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

NREL Pyrheliometer Comparisons: 

September 22–26, 2014 (NPC-2014)

Ibrahim Reda, Mike Dooraghi, and Aron Habte

Technical Report

NREL/TP-3B10-63050

October 2014
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

NREL Pyrheliometer Comparisons: 

September 22–26, 2014 (NPC-2014)

Ibrahim Reda, Mike Dooraghi, and Aron Habte

Prepared under Task No. SS13.3511

Technical Report

NREL/TP-3B10-63050

October 2014
NOTICE

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Cover Photos: (left to right) photo by Pat Corkery, NREL 16416, photo from SunEdison, NREL 17423, photo by Pat Corkery, NREL 

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iii

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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 Quality 

Management Systems & Assurance center (QMS&A). Our thanks also go to all the participants

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

Figure 1. NPC-2014 Participants. Not shown: Tom Stoffel and Cary Thompson 
iv

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List of Acronyms

BMS Baseline Measurement System

BORCAL Broadband Outdoor Radiometer Calibration

DOE U.S. Department of Energy

IPC International Pyrheliometer Comparison

IPC-XI Eleventh International Pyrheliometer Comparisons

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
v

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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, improving our 

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

technology applications involving solar flux. Providing these measurements places many 

demands on the quality system used by the operator 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 seven 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 and other types of pyrheliometers 

are subject to performance changes over time. Therefore, every five years the PMOD/WRC in 

Davos, Switzerland, hosts an International Pyrheliometer Comparison (IPC) for transferring the 

WRR to participating radiometers. NREL has represented 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 ISO 17025 accredited Broadband Outdoor 

Radiometer Calibration (BORCAL) process (Reda et al. 2008).

National Renewable Energy Laboratory (NREL) pyrheliometer comparisons (NPCs) are held 

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

pyrheliometer 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-2014 was held September 22-26, 2014. Participants operated 36 absolute cavity 

radiometers and 22 conventional thermopile-based pyrheliometers to simultaneously measure 

clear-sky direct normal solar irradiance during this period. The Transfer Standard Group (TSG) 

of reference radiometers for NPC-2014 consisted of four NREL radiometers with direct 

traceability to the WRR, having participated in the Eleventh International Pyrheliometer 

Comparisons (IPC-XI) in the fall of 2010. As a result of NPC-2014, each participating absolute 

cavity radiometer was assigned a new WRR-TF, computed as the reference irradiance 

 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.
vi

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determined by the TSG divided by the observed irradiance from the participating radiometer. 

The performance of the TSG during NPC-2014 was consistent with previous comparisons, 

including IPC-XI. The measurement performance of the TSG allowed the transfer of the WRR to 

each participating radiometer with an estimated uncertainty of ± 0.33% with respect to the 

International System of Units.

The comparison protocol is based on data collection periods called runs. Each measurement run

consists of an electrical self-calibration requiring 6 minutes for the AHF cavities, a series of 37 

solar irradiance measurements at 20-second intervals, and a post-calibration. More than 2,000

reference irradiance measurements were collected by the TSG during NPC-2014. Clear-sky daily 

maximum direct normal irradiance levels ranged from 950 Wm-2 to 1000 Wm-2.

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

humidity, wind speed, and vertical wind sheer) 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.
vii

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Table of Contents

Acknowledgments ..................................................................................................................................... iii

List of Acronyms........................................................................................................................................ iv

Executive Summary.................................................................................................................................... v

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

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

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

4 Transferring the World Radiometric Reference ................................................................................ 3

4.1 Calibration Requirements.............................................................................................................. 3

4.2 Determining the Reference Irradiance .......................................................................................... 4

4.3 Data Analysis Criteria ................................................................................................................... 4

4.4 Measurements................................................................................................................................ 4

4.5 Results........................................................................................................................................... 5

4.6 Recommendations....................................................................................................................... 36

5 Ancillary Data...................................................................................................................................... 36

References................................................................................................................................................. 37

Appendix A: List of Participants and Pyrheliometers........................................................................... 38

Appendix B: Ancillary Data Summaries ................................................................................................. 40
1 

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

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 Eleventh IPC (IPC-XI) was completed in 2010 (Finsterle 2011). 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 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, for non-IPC 

years. The eighteenth National Renewable Energy Laboratory Pyrheliometer Comparisons (NPC2014) was held September 22-26, 2014, at the SRRL. Participants operated 36 absolute cavity 

radiometers and 22 conventional thermopile-based pyrheliometers 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 participated 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 NPC-2014 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 2010

(Finsterle 2011).

Table 1. IPC-XI Results Summary for the NPC-2014 TSG

Serial Number WRR Factor

(IPC-XI)

Standard Deviation

(%)

Number of

Readings

AHF 28968 0.99773 0.0656 420

AHF 29220 0.99769 0.0669 418

AHF 30713 0.99755 0.0679 421

ATMI 68018 0.99680 0.0642 415

Mean WRR for the TSG 0.99744 SDp for the TSG: 0.07%

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

SSp = �∑ ni m

i=1 ∗ Si

2

∑ ni m

i=1

where,

i = i

th cavity

m = number of reference cavities

Si = standard deviation of the ith cavity, from IPC-XI

ni = number of readings of the ith cavity, from IPC-XI
3

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

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 37 observations at 20-second intervals (PMO6 used a 40-second open-

/closed-shutter cycle). 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 6-minute 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 WRRTF for an NPC. Participants also agreed that a minimum of 10 runs should be made during a period 

of at least 3 days to provide a variety of temperature and spectral irradiance conditions when 

computing the WRR-TF. A statistically significant dataset was required to derive the WRR-TF for 

each pyrheliometer.

Data from each pyrheliometer/operator system were emailed or collected at the end of the day using 

USB flash memory.

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-2014:

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

• Digital multimeters with accuracy > 0.05% reading were 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 

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4.2 Determining the Reference Irradiance 

Four absolute cavity radiometers that are maintained by NREL and that participated in IPC-XI 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-2014 is 

recalculated for each of the TSG cavities. (See Figure .) 

3. The reference irradiance for each 20-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 2 seconds 

centered on the comparison reading—i.e., 1 second before and 1 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 precalibration, or similar cause for a bias greater than expected from a properly functioning absolute 

cavity radiometer. 

Note that the ratios of windowed pyrheliometers do not have a normal distribution (see note under 

the histogram in the data figures), yet their uncertainty is calculated using a normal distribution for 

consistency with the NPC protocol for un-windowed pyrheliometers. Users must recalculate the 

uncertainty of their windowed pyrheliometers based on the actual distribution and their knowledge 

about the spectral effect due to the specifications of their respective windows. 

4.4 Measurements 

NPC-2014 was held September 22-26, 2014. The comparisons were completed on September 26, 

after more than 2,000 data points were collected by the reference cavities during the requisite clearsky 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. 
5

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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-2014 WRR-TF did not change by more than a fraction of the standard 

deviation derived during IPC-XI in 2010. (See Figure 2.)

• For the control standards—i.e., cavities that participated in IPC-XI and NPC-2014—their 

new WRR-TF, from NPC-2014, were consistent with their IPC-XI results. (See Table 2.)

Results for each radiometer participating in NPC-2014 are presented in Table 3.

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

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
6 

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Table 2. Summary Results for the Control Standards for NPC-2014

S/N WRR

(IPC-XI)

WRR

(NPC-2014)

SD% 

(NPC-2014)

WRRIPC - WRR 

NPC

%

AHF 14915 0.999682 1.00009 0.07 -0.040

AHF 17142 0.998358 0.99825 0.05 0.011

AHF 23734 0.998281 0.99809 0.04 0.019

AHF 28553 0.996842 0.99781 0.19 -0.097

AHF 31041 0.996286 0.99663 0.06 -0.035

AHF 31105 0.999964 0.99919 0.05 0.077

AHF 31114AWX 1.001244 1.00107 0.16 0.017

AHF 32448AWX 0.999939 1.00082 0.08 -0.088

AHF 32455 1.000276 1.00087 0.07 -0.060

PMO6 81109 0.998577 0.99805 0.06 0.053

PMO6 911204 0.999711 0.99963 0.08 0.008

PMO6cc 0103* 0.999424 0.99828 0.07 0.114

PMO6cc 0401 1.020979 1.02135 0.05 -0.036

TMI 67502 0.999294 0.99994 0.07 -0.065

TMI 68835 1.00098 1.00097 0.06 0.001

* from IPC-X

Table 3. Results for Radiometers Participating in NPC-2014

S/N 

WRRReduction 

Factor 

(Testcav)

%uA NRdg uC Eff DF %U95

AHF 14915 1.00009 0.07 2002 0.20 ∞ 0.39

AHF 17142 0.99825 0.05 1200 0.19 ∞ 0.38

AHF 21182 1.00029 0.06 2048 0.20 ∞ 0.38

AHF 23734 0.99809 0.04 2222 0.19 ∞ 0.37

AHF 28553 0.99781 0.00 2661 0.19 ∞ 0.36

AHF 28556 0.99524 0.04 1053 0.19 ∞ 0.37

AHF 29219-Window 1.05990 0.06 2194 0.19 ∞ 0.38

AHF 29222-Window 1.05937 0.07 3130 0.20 ∞ 0.39

AHF 30110-Window 1.06348 0.06 2944 0.19 ∞ 0.38

AHF 30494 0.99797 0.12 2148 0.22 ∞ 0.43

AHF 30495 0.99809 0.05 3120 0.19 ∞ 0.38

AHF 31041 0.99663 0.06 2984 0.20 ∞ 0.38

AHF 31104 0.99909 0.04 2225 0.19 ∞ 0.37

AHF 31105 0.99919 0.05 2990 0.19 ∞ 0.38

AHF 31107-Window 1.04620 0.08 2964 0.20 ∞ 0.40
7 

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S/N 

WRRReduction 

Factor 

(Testcav)

%uA NRdg uC Eff DF %U95

AHF 31108 0.99726 0.06 2134 0.19 ∞ 0.38

AHF 31114AWX 1.00107 0.00 2657 0.19 ∞ 0.36

AHF 32448AWX 1.00082 0.08 2666 0.20 ∞ 0.40

AHF 32452AWX-Window 1.03156 0.08 2246 0.20 ∞ 0.40

AHF 32455 1.00087 0.07 2943 0.20 ∞ 0.39

AHF 34926 1.00054 0.06 2150 0.20 ∞ 0.38

CH1 040370 0.99388 0.15 2891 0.24 ∞ 0.46

CH1 060460 1.00130 0.14 3051 0.23 ∞ 0.46

CH1 070571 0.99884 0.15 2903 0.24 ∞ 0.46

CH1 930018 1.00288 0.28 3096 0.33 ∞ 0.65

CHP1 090062 0.99942 0.11 2234 0.22 ∞ 0.42

CHP1 090127 1.00230 0.11 3052 0.21 ∞ 0.42

CHP1 10533 0.99936 0.14 2969 0.23 ∞ 0.46

CHP1 120967 1.00377 0.19 2204 0.26 ∞ 0.52

CHP-1 131060 0.99994 0.12 3041 0.22 ∞ 0.43

CHP1 131132 0.99285 0.12 2198 0.22 ∞ 0.43

CHP1 REF1 0.99937 0.10 2975 0.21 ∞ 0.41

DR02 SN-0041 ST 0.99983 0.16 2234 0.25 ∞ 0.49

DR03 SN-10012 0.99911 0.19 2219 0.26 ∞ 0.52

EKO_MS56_P13019 0.99654 0.19 2384 0.27 ∞ 0.52

EPP_NIP_21620E6 1.00144 0.49 2429 0.52 ∞ 1.02

KZ_CH1_070571 0.98497 0.21 2422 0.28 ∞ 0.55

MS56_PRH-REF.03 0.99850 0.14 2075 0.23 ∞ 0.45

MS56_PRH-REF.04 1.00035 0.24 2171 0.30 ∞ 0.59

NIP 30174E6 0.93123 0.49 3124 0.53 ∞ 1.03

PM06-CC 0816 1.00027 0.11 533 0.22 ∞ 0.42

PMO6 0103 0.99828 0.07 293 0.20 ∞ 0.39

PMO6 81109 0.99805 0.06 819 0.19 ∞ 0.38

PMO6 911204 0.99963 0.08 814 0.20 ∞ 0.40

PMO6cc 0401 1.02135 0.05 390 0.19 ∞ 0.38

PMO6cc 0807 1.00450 0.06 400 0.20 ∞ 0.38

sNIP 36477 1.00066 0.16 2049 0.25 ∞ 0.48

sNIP 37441 0.99817 0.19 1871 0.26 ∞ 0.52

sNIP 37882E6 1.00053 0.14 3057 0.23 ∞ 0.45

TMI 67502 0.99994 0.07 2085 0.20 ∞ 0.39

TMI 67603 0.99998 0.06 2126 0.19 ∞ 0.38
8 

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S/N 

WRRReduction 

Factor 

(Testcav)

%uA NRdg uC Eff DF %U95

TMI 68020 0.99930 0.08 833 0.20 ∞ 0.39

TMI 68835 1.00097 0.08 3117 0.20 ∞ 0.40

TMI 69036 1.00036 0.06 2208 0.19 ∞ 0.38

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

was calculated using the following formula: 

U95 = ±1.96 ∗ �uA

2 + uB

2

where,

U95 = Uncertainty of the WRR-TF (in percent) determined at NPC-2014 with 95% confidence 

level

1.96 = Coverage factor

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

determined at NPC-2014 

uB = Type B standard uncertainty

uB = ± �(

0.3

√3

 )2 + 0.072

where, 

0.3 = Estimated expanded uncertainty (± %) of the WRR scale with respect to SI

√3 = Coverage factor for rectangular distribution

0.07 = Pooled standard deviation of the four reference radiometers (TSG) that participated in 

IPC-XI (September/October 2010). 

The statistical analyses of WRR-TF for 51 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-2014. 
9

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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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 14915

0.9965

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 17142
10

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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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 21182

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 23734
11

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

0.9955

0.9960

0.9965

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 28553

0.9935

0.9940

0.9945

0.9950

0.9955

0.9960

0.9965

0.9970

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 28556
12

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

1.0575

1.0580

1.0585

1.0590

1.0595

1.0600

1.0605

1.0610

1.0615

1.0620

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 29219-

Windowed

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

1.0560

1.0570

1.0580

1.0590

1.0600

1.0610

1.0620

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 29222-

Windowed

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
13

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

1.0610

1.0615

1.0620

1.0625

1.0630

1.0635

1.0640

1.0645

1.0650

1.0655

1.0660

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 30110-

Window

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

0.9940

0.9950

0.9960

0.9970

0.9980

0.9990

1.0000

1.0010

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 30494
14

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

0.9960

0.9965

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 30495

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 31041
15

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

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 31104

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 31105
16

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

1.0430

1.0440

1.0450

1.0460

1.0470

1.0480

1.0490

1.0500

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 31107-

Window

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 31108
17

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

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 31114AWX

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 32448AWX
18

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

1.0260

1.0270

1.0280

1.0290

1.0300

1.0310

1.0320

1.0330

1.0340

1.0350

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 

32452AWXWindowed

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 32455
19

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

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

1.0020

1.0025

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

AHF 34926

0.9890

0.9900

0.9910

0.9920

0.9930

0.9940

0.9950

0.9960

0.9970

0.9980

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

CH1 040370

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
20

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

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

CH1 060460

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

0.9940

0.9950

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

CH1 070571

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
21

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

0.9920

0.9940

0.9960

0.9980

1.0000

1.0020

1.0040

1.0060

1.0080

1.0100

1.0120

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

CH1 930018

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

0.9950

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

CHP1 090062

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
22

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

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

CHP1 090127

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

0.9940

0.9950

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

CHP1 10533

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
23

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

0.9960

0.9980

1.0000

1.0020

1.0040

1.0060

1.0080

1.0100

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

CHP1 120967

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

CHP1 131060

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
24

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

0.9890

0.9900

0.9910

0.9920

0.9930

0.9940

0.9950

0.9960

0.9970

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

CHP1 131132

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

CHP1 REF1

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
25

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

0.9940

0.9960

0.9980

1.0000

1.0020

1.0040

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

DR02-0041 ST

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

0.9920

0.9940

0.9960

0.9980

1.0000

1.0020

1.0040

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

DR03 10012

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
26

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

0.9900

0.9920

0.9940

0.9960

0.9980

1.0000

1.0020

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

EKO_MS56_P13019

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

0.9850

0.9900

0.9950

1.0000

1.0050

1.0100

1.0150

1.0200

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

EPP_NIP_21620E6

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
27

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

0.9780

0.9800

0.9820

0.9840

0.9860

0.9880

0.9900

0.9920

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

KZ_CH1_070571

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

0.9940

0.9950

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

MS56_PRH-REF.03

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
28

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

0.9920

0.9940

0.9960

0.9980

1.0000

1.0020

1.0040

1.0060

1.0080

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

MS56_PRH-REF.04

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

0.9150

0.9200

0.9250

0.9300

0.9350

0.9400

0.9450

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

NIP 30174E6

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
29

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

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

PM06_0816

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

PMO6 0103
30

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

0.9960

0.9965

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

PMO6 81109

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

PMO6 911204
31

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

1.0190

1.0195

1.0200

1.0205

1.0210

1.0215

1.0220

1.0225

1.0230

1.0235

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

PMO6cc 0401

1.0025

1.0030

1.0035

1.0040

1.0045

1.0050

1.0055

1.0060

1.0065

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

PMO6cc 0807
32

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

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

sNIP 36477

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

0.9920

0.9940

0.9960

0.9980

1.0000

1.0020

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

sNIP 37441

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.
33

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

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

sNIP 37882E6

Average & 2*SD for windowed pyrheliometers are only shown 

for consistency with NPC reporting method.

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

TMI 67502
34 

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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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

TMI 67603

0.9965

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

TMI 68020
35

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

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

TMI 68835

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

WRR-TF

MST

WRR-Transfer Factor vs Mountain Standard Time 

NPC-2014

WRR-TF +/- 2 * SD

Histogram

TMI 69036
36

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

4.6 Recommendations

As a result of these comparisons, we suggest that participants observe the following measurement 

practices:

• For the purpose of pyrheliometer comparisons, such as NPC-2014, we recommend 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. We recommend this approach to realize the benefits of participating in the NPC. 

• For future pyrheliometer comparisons, we strongly urge participants to provide their irradiance 

readings in the following format:

Serial number

##, MM/DD/YYYY, HH:MM:SS, IRR

where,

Serial number = Instrument serial number (first line only)

## = Reading number (1 to 37) within the run

MM/DD/YYYY = Month, Day, Year of the reading 

HH:MM:SS = Hour, minute, and second of the reading (local standard time, 24-hour clock)

IRR = Computed irradiance (Wm-2) with resolution of XXXX.XX

The file naming convention is suggested to include the radiometer serial number and date of 

observations (e.g., AHF30713_09202013 would correspond to data from AHF30713 on September 

20, 2013).

5 Ancillary Data

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

and vertical wind sheer, 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.
37

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

References

Finsterle, W. (2011). WMO International Pyrheliometer Comparison, IPC-XI, 27 September – 15 

October 2010: Final Report. WMO IOM Report No. 108. Davos, Switzerland. 86 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. (2008). “Uncertainty Estimate for the Outdoor Calibration of Solar 

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

(3:4), December 2008; pp. 58–66. NREL/JA-581-41370.

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

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

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

Reference and the SI Radiometric Scale.” Metrologia (32:6) May; 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.
38

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 17142 Erik Naranen Atlas Weathering Services Group

AHF 28556 Erik Naranen Atlas Weathering Services Group

EKO_MS56_P13019 James Augustyn Augustyn + Company

KZ_CH1_070571 James Augustyn Augustyn + Company

EPP_NIP_21620E6 James Augustyn Augustyn + Company

sNIP 37882E6 Afshin Andreas Baseline Measurement System

CHP1 131060 Scott Smith Brookhaven National Laboratory

PMO6cc 0807 Stefan Wilbert DLR German Aerospace

AHF 29222 Craig Webb DOE Atmospheric Radiation Measurement 

(ARM) Program

AHF 30495 Craig Webb DOE Atmospheric Radiation Measurement 

(ARM) Program

MS56_PRH_REF03 Tsukasa Kobashi Akihito Akiyama EKO Instruments USA, Inc.

MS56A_PRH_REF04 Tsukasa Kobashi Akihito Akiyama EKO Instruments USA, Inc.

PMO6_0816 Tsukasa Kobashi Akihito Akiyama EKO Instruments USA, Inc.

NIP 30174E6 Abdulrahman S. 

Binsufayran

Ayman K. Sheikho Energy Research Institute King Abdulaziz City 

for Science and Technology

AHF 14915 Tom Kirk Eppley Laboratory, Inc.

sNIP 36477 Tom Kirk Eppley Laboratory, Inc.

sNIP 37441 Tom Kirk Eppley Laboratory, Inc.

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

CH1 930018 Wim Zaaiman European Commission Directorate General 

JRC

CH1 040370 Wim Zaaiman European Commission Directorate General 

JRC

CH1 060460 Wim Zaaiman European Commission Directorate General 

JRC

CHP1 10533 Wim Zaaiman European Commission Directorate General 

JRC

AHF 21182 Caleb Reninger Florida Solar Energy Center

CHP1 090062 Justin Robinson Groundworks

CHP1 120967 Justin Robinson Groundworks

CHP1 131132 Justin Robinson Groundworks

DR02 SN-0041 ST Robert Dolce Jörgen Konings HUKSEFLUX

DR03 SN-10012 Robert Dolce Jörgen Konings HUKSEFLUX
39

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

AHF 30110 Hussain Shibli Yaser Al Jnoobi King Abdullah City of Atomic and Renewable 

Energy

AHF 31107 Hussain Shibli Yaser Al Jnoobi King Abdullah City of Atomic and Renewable 

Energy

PMO6 0103 Victor Cassella Joop Mes Kipp&Zonen

CHP1 REF1 Victor Cassella Joop Mes Kipp&Zonen

CHP1 090127 Victor Cassella Joop Mes Kipp&Zonen

CH1 070541 Victor Cassella Joop Mes Kipp&Zonen

TMI 68020 Cary Thompson Lockheed Martin

AWX31114 Jim Wendell Emiel Hall National Oceanic & Atmospheric 

Administration

AWX32448 Jim Wendell Emiel Hall National Oceanic & Atmospheric 

Administration

AHF 28553 Jim Wendell Emiel Hall National Oceanic & Atmospheric 

Administration

TMI 67502 Jim Wendell Emiel Hall National Oceanic & Atmospheric 

Administration

AHF 28968 Ibrahim Reda Preston Morse/ 

Afshin Andreas

National Renewable Energy Laboratory

AHF 29220 Ibrahim Reda Preston Morse/ 

Afshin Andreas

National Renewable Energy Laboratory

AHF 30713 Ibrahim Reda Preston Morse/ 

Afshin Andreas

National Renewable Energy Laboratory

AHF 32452 Ibrahim Reda Preston Morse/ 

Afshin Andreas

National Renewable Energy Laboratory

AHF 23734 Ibrahim Reda Preston Morse/ 

Afshin Andreas

National Renewable Energy Laboratory

AHF 31104 Ibrahim Reda Preston Morse/ 

Afshin Andreas

National Renewable Energy Laboratory

AHF 29219 Ibrahim Reda Preston Morse/ 

Afshin Andreas

National Renewable Energy Laboratory

AHF 30494 Ibrahim Reda Preston Morse/ 

Afshin Andreas

National Renewable Energy Laboratory

AHF 69036 Ibrahim Reda Preston Morse/ 

Afshin Andreas

National Renewable Energy Laboratory

AHF 68018 Ibrahim Reda Preston Morse/ 

Afshin Andreas

National Renewable Energy Laboratory

AHF 32455 Nathan Mingard Physikalisch-Meteorologisches 

Observatorium Davos (PMOD)

PMO6cc 0401 Nathan Mingard Physikalisch-Meteorologisches 

Observatorium Davos (PMOD)

AHF 31108 Bill Boyson Craig Carmignani Sandia National Laboratories

TMI 67603 Bill Boyson Craig Carmignani Sandia National Laboratories

AHF 31105 Fred Denn Science Systems & Applications, Inc.

AHF 31041 Fred Denn Science Systems & Applications, Inc.

AHF 34926 Rich Kessler Josh Peterson University of Oregon
40

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 1985. 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/aim/npc.html)

Time-series plots and other graphical presentations of these data acquired during the NPC-2014

measurements are presented here.

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] on September 22 to 26, 2014

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

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

Broadband Turpidity on September September 22 to 26, 2014
41

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0

5

10

15

20

25

30

35

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

Deck Temp [deg C] on September September 22 to 26, 2014

0

10

20

30

40

50

60

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

Deck RH [%] on September September 22 to 26, 2014

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

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

Avg Wind Speed @ 6 ft [m/s] on September September 22 to 26, 2014