Systems & 

Applications, Inc.

AHF 31105 Fred Denn Science Systems & 

Applications, Inc.

AHF28968 Ibrahim Reda Mike Dooraghi NREL

(Metrology and SePA)

AHF29220 Ibrahim Reda Mike Dooraghi NREL

(Metrology and SePA)

AHF30713 Ibrahim Reda Mike Dooraghi NREL

(Metrology and SePA)

TMI68018 Ibrahim Reda Mike Dooraghi NREL

(Metrology and SePA)

TMI 68020 Jim Goza Gian Mazzadi-Smith Lockheed Martin

AHF 28486 Jose Balenzategui CIEMAT

AHF 34926AWX Josh Peterson SRML University of Oregon

PMO6 1611-Linard-00 Markus Suter Davos Instruments 

AFH 30110 Mohammed Al Harbi

King Abdullah City for 

Atomic and Renewable 

Energy (KACARE)

AHF 31107 Mohammed Al Harbi

King Abdullah City for 

Atomic and Renewable 

Energy (KACARE)

AHF 17142 Patrick Smith Atlas Material Testing 

Technology, LLC

AHF 28556 Patrick Smith Atlas Material Testing 

Technology, LLC

TMI 67811 Tim Moss Alan Nelson

Sandia National Labs

(Concentrating Solar 

Energy)
31

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

S/N Operator 1 Operator 2 Affiliation

TMI 68022 Tim Moss Alan Nelson

Sandia National Labs

(Concentrating Solar 

Energy)

AHF 14915 Tom Kirk EPPLEY LAB

Pmo6cc 0103 Victor Cassella Joop Mes Kipp & Zonen USA, Inc

AHF 0000 Wim Zaaiman European Commission

Directorate General JRC

PMO6 81109 Wim Zaaiman European Commission

Directorate General JRC

PMO6 911204 Wim Zaaiman European Commission

Directorate General JRC

TMI 68835 Wim Zaaiman European Commission

Directorate General JRC
32

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Appendix B: Ancillary Data Summaries

The measurement performance of an absolute cavity can be affected by several environmental parameters.

Potentially relevant meteorological data collected during the NPC are presented in this appendix. The 

BMS has been in continuous operation at the SRRL since 1981. BMS data are recorded as 1-minute 

averages of 3-second samples for each instrument. (Additional information about SRRL and the BMS can 

be found at the Measurement and Instrumentation Data Center: http://www.nrel.gov/midc/srrl_bms/).

Time-series plots and other graphical presentations of these data acquired during the NPC-2018

measurements are presented here.
33

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

650

700

750

800

850

900

950

1000

1050

1100

6:43:12 7:55:12 9:07:12 10:19:12 11:31:12 12:43:12 13:55:12 15:07:12 16:19:12

MST

Reference Irradiance [W/m2]

0

0.05

0.1

0.15

0.2

6:43 7:55 9:07 10:19 11:31 12:43 13:55 15:07 16:19

MST

Est. Broadband Turbidity

0

5

10

15

20

25

30

35

6:43 7:55 9:07 10:19 11:31 12:43 13:55 15:07 16:19

MST

Tower Temp @ 2m [deg C]
34

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

0

10

20

30

40

50

60

70

80

6:43 7:55 9:07 10:19 11:31 12:43 13:55 15:07 16:19

MST

Tower RH @ 2m [%]

0

1

2

3

4

5

6

7

6:43 7:55 9:07 10:19 11:31 12:43 13:55 15:07 16:19

MST

Avg Wind Speed @ 2m [m/s]

0

4

8

12

16

20

6:43 7:55 9:07 10:19 11:31 12:43 13:55 15:07 16:19

MST

Est. Precipitable Water [mm]
35

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

3003504004505005506006507007508008509009501000105011001150120012501300135014001450150015501600165017001750180018501900195020002050210021502200225023002350240024502500

Spectral Irradiance (W/m2/nm)

Wavelenth (nm)

30-Minute Spectral Irradiances for September 24, 2018

6:30

7:00

7:30

8:00

8:30

9:00

9:30

10:00

10:30

11:00

11:30

12:00

12:30

13:00

13:30

14:00

14:30

15:00

15:30

16:00

16:30

17:00

Mountain

Standard 

Times

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

3003504004505005506006507007508008509009501000105011001150120012501300135014001450150015501600165017001750180018501900195020002050210021502200225023002350240024502500

Spectral Irradiance (W/m2/nm)

Wavelenth (nm)

30-Minute Spectral Irradiances for September 25, 2018

6:30

7:00

7:30

8:00

8:30

9:00

9:30

10:00

10:30

11:00

11:30

12:00

12:30

13:00

13:30

14:00

14:30

15:00

15:30

16:00

16:30

17:00

Mountain

Standard 

Times

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

3003504004505005506006507007508008509009501000105011001150120012501300135014001450150015501600165017001750180018501900195020002050210021502200225023002350240024502500

Spectral Irradiance (W/m2/nm)

Wavelenth (nm)

30-Minute Spectral Irradiances for September 26, 2018

6:30

7:00

7:30

8:00

8:30

9:00

9:30

10:00

10:30

11:00

11:30

12:00

12:30

13:00

13:30

14:00

14:30

15:00

15:30

16:00

16:30

17:00

Mountain

Standard 

Times
36

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

3003504004505005506006507007508008509009501000105011001150120012501300135014001450150015501600165017001750180018501900195020002050210021502200225023002350240024502500

Spectral Irradiance (W/m2/nm)

Wavelenth (nm)

30-Minute Spectral Irradiances for September 27, 2018

6:30

7:00

7:30

8:00

8:30

9:00

9:30

10:00

10:30

11:00

11:30

12:00

12:30

13:00

13:30

14:00

14:30

15:00

15:30

16:00

16:30

17:00

Mountain

Standard 

Times

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

3003504004505005506006507007508008509009501000105011001150120012501300135014001450150015501600165017001750180018501900195020002050210021502200225023002350240024502500

Spectral Irradiance (W/m2/nm)

Wavelenth (nm)

30-Minute Spectral Irradiances for September 29, 2018

6:30

7:00

7:30

8:00

8:30

9:00

9:30

10:00

10:30

11:00

11:30

12:00

12:30

13:00

13:30

14:00

14:30

15:00

15:30

16:00

16:30

17:00

Mountain

Standard 

Times

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

3003504004505005506006507007508008509009501000105011001150120012501300135014001450150015501600165017001750180018501900195020002050210021502200225023002350240024502500

Spectral Irradiance (W/m2/nm)

Wavelenth (nm)

30-Minute Spectral Irradiances for October 3, 2018

6:30

7:00

7:30

8:00

8:30

9:00

9:30

10:00

10:30

11:00

11:30

12:00

12:30

13:00

13:30

14:00

14:30

15:00

15:30

16:00

16:30

17:00

Mountain

Standard 

Times