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Contract No. DE-AC36-08GO28308

Evaluation of Radiometers 

Deployed at the National 

Renewable Energy Laboratory’s

Solar Radiation Research 

Laboratory

A. Habte, S. Wilcox, and T. Stoffel

National Renewable Energy Laboratory

Technical Report

NREL/TP-5D00-60896

Revised December 2015
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

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Evaluation of Radiometers 

Deployed at the National 

Renewable Energy 

Laboratory’s Solar Radiation 

Research Laboratory

A. Habte, S. Wilcox, and T. Stoffel

National Renewable Energy Laboratory

Prepared under Task No. SS13.3514

Technical Report

NREL/TP-5D00-60896

Revised December 2015
NOTICE

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Errata

Figure 15 was revised in December 2015 to correct an error in the calculation to derive the 

values. The error does not affect any other parts of the publication.
iii

Acknowledgments 

We acknowledge Daryl Myers, retired National Renewable Energy Laboratory employee, for his 

insightful support and suggestions during the development of this report. We are grateful to 

Afshin Andreas, Ibrahim Reda, Mike Dooraghi, and Mark Kutchenreiter for maintaining the 

world’s largest array of web-accessible, continuously-operating, radiometric measurements at the 

National Renewable Energy Laboratory’s Solar Radiation Research Laboratory. We thank Peter 

Gotseff for reviewing the report. Finally, the leadership of Manajit Sengupta, acting Solar 

Resources and Forecasting Group Manager, in setting higher standards for solar radiation 

resource measurements is appreciated.

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iv

List of Acronyms

ATI Ascension Technology, Inc. 

DHI diffuse horizontal irradiance 

DNI direct normal irradiance

GHI global horizontal irradiance

MBE mean bias error 

NREL National Renewable Energy Laboratory

PSP precision spectral pyranometer 

RMSE root mean square error 

RSR rotating shadowband radiometer 

TSI total sky imager

TSR thermopile shadowband radiometer 

UUT unit under test 

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v 

Executive Summary 

Solar radiation resource measurements from radiometers are used to predict and evaluate the 

system performance of photovoltaic cells, modules, and arrays. This study analyzes the 

performance of various commercially available pyranometers used for measuring global 

horizontal irradiance and pyrheliometers, rotating shadowband radiometers, and a pyranometer 

with fixed internal shading to provide direct normal irradiance deployed at the National 

Renewable Energy Laboratory’s Solar Radiation Research Laboratory. Data from 32 global 

horizontal irradiance and 19 direct normal irradiance radiometers are presented in this report.

The radiometers in this study were deployed for one year (from April 1, 2011, through March 31, 

2012) and compared to measurements from radiometers with the lowest values of estimated

measurement uncertainties for producing reference global horizontal irradiances and direct 

normal irradiances. The reference radiometers were calibrated with absolute cavity radiometers 

traceable to the World Radiometric Reference using the National Renewable Energy 

Laboratory’s Transfer Standard Group and Broadband Outdoor Radiometer Calibration process. 

The reference irradiance data were corrected for radiometer temperature response based on 

information from the instrument manufacturer. These corrected reference data were then quality 

assessed using the National Renewable Energy Laboratory’s SERI_QC (Maxwell, Wilcox, and 

Rymes 1993). Data from the radiometers under test were normalized for a minimum bias error at 

a 45-degree solar zenith angle data with respect to the reference instrument. 

The intent of this report is to present a general overview of each radiometer’s performance. The 

manufacturers were allowed to select the instruments they provided, and the National Renewable 

Energy Laboratory made no effort to ensure that the participating radiometers were 

representative units; therefore, this report does not guarantee the same result for all radiometers 

from the same manufacturer or model.

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vi

Table of Contents 

List of Figures ......................................................................................................................................... vii

List of Tables ........................................................................................................................................... vii

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

2 Method................................................................................................................................................. 2

2.1 Reference Data............................................................................................................................... 2

2.2 Data Quality................................................................................................................................... 2

2.2.1 SERI_QC ........................................................................................................................ 3

2.2.2 Temperature Dependency................................................................................................ 4

2.2.3 Operation and Maintenance............................................................................................. 4

2.2.4 Data Exclusion ................................................................................................................ 5

2.2.5 Data Normalization ......................................................................................................... 5

2.2.6 Data Filtering for Clear-, Partly-Cloudy, and Mostly-Cloudy Sky Conditions................ 7

2.2.7 Uncertainty...................................................................................................................... 7

3 Results ................................................................................................................................................ 9

3.1 GHI Radiometers (Pyranometers).................................................................................................. 9

3.2 DNI Radiometers (Pyrheliometers).............................................................................................. 20

4 Summary ........................................................................................................................................... 28

5 References ........................................................................................................................................ 29

6 Appendix A: GHI Comparisons ....................................................................................................... 31

7 Appendix B: DNI Comparisons ..................................................................................................... 124

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vii

List of Figures 

Figure 1. Example of SERI_QC software output .................................................................................... 3

Figure 2. Example of SERI-QC Gompertz curve..................................................................................... 4

Figure 3. Temperature dependency for the (left) DNI and (right) DHI reference instruments ............ 4

Figure 4. Example plots showing how the TSI-880 is used to partition the sky condition: (left) 

instantaneous image and (right) results for all data ....................................................................... 7

Figure 5. Bias error in percent for the 10-minute average for all GHI data under study. The red line 

signifies the mean value of the MBE for the 95% confidence level. See Table 5 to obtain the 

instrument name and model by using the instrument number (x-axis)....................................... 12

Figure 6. MBE in percent for the hourly average for all GHI data under study. The red line signifies 

the mean value of the MBE for the 95% confidence level. See Table 5 to obtain the instrument 

name and model by using the instrument number (x-axis). ......................................................... 13

Figure 7. RMSE in percent for the hourly average for all GHI data under study. The red line 

signifies the mean value of the RMSE for the 95% confidence level. Refer to Table 5 to obtain 

the instrument name and model using the instrument number (x-axis). .................................... 16

Figure 8. Thermopile and silicon photodiode pyranometers spectral response (modified from 

Myers 2011 and Myers 2013) ........................................................................................................... 17

Figure 9. (Top) Comparison between the uncorrected silicon photodiode sensor and (bottom) 

corrected silicon photodiode—NREL-CRADA-RSR2 .................................................................... 18

Figure 10. Understanding nighttime thermal offset (top) under all sky conditions and (bottom) 

normalized differences (UUT minus reference) versus temperature for (left) ventilated and 

(right) unventilated Eppley Laboratory, Inc., PSP radiometers under clear-sky conditions (1-

minute data) ...................................................................................................................................... 19

Figure 11. Asymmetric conditions caused by an unleveled radiometer............................................ 20

Figure 12. Bias error in percent for the 10-minute average for all DNI data under study. The red line 

signifies the mean value of the MBE for the 95% confidence level. See Table 7 to obtain the 

instrument name and model by using the instrument number (x-axis)....................................... 22

Figure 13. MBE in percent for the hourly average for all DNI data under study. The red line 

signifies the mean value of the MBE for the 95% confidence level. See Table 7 to obtain the 

instrument name and model by using the instrument number (x-axis)....................................... 23

Figure 14. Effect of temperature on DNI measurement ....................................................................... 24

Figure 15. RMSE in percent for the hourly average for all DNI data under study. The red line 

signifies the mean value of the RMSE for the 95% confidence level. See Table 5 to obtain the 

instrument name and model by using the instrument number (x-axis)....................................... 27

List of Tables 

Table 1. GHI Data Normalization Ratio for Each Instrument Using the 10-Minute Data...................... 6

Table 2. DNI Data Normalization Ratio for Each Instrument Using the 10-Minute Average Data....... 7

Table 3. Broadband Outdoor Radiometer Calibration Uncertainty Values for Reference Instruments 

Transferred to the Average Data for the Period of Record (April 1, 2011, to March 31, 2012). 

Calibration Date: May 4, 2012............................................................................................................ 8

Table 4. Example Uncertainty Estimates for Solar Radiometer Responsivity (Reda 2011)................ 8

Table 5. GHI Instrument List .................................................................................................................... 9

Table 6. Hourly Average MBE in Percent Under Various Solar Zenith Angle Ranges for GHI Data 14

Table 7. DNI Instrument List................................................................................................................... 21

Table 8. Hourly Average MBE in Percent Under Various Solar Zenith Angle Ranges for DNI Data 25

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1 

1 Introduction 

Accurate measurements of solar resources are essential for the successful deployment of 

photovoltaic and concentrating solar power systems (Stoffel et al. 2010). Accurate solar resource 

data reduce investment risks for designing, installing, operating, and maintaining solar energy 

conversion systems. Solar resource data are generally available from satellite-based models 

developed and validated with ground-based measurements of solar radiation. Understanding the 

measurement performance of commercially available radiometers is important for properly 

interpreting solar resource data.

The National Renewable Energy Laboratory’s (NREL’s) Solar Radiation Research Laboratory 

collects and disseminates solar irradiance and meteorological data through the Measurement and 

Instrumentation Data Center.

1 The center provides historical solar data from pyrheliometers and 

pyranometers with calibration traceability to the World Radiometric Reference and the Système 

International d’Unités (Stoffel and Reda 2013). This paper provides a comprehensive estimation 

of differences associated with radiometric data obtained from various radiometers under all sky 

conditions. Some of the variables contributing to the differences include radiometer calibrations 

and each instrument’s response characteristics to variations in solar zenith angle, spectral 

irradiance distributions, temperature, installation (e.g., tilt), aging, nonlinearity, and incomplete 

knowledge of environmental conditions, which could also affect each instrument’s performance. 

This report presents comparisons among 51 data sets of global horizontal irradiance (GHI) and 

direct normal irradiance (DNI) from various pyranometer and pyheliometer radiometers. The 

radiometers were deployed at NREL’s Solar Radiation Research Laboratory in continuous 

operation for one year, from April 1, 2011, through March 31, 2012. Data were recorded as 1-

minute averages of 3-second samples. All instruments at the Solar Radiation Research 

Laboratory are maintained daily (Monday through Friday), except on holidays. The differences 

among radiometer measurements were calculated using the mean bias error (MBE) and root 

mean square error (RMSE) methods, in which the GHI and DNI values from individual 

instruments were compared with concurrently computed GHI reference and measured reference 

DNI. The differences were calculated as a percent and W/m2

 for solar zenith angles ranging from 

17.5 degrees to 85 degrees (the range of available solar zenith angles varies throughout the year). 

The solar measurements from the radiometers were quality assessed to minimize erroneous data; 

however, these data are presented with some caveats: (1) There was not an ample number of 

radiometers from each model to provide representative sample data for each manufacturer’s 

product. (2) In our professional experience, we have found that each instrument responds 

differently under various climatic/weather conditions; thus, these results are limited to the 

conditions encountered at the Solar Radiation Research Laboratory during the period of analysis. 

(3) Solar irradiance on the Earth’s surface is extremely variable, and these instruments have 

variable time responses, spectral components, cosine response, and temperature sensitivity; thus, 

the limited data set and specific location used for this study are not intended to be used by the 

reader to infer any general conclusions of radiometer performance. This report intends to provide 

the reader with a general understanding about how each radiometer behaves under specific 

documented conditions compared to the selected reference data. 

 

1 http://www.nrel.gov/midc 

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2 

2 Method 

Most of the instruments in this study are owned by NREL as part of our ongoing Baseline 

Measurement System.

2

 For this study, additional instruments were provided by the 

manufacturers at the request of NREL. A complete list of instruments is provided as Table 1 and 

Table 2. The manufacturers were allowed to select the instruments they provided, and NREL 

made no effort to ensure that they were representative units. Prior to the start of the evaluation 

period, the manufacturers were allowed to ensure that their instruments were operating to their 

satisfaction. After the evaluation period began, the manufactures were not allowed any contact or 

interaction with the instruments. All data collected from the instruments for this study were 

downloaded directly to NREL computers. The Solar Millennium instrument (Solar-Mil.-

CRADA-RSR) was set up with special internal processing to take the place of the external 

processing that is usually provided by the manufacturer on corporate servers. As noted below, 

this necessitated the omission of the instrument’s usual 1-minute data output.

The performance of each test instrument relative to the reference instrument was analyzed using 

the MBE and RMSE methods to obtain the 1-minute, 10-minute, hourly, daily, and monthly 

mean daily totals.

One-minute data records were averaged to produce 10-minute values—with the exception of the 

Solar-Mil.-CRADA-RSR, which recorded the data as 10-minute averages. The hourly data were 

averaged from the 10-minute data sets. Daily totals were calculated by summing all hourly data 

for solar zenith angles less than 85 degrees to avoid the low solar irradiance values near sunrise 

and sunset. Mean monthly daily totals were calculated by averaging the daily total data in the 

month. 

2.1 Reference Data

Reference data were obtained using a Kipp and Zonen CH1 (DNI) instrument and an Eppley 

Laboratory, Inc., black-and-white model 8-48 (diffuse horizontal irradiance, or DHI) instrument. 

A reference GHI was calculated from a component sum method using the CH1 and 8-48 

radiometers (Equation 1). 

GHI = DNI * Cosine (Solar Zenith Angle) + DHI (1) 

These reference radiometers were calibrated with absolute cavity radiometers traceable to the 

World Radiometric Reference using NREL’s Broadband Outdoor Radiometer Calibration 

process. Further, they have lowest calibration and measurement uncertainties for both GHI and 

DNI irradiances.

2.2 Data Quality

Various data quality schemes were applied to the radiometric data. Some of these schemes target 

detecting equipment and operational related errors, data issues as a result of unclean instruments, 

instrument limitations provided by manufacturer specifications, and the estimated measurement 

uncertainty of data from these instruments. 

 

2 http://www.nrel.gov/midc/srrl_bms 

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3 

2.2.1 SERI_QC 

SERI_QC was developed by NREL in 1993 as a data quality assessment tool to test each of the 

three irradiance measurements: GHI, DNI, and DHI. The process generates a data quality value, 

or flag, for each data point based upon a normalization process involving the dimensionless 

parameters (clearness or cloudiness index (Kt), effective diffuse horizontal transmittance (Kd) 

and direct beam transmittance (Kn)) derived from the corresponding extraterrestrial radiation 

(Maxwell, Wilcox, and Rymes 1993). Using an empirical approach, the process generates 

acceptable physical limits of the measurements (Figure 1 and Figure 2). This data quality 

assessment tool was applied to the reference data in this study. Data that were outside of the 

physical limits were excluded from the analysis.

Figure 1. Example of SERI_QC software output

Figure 1 shows the days of the month on the vertical axis plotted against the hours of the day on 

the horizontal axis. The leftmost chart shows the most severe flags from among the three 

components at each time interval, the least error in the dark blue, and the greatest error in red. 

The remaining three charts present the relative solar irradiance for each of the three major

components. The Kt, Kn, and Kd charts show clearness ranges in which the dark blue represents 

the least clear and the red shows the highest radiance (Wilcox and McCormack 2011). 

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4 

Figure 2. Example of SERI-QC Gompertz curve

The SERI_QC software package includes a graphical representation of component tests as 

described in Figure 2. The polynomial curve boundaries in Figure 2 are called Gompertz curves. 

They show acceptable values for global and direct components (Maxwell, Wilcox, and Rymes 

1993). Data failing three-component coupling were flagged as unacceptable.

2.2.2 Temperature Dependency

The temperature dependencies of the reference radiometers were provided by the manufacturer 

specifications. Figure 3 shows the temperature dependencies of the pyranometer and 

pyrheliometers used in our study. The temperature dependency function from these figures was

used to correct the reference irradiance data obtained from these radiometers.

Figure 3. Temperature dependency for the (left) DNI and (right) DHI reference instruments 

2.2.3 Operation and Maintenance

Regularly cleaning and performing operational checks of the radiometers, data acquisition 

systems, and ancillary instruments is essential for acquiring reliable solar resource data (Myers et 

al. 2001). 

Proper operation and maintenance of the instruments is critical for measuring accurate solar 

resources. Cleaning radiometer optical surfaces on a regular basis prevents the attenuation of the 

signals that are generated by contaminates on the radiometers. However, in some cases, 

accumulation of snow or frost could create greater statistical differences between the reference 

and unit under test (UUT) irradiances because some instruments tend to accumulate more snow 

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5 

or frost than others. To minimize the effect of debris accumulation, some of the pyranometers are 

equipped with ventilators. Leveling the radiometers is essential to avoid any asymmetric 

responses by the radiometers throughout the day. Checking desiccants is important to prevent 

condensation inside the optical surfaces, which would induce signal attenuation. Further, 

regularly checking the signal cable condition of the radiometers helps prevent data loss. 

However, these problems might not have been completely removed by the SERI_QC software.

Some instruments have built-in correction algorithms for known responses in the data logger. 

These corrections are applied by the manufacturer during routine data acquisitions and were 

available in the instrument output data set used for this study. Further, some radiometers, such as 

the Eppley Laboratory, Inc., model PSP (precision spectral pyranometer), have correction 

algorithms implemented by NREL. A thermal correction using a pyrgeometer and a correction 

for the responsivity as a function of the solar zenith angle were implemented in the study for 

consideration by the user. 

2.2.4 Data Exclusion

Data points were excluded from the analysis for various reasons, including to help maintain a 

consistent comparison among instruments for the 12-month period. If data were excluded from 

one instrument, data from the same period were excluded from all instruments. The exception 

was the thermopile shadowband radiometer 591 (TSR-591), which had incorrect or no data for 

the whole month of November 2011. To minimize the loss of data in the analysis from all the 

UUT, we avoided using the TSR-591 in the filtering process for the suspected month. In the 

analysis, all test instruments for November 2011 were accounted for with the exception of TSR591. 

The study included only daytime data (e.g., solar zenith angles less than or equal to 85 degrees). 

Some data points at higher solar zenith angles (low solar irradiance levels) were excluded from 

the analysis to avoid the effects of large relative differences during the low absolute magnitude 

of irradiance near sunrise and sunset. 

Differences greater than +100% were excluded from the final analysis for all UUT radiometers. 

This criterion is consistent with our previous study (Wilcox and Myers 2008). 

2.2.5 Data Normalization

The solar irradiance data acquired from the test radiometers were normalized to the reference 

data to remove potential instrument calibration biases. Although an accurate instrument 

calibration is a critical component for producing accurate measurements, differences among 

instruments in calibration methods and times since the last calibrations are the subject of another 

exhaustive study. Normalizing the data to a common reference allowed us to compare the 

environmental effects of each instrument’s performance, which was the stated focus of this 

study. 

Normalization was carried out by isolating all the irradiance data for the study period (April 1, 

2011, through March 31, 2012) between 44- and 46-degree solar zenith angles and summing all 

the data in the range of the solar zenith angles for each radiometer. The specified solar zenith 

angle range was selected to conform to the NREL convention of reporting all broadband 

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6 

radiometer calibrations at a 45-degree solar zenith angle. A ratio was then obtained by dividing 

each UUT radiometer summation by the sum of the reference data for the same conditions. The 

ratio for each UUT radiometer was then used to acquire the new normalized irradiance value 

(IUUT(New)) by dividing each UUT irradiance value for the time interval by the normalization ratio

(Equation 2): 

Ratio = 

∑ IUUT44° to 46°

∑ IRef44° to 46°

(2) 

where IUUT 44o

 to 46o is the irradiance data for the UUT within the two-degree solar zenith angle bin 

and IRef 44o

 to 46o is the irradiance data of the reference instrument within the same solar zenith 

angle range.

The new normalized irradiance data from the UUT were then computed as (Equation 3): 

IUUT(New) = 

IUUT

Ratio (3) 

The adjustments used to normalize the data for each instrument are included in Table 1. Results 

listed in the table provide users with a range of possible variances among calibration techniques 

that could affect the accuracy of their measurements.

A ratio greater than one indicated that the UUT data were higher than the reference radiometer 

measurements. A ratio lower than one meant that the UUT was underestimating irradiance 

relative to reference data. 

Overall, as shown by the values in Table 1 and Table 2, the data normalization process presented 

an approximate 2% difference on average compared to the unnormalized data for both the GHI 

and DNI data sets. 

Table 1. GHI Data Normalization Ratio for Each Instrument Using the 10-Minute Data

Instrument 

Number Model Ratio Instrument Number Model Ratio

1 CM22* 1.01 17 RSR2 (secondary) 0.99

2 CM6b* 1.01 18 TSR-590 0.96

3 CM3-CNR1* 1.01 19 TSR-591 0.95

4 PSP* 1.00 20 TSR-592 0.96

5 PSP* 1.00 21 TSR-590-LH 0.97

6 PSP* 0.99 22 TSR-591-LH 0.97

7 PSP* 1.00 23 TSR-592-LH 0.97

8 TSP-700* 1.01 24 TSR-590-JM 0.99

9 TSP-1* 1.00 25 TSR-591-JM 0.99

10 SPN1* 1.03 26 TSR-592-JM 0.98

11 SPLite* 1.01 27 SR11-7196 0.97

12 SP-110* 1.06 28 SR11-7242 0.98

13 LI-200* 1.01 29 LP02-41120 0.99

14 P007 1.00 30 LP02-41272 0.97

15 ATI 1.00 31 NREL-CRADA-RSR2 0.98

16 RSR2 0.98 32 Solar-Mil.-CRADA-RSR 1.00 *

 Current NREL calibration factor used during the period of the evaluation.

The same method was applied for both the GHI and DNI radiometers. The ratios for the DNI 

radiometers are listed in Table 2. 

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7 

Table 2. DNI Data Normalization Ratio for Each Instrument Using the 10-Minute Average Data

Instrument 

Number Model Ratio Instrument Number Model Ratio

1 NIP2* 0.99 11 TSR-592LH 0.99

2 NIP1* 0.99 12 TSR-590JM 0.98

3 LI-201* 0.94 13 TSR-591JM 0.97

4 ATI 1.04 14 TSR-592JM 0.97

5 RSR2 0.98 15 NREL-CRADA-RSR2 0.98

6 TSR-590 0.99 16 SPN1* 1.05

7 TSR-591 0.97 17 DR018066 0.97

8 TSR-592 0.99 18 DR018068 0.98

9 TSR-590LH 0.99 19 Solar-Mil.-CRADA-RSR 1.00

10 TSR-591LH 0.98 *

 Current NREL calibration factor used during the period of the evaluation.

2.2.6 Data Filtering for Clear-, Partly-Cloudy, and Mostly-Cloudy Sky Conditions

Concurrent data from a Yankee Environmental Systems, Inc., total sky imager (TSI) model TSI880 were used to categorize irradiance data from all the instruments according to clear-, partlycloudy, and mostly-cloudy sky conditions (Figure 4). The total cloud cover percentage from the 

TSI-880 instrument was used for this classification. The instrument provides high-resolution, 1-

minute data that coincide with the radiometric data. In our experience, the instrument tends to 

underestimate cloud cover, and for this reason a simple criterion that mitigates the problem was 

developed to distinguish cloud covers. Sky condition categories were determined as follows: 

clear when TSI total cloud cover data were less than 1.2%; partly cloudy for the range from 1.2% 

to 60%; and mostly cloudy when TSI total cloud cover data were greater than 60%.

Figure 4. Example plots showing how the TSI-880 is used to partition the sky condition: (left) 

instantaneous image and (right) results for all data 

2.2.7 Uncertainty

Each instrument, including the reference instruments, had its own estimated measurement

uncertainty. The calibration uncertainty for the reference instruments is summarized below, and 

is provided to show the expected uncertainty in determining irradiance values in the range of 

zenith angles from 30 degrees to 60 degrees. According to each Broadband Outdoor Radiometer 

Calibration report,3 the uncertainty value for the DNI reference instrument is +0.72%/-0.66%. 

 

3 Broadband Outdoor Radiometer Calibration reports are available from http://www.nrel.gov/aim/borcal.html

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
8 

The uncertainty in the calculated reference GHI is +2.1% (see Table 3), as calculated from the 

highest calibration uncertainty values from the Broadband Outdoor Radiometer Calibration 

report of the DNI and DHI reference pyrheliometer and pyranometer, respectively. Therefore, 

the reported differences in the analysis could have varied by an additional +2.1% for GHI and 

+0.72% for DNI. That is, reported differences greater than these uncertainties could be 

considered measurable.

Table 3. Broadband Outdoor Radiometer Calibration Uncertainty Values for Reference Instruments 

Transferred to the Average Data for the Period of Record (April 1, 2011, to March 31, 2012). 

Calibration Date: May 4, 2012

CH1 (Pyrheliometer)—Direct Irradiance

S/N: 10256

Responsivity: 10.841 (μV/W/m2)

Uncertainty: + 0.72 %

Average direct irradiance (DNI): 494 W/m^2

Contribution to the average global irradiance uncertainty: ~+0.0072* 494 +3.5 W/m^2

Eppley Laboratory, Inc., 8-48—Diffuse Irradiance

S/N: 21096

Responsivity: 11.554 (μV/W/m2)

Uncertainty: + 4.05/-1.55 %

Average diffuse irradiance: 132.5 W/m^2

Contribution to the average calculated global irradiance uncertainty: ~+0.04* 132.47 +5 .3W/m^2

Average calculated global irradiance: 412W/m^2

Average percentage uncertainty to calculated global irradiance: (+8.8/412)*100 ~2.1%

Sources of radiometer measurement uncertainty include calibration, zenith angle response, 

azimuth response, spectral response, temperature response, detector aging, instrument 

maintenance, data logger function, etc. Table 4 describes the estimated expanded uncertainty 

values of thermopile and semiconductor radiometers (Reda 2011). These estimates do not 

necessarily represent uncertainties among the reference or test instruments in this study.

Table 4. Example Uncertainty Estimates for Solar Radiometer Responsivity (Reda 2011)

Uncertainty Source Thermopile 

Pyranometer

Semiconductor 

Pyranometer

Thermopile 

Pyrheliometer

Semiconductor 

Pyrheliometer

Calibrationa 3% 5% 2% 3%

Zenith Responseb 2% 2% 0.5% 1%

Azimuth Response 1% 1% 0% 0%

Spectral Response 1% 5% 1.5% 8%

Tiltc 0.2% 0.2% 0% 0%

Nonlinearity 0.5% 1% 0.5% 1%

Temperature Response 1% 1% 1% 1%

Aging Per Year 0.2% 0.5% 0.1% 0.5%

Total U = Sum 8.9% 15.7% 5.6% 14.5%

Total U = RSS 4.1% 8.0% 2.7% 8.9% a Includes zenith angle response from 30 degrees to 60 degrees b Includes zenith angle response from 0 degrees to 30 degrees and 60 degrees to 90 degrees c This uncertainty was set to zero for untilted radiometers. 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
9 

Campbell Scientific data loggers were used during the period of study. The uncertainty from the 

data loggers could be included in the above expanded uncertainty estimate table. Wilcox and 

Myers (2008) described the uncertainty of the data logger as often less than 1%. 

3 Results 

Except for the Solar-Mil.-CRADA-RSR, irradiance measurements from the radiometers were 

recorded as 1-minute averages of 3-second samples using Campbell Scientific, Inc., data loggers.

The 1-minute data were then averaged to produce the 10-minute and hourly data used in our 

evaluations. Daily total data were obtained by summing the sunup hourly averaged data for a 

specific day (maximum solar zenith angle less than 85 degrees). Consequently, monthly mean 

daily total data were calculated using the averages of the daily totals for each month.

3.1 GHI Radiometers (Pyranometers)

Thirty-two GHI data sets were evaluated for this study. Reference values of GHI were calculated 

using a reference DNI and a reference DHI following the component sum method (Equation 1). 

The two reference radiometers used in our study were a Kipp & Zonen model CH1 

pyrheliometer (serial number 10256) and an Eppley model 8-48 pyranometer (serial number 

21096). Both radiometers were mounted on an automatic solar tracker to provide DNI and DHI 

solar irradiance measurements. Previous studies (Wilcox and Myers 2008, Michalsky et al. 2011) 

and Broadband Outdoor Radiometer Calibration results from these instruments confirm that the 

CH1 exhibits relatively lower calibration and measurement uncertainty compared to others in the 

sample. For consistency with our previous study, the model 8-48 pyranometer was selected 

instead of an available model CM22 pyranometer to minimize a possible systematic design bias 

that might favor the evaluation of the CM22 global instrument. Each GHI instrument under test 

was then compared to the calculated reference GHI values.

One-minute reference GHI data were calculated using the reference DNI and DHI instrument. 

The reference GHI data and UUT GHI data were then averaged to obtain hourly data. 

Table 5. GHI Instrument List

Instrument 

Number

Instrument 

Type Model Manufacturer Comment

1 Thermopile CM22 Kipp & Zonen Ventilated

2 Thermopile CM6b Kipp & Zonen ---

3 Thermopile CM3-CNR1

Kipp & Zonen Does not have a relatively clear view of the sky (current 

location)

4 Thermopile PSP

Eppley Laboratory, 

Inc. Ventilated, thermal, and zenith corrected

5 Thermopile PSP

Eppley Laboratory, 

Inc. Thermal and zenith corrected

6 Thermopile PSP

Eppley Laboratory, 

Inc. Ventilated

7 Thermopile PSP

Eppley Laboratory, 

Inc. ---

8 Thermopile TSP-700 YES, Inc.

Platinum resistance thermal detectors

- Ventilated

9 Thermopile TSP-1 YES, Inc.

Platinum resistance thermal detectors

Ventilated

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10 

Table 5. GHI Instrument List

Instrument 

Number

Instrument 

Type Model Manufacturer Comment

10 Thermopile SPN1 Delta-T ---

11 Semiconductor SPLite Kipp & Zonen ---

12 Semiconductor SP-110 Apogee ---

13 Semiconductor LI-200 LI-COR ---

14 Semiconductor P007 David Brooks

Experimental sensor

http://www.instesre.org/construction/pyranometer/pyrano

meter.htm/

15 Semiconductor ATI

Ascension 

Technology, Inc. 

RSR/LI-COR Ascension Technology, Inc. RSR

16 Semiconductor RSR2

Irradiance, Inc./

LI-COR Experimental instrument. Not for sale.

17 Semiconductor RSR2 (secondary)

Irradiance, Inc./

LI-COR Experimental Instrument. Not for sale.

18 Thermopile TSR-590 YES, Inc. TSR

19 Thermopile TSR-591 YES, Inc. TSR

20 Thermopile TSR-592 YES, Inc. TSR

21 Thermopile TSR-590-LHa YES, Inc.

 TSR

(Correction applied)

22 Thermopile TSR-591-LH YES, Inc.

 TSR

(Correction applied)

23 Thermopile TSR-592-LH YES, Inc.

 TSR

(Correction applied)

24 Thermopile TSR-590-JMb YES, Inc. TSR

(Correction applied)

25 Thermopile TSR-591-JM YES, Inc.

 TSR

(Correction applied)

26 Thermopile TSR-592-JM YES, Inc.

 TSR

(Correction applied)

27 Thermopile SR11-7196 Hukseflux ---

28 Thermopile SR11-7242 Hukseflux ---

29 Thermopile LP02-41120 Hukseflux ---

30 Thermopile LP02-41272 Hukseflux ---

31 Semiconductor

NREL-CRADARSR2

Irradiance Inc./

LI-COR http://www.irradiance.com/

32 Semiconductor

Solar-Mil.-

CRADA-RSR

Solar Millennium 

AG./LI-COR

Not available (Solar Millennium, Inc., is out of business)

a Lee Harrison b Joseph Michalsky

The figures and tables discussed in the results section show the performance of each test 

instrument relative to the reference instrument under various sky conditions. To assess the 

performance of each instrument relative to the reference instruments, MBE and RMSE were used 

as basic set of performance measures. MBE calculations both in percent and watts per square 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
11 

meter square (W/m2

) were used to assess radiometer performance (Equation 4). Further, 

supplemental meteorological measurements such as temperature and wind speed from the 

collocated instruments were used to assist in the analysis. Each radiometric data value from the 

test instruments demonstrated systematic differences as a function of solar zenith angle, 

temperature, and wind speed. Therefore, plots in the appendices contain comparisons as 

functions of solar zenith angle, temperature, and wind speed. 

MBE(W m2 ⁄ ) = 1

n ∑ (IUUT(New) n

i=1 − IRef) and MBE(%) = � 1

n ∑n

i=1

(IUUT(New)−IRef)

IRef � ∗ 100 (4) 

where, IUUT(New) is the irradiance value for the UUT in W/m2 and IRef is the irradiance value for 

the reference instrument in W/m2

. 

RMSE demonstrates the randomness in the data, and the calculation was performed for both 

W/m2 and percent differences using Equation 5. 

RMSE(W m2 ⁄ ) = �� 1

n ∑n

i=1 �IUUT(New) − IRef�

2

� and RMSE(%) = �� 1

n ∑n

i=1 �

IUUT(New)−IRef

IRef �

2

� ∗ 100 (5) 

Figure 5 and Figure 6 demonstrate the 10-minute and hourly average GHI data comparsion, 

respectively, for all instruments under test. The MBE and RMSE in percent are scattered along 

the x axis for each instrument. The yellow box illustrates a 95% confidence interval coverage, 

and the red line is the mean point for the 95% confidenec interval area. These plots demonstrate 

the relative tendencies or performance of each instrument with respect to the references used in 

our study. In these plots, a +20% MBE for GHI and +50% MBE for DNI bounds were used. 

Data points outside of these bounds were not included. These criteria helped limit the 

contribution of outlying high-percentage differences that could have resulted from time response 

differences in the instruments, routine cleaning events, and/or erroneous observations for 

unknown reasons. Further, Figure 5 and Figure 6 illustrate the skewness of the differences when 

the MBE percent tended to the positive on some of the instruments and negative on others. These 

characterisitic differences assisted in identifying the overestimation or underestimation of 

irradiance by each UUT instrument. Under clear-sky condtions, the relative difference appeared

to exhibit less bias and the mean diffferences (red line) for most instruments (UUT) tended to 

have less variability than the partly-cloudy or mostly-cloudy sky condition. One important aspect 

to note from Figure 5 and Figure 6 is that the sizes of the 95% confidence interval yellow boxes 

provide valuable information about the scatter of the differences between the reference 

instrument and the UUT. Smaller yellow boxes depict small differences in the data within the 

95% confidence interval. Bigger yellow boxes of the 95% confidence interval demonstrate larger 

differences within the interval. Further, instruments (UUT) that showed higher varibility above 

the confidence interval box either to the positive or negative trend tended to have relatively 

lower performance in the higher zenith angle. (See Appendix A for individual instrument 

performance plots.) 

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12 

Figure 5. Bias error in percent for the 10-minute average for all GHI data under study. The red line 

signifies the mean value of the MBE for the 95% confidence level. See Table 5 to obtain the 

instrument name and model by using the instrument number (x-axis).

The 10-minute average data showed more scatter, and the scatter was reduced as the averaging 

time increased. (Compare the ranges among the 10-minute plots in Figure 5 and the hourly plots 

in Figure 6). In general, the average MBE (%) for all zenith angles (17 degrees to 85 degrees) 

appeared to fall within +5% for almost all instruments using the 95% confidence coverage for 

the 10-minute and hourly averaging time. 

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13 

Figure 6. MBE in percent for the hourly average for all GHI data under study. The red line signifies 

the mean value of the MBE for the 95% confidence level. See Table 5 to obtain the instrument 

name and model by using the instrument number (x-axis).

Sorting out the MBE differences using solar zenith angle ranges (Table 6) provides a clear 

understanding of how each instrument performs within a specific range of zenith angles. In Table 

6, the solar zenith angle was subdivided into 10-degree bins in which the center of the bin 

represented the independent variable. The only caveats were the 10-degree to 20-degree and 80-

degree to 90-degree bins; during the study period the lowest solar zenith angle observed was

approximately 17.5 degrees, therefore the bin range had an approximate 18.5-degree center 

point. Because data above an 85-degrees zenith angle were removed from the analysis, the center 

point for this bin was an 82.5-degree zenith angle. 

The MBE values shown in Table 6 present all differences within a +100% data exclusion range. 

There could have been outliers that were close to the data exclusion range that might have 

affected the overall MBE average in each bin of the higher solar zenith angle and mostly-cloudy sky

conditions. These two situations included irradiance values of a very small magnitude, and ratios 

of two small irradiance numbers tended to produce high MBE in percent. Values greater than 5% 

are highlighted in Table 6. 

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14 

Table 6. Hourly Average MBE in Percent Under Various Solar Zenith Angle Ranges for GHI Data 

Inst. 

# Instrument Model Sky Condition

Solar Zenith Angle Ranges in Degrees

17 to 20 20 to 30 30 to 40 40 to 50 50 to 60 60 to 70 70 to 80 80 to 85

1 CM22

Clear -1.27 -1.03 -0.81 -0.69 -0.51 -0.07 0.80 3.06

Mostly cloudy 0.70 1.52 1.43 1.06 0.51 0.23 -0.25 -0.50

2 CM6b

Clear -1.98 -1.32 -0.77 -0.60 -0.72 -1.46 -1.13 -1.04

Mostly cloudy 1.06 1.71 1.23 0.93 0.05 -0.11 -0.75 -1.04

3 CM3-CNR1

Clear 0.57 0.42 0.26 -0.45 -0.88 -2.40 -3.92 -5.72

Mostly cloudy 0.96 1.27 0.84 0.08 -1.13 -2.13 -3.03 -5.03

4 

PSP (ventilated, 

thermal, and 

zenith corrected)

Clear 0.87 0.44 -0.15 -0.56 -1.05 -2.75 -2.91 -2.15

Mostly cloudy -0.10 -1.22 -0.93 -0.61 -0.71 -1.69 -4.33 -13.62

5 PSP (thermal and 

zenith corrected)

Clear 0.17 -0.09 -0.48 -0.85 -0.90 -1.27 0.99 4.18

Mostly cloudy 0.45 0.95 1.02 1.50 2.37 4.91 12.99 26.02

6 PSP (ventilated)

Clear 3.32 2.41 1.02 -0.65 -2.78 -5.55 -12.32 -23.22

Mostly cloudy 1.38 -0.17 -0.47 -1.57 -3.57 -6.00 -15.70 -31.77

7 PSP

Clear 2.98 2.27 1.14 -0.65 -2.96 -6.51 -10.06 -16.97

Mostly cloudy 2.41 2.27 1.43 0.40 -1.48 -2.29 -4.04 -6.48

8 TSP-700

Clear -0.68 -0.93 -0.22 0.27 0.51 1.07 2.49 5.95

Mostly cloudy 1.13 -0.56 1.05 0.67 1.25 1.31 2.03 2.69

9 TSP-1 

Clear -2.15 -1.51 -0.01 0.48 0.28 -0.01 1.90 5.22

Mostly cloudy -1.30 2.08 1.72 1.89 1.64 3.47 5.08 17.49

10 SPN1

Clear 1.19 -0.40 -0.50 -0.22 0.59 1.39 3.98 6.08

Mostly cloudy 3.62 2.45 2.73 3.52 4.51 6.25 16.36 56.87

11 SPLite

Clear -2.41 -2.24 -1.51 -1.03 -0.74 -1.40 -0.32 7.03

Mostly cloudy 3.62 4.87 3.36 2.88 1.87 1.18 1.30 4.12

12 SP-110

Clear -0.20 -1.25 -1.23 0.13 -0.14 1.16 5.83 19.22

Mostly cloudy 4.16 3.61 2.53 2.91 2.05 1.86 3.63 9.05

13 LI-200

Clear 0.64 0.38 -0.20 -1.04 -2.44 -4.15 -5.50 -4.72

Mostly cloudy 2.77 3.67 2.44 1.22 -0.53 -1.72 -2.09 0.02

14 P007

Clear -1.82 -1.70 -1.28 -1.70 -3.36 -8.39 -16.59 -27.75

Mostly cloudy 5.04 6.92 5.60 4.72 3.31 2.00 2.57 6.89

15 ATI

Clear -0.74 -0.94 -1.09 -1.46 -1.85 -1.85 -1.55 -1.28

Mostly cloudy 3.27 4.83 4.01 3.21 2.19 1.65 1.87 6.88

16 RSR2

Clear -0.60 -0.94 -0.92 -0.97 -0.63 0.37 2.70 4.40

Mostly cloudy 4.11 4.14 3.30 2.75 2.18 2.52 2.11 1.07

17 RSR2 (secondary) 

Clear -2.30 -1.88 -1.43 -1.12 -1.09 -0.67 1.15 3.20

Mostly cloudy 2.74 3.97 3.22 2.50 1.34 0.72 -0.05 0.29

18 TSR-590

Clear 1.38 1.39 1.31 1.39 1.05 0.69 -1.07 -3.29

Mostly cloudy 1.44 -0.57 0.33 0.32 -0.18 -0.75 -0.93 0.28

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15 

Table 6. Hourly Average MBE in Percent Under Various Solar Zenith Angle Ranges for GHI Data 

Inst. 

# Instrument Model Sky Condition

Solar Zenith Angle Ranges in Degrees

17 to 20 20 to 30 30 to 40 40 to 50 50 to 60 60 to 70 70 to 80 80 to 85

19 TSR-591

Clear 0.70 0.89 0.97 0.95 0.77 0.13 -0.72 -1.74

Mostly cloudy 1.17 -0.67 0.36 0.17 -0.26 -0.82 1.14 7.69

20 TSR-592

Clear 2.10 2.13 1.94 2.06 1.81 0.97 0.43 -0.83

Mostly cloudy 2.36 0.29 1.29 1.20 0.71 -0.10 -0.13 0.44

21 TSR-590-LH

Clear 1.73 1.73 1.62 1.71 1.33 0.91 -0.97 -3.47

Mostly cloudy 0.38 -1.77 -0.84 -0.82 -1.38 -2.00 -2.25 -1.09

22 TSR-591-LH

Clear 1.14 1.31 1.38 1.35 1.15 0.42 -0.58 -1.96

Mostly cloudy -0.15 -2.17 -1.10 -1.26 -1.76 -2.41 -0.56 5.83

23 TSR-592-LH

Clear 2.28 2.30 2.10 2.23 1.96 1.08 0.49 -0.91

Mostly cloudy 1.83 -0.31 0.70 0.63 0.11 -0.73 -0.79 -0.25

24 TSR-590-JM

Clear 1.32 1.34 1.25 1.34 0.99 0.64 -1.12 -3.34

Mostly cloudy 1.38 -0.63 0.27 0.27 -0.23 -0.80 -0.98 0.23

25 TSR-591-JM

Clear 0.63 0.82 0.90 0.88 0.70 0.06 -0.79 -1.81

Mostly cloudy 1.10 -0.74 0.29 0.10 -0.33 -0.89 1.07 7.61

26 TSR-592-JM

Clear 0.78 0.58 0.42 0.05 -0.34 -0.34 -0.04 0.24

Mostly cloudy 4.86 6.44 5.61 4.79 3.76 3.21 3.44 8.52

27 SR11-7196

Clear -1.67 -1.48 -0.86 -0.51 -0.28 0.48 3.07 9.42

Mostly cloudy 1.09 2.00 1.43 1.21 0.38 0.34 0.50 1.40

28 SR11-7242

Clear -1.36 -1.31 -0.94 -0.58 -0.34 0.49 3.04 9.54

Mostly cloudy 1.22 1.98 1.41 1.14 0.30 0.14 0.30 1.25

29 LP02-41120

Clear -0.15 -0.17 -0.19 -0.46 -1.12 -1.92 -0.91 0.18

Mostly cloudy 1.04 1.35 0.85 0.17 -0.68 -1.40 -2.20 -1.17

30 LP02-41272

Clear -0.74 -0.83 -0.41 -0.43 -0.71 -0.92 0.63 2.15

Mostly cloudy 0.92 1.76 1.17 0.70 0.05 -0.35 -1.11 -0.18

31 NREL-CRADA-RSR2

Clear -0.32 -0.43 -0.69 -0.90 -1.00 -0.17 0.60 2.52

Mostly cloudy 3.73 4.69 3.59 2.37 1.39 1.72 1.66 -0.52

32 RSR-Mil.-CRADARSR

Clear -2.47 -2.29 -2.07 -2.28 -2.53 -2.53 -2.69 -3.32

Mostly cloudy 1.30 2.42 1.17 0.71 -0.37 -1.15 -2.51 -3.46

Note: Highlighted values are MBE greater than +5% and the data exclusion threshold was +100% MBE.

Figure 7 presents the RMSE in percent, and the clear-sky conditions for all instruments showed

relatively less RMSE. This implies that the comparison of the UUT and reference data produced 

less randomness than the other sky conditions. The higher RMSE in the partly- and mostlycloudy sky conditions was largely attributed to changing sky conditions and associated with 

differing time responsivity of the radiometers (Vignola, Stoffel, and Michalsky 2012). 

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16 

Further, both the MBE and RMSE values should be taken into consideration (e.g., Figure 5

through Figure 7) to understand the performance of each instrument relative to the reference 

instrument. For instance, one instrument could have less bias but higher RMSE; therefore, an 

instrument with less bias and less RMSE would perform similarly to the reference data. This also 

applies to the DNI data set comparison, which is discussed in Section 3.2. 

Figure 7. RMSE in percent for the hourly average for all GHI data under study. The red line 

signifies the mean value of the RMSE for the 95% confidence level. Refer to Table 5 to obtain the 

instrument name and model using the instrument number (x-axis).

Silicon photodiode radiometers such as LI-COR (LI-200), which are fast responding sensors, 

sample a smaller portion of the solar spectrum (400 nm to 1,100 nm), but they are calibrated to a 

thermopile output range (300 nm to 4,000 nm). This is illustrated in Figure 8. 

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17 

Figure 8. Thermopile and silicon photodiode pyranometers spectral response (modified from 

Myers 2011 and Myers 2013)

These silicon instruments tend to have reasonable accuracy under the calibration conditions, but 

have spectral, angular, and temperature sensitivity issues that contribute to significant differences 

under conditions other than the calibration conditions (Myers 2011, Vignola 2006, King and 

Myers 1997). Many methods are available to correct these problems. The RSR2 instrument has a 

built-in correction algorithm in the data acquisition system supplied by the manufacturer4

. These 

algorithms apply different correction methods to minimize the above-mentioned issues. It 

appears that the application of such algorithms in the RSR2 minimized the deviation of the data 

relative to the reference instrument (Refer to Table 6 for NREL CRADA RSR2). The RSR2 uses 

a LI-COR LI-200 sensor, and Figure 9 demonstrates the comparison results between the RSR2 

and LI-200 sensors for GHI irradiance relative to the reference instrument data. The LI-200 

instrument does not have spectral, angular, or temperature sensitivity corrections. Between the 

two instruments, there was a clear shift of the probability distribution for the clear-sky condition 

from negative errors for the LI-200 to less and relatively normally distributed errors for the 

RSR2. However, in the mostly-cloudy sky condition, the probability distribution errors shifted

more to the positive MBE for the RSR2. 

 

4 http://www.irradiance.com/

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18 

Figure 9. (Top) Comparison between the uncorrected silicon photodiode sensor and (bottom) 

corrected silicon photodiode—NREL-CRADA-RSR2

Obviously, the temperature and zenith dependencies appeared to be less in the NREL-CRADARSR2 than the LI-200. However, these improvements were more conspicuous for the clear-sky 

condition than the partly- or mostly-cloudy sky conditions. Under clear-sky conditions, NRELCRADA-RSR2 tended to have better temperature sensitivity performance than the LI-200. This 

could be related to the temperature correction algorithm applied in the NREL-CRADA-RSR2 

system. Under mostly-cloudy sky conditions, the NREL-CRADA-RSR2 data did not show 

significant improvement from the internal corrections. This could be related to the spectral 

sensitivity of the sensors in which the instrument is susceptible to changes of broadband spectral 

distribution by changes of air mass, humidity, clouds, and aerosols (Vignola 2006). The angular 

dependency for the NREL-CRADA-RSR2 appeared noticeable above an 80-degree solar zenith 

angle but relatively less evident than the uncorrected LI-200 data. This situation was related to 

the geometric response of the sensor, which is called a “cat ear error” (Augustyn et al. 2004). 

Augustyn applied a correction to mitigate the problem. 

Generally, surface temperatures are higher than effective sky temperatures. Likewise, thermopile 

radiometers equilibrate to ambient temperature, which is typically higher than the sky 

temperature, and this creates an infrared energy imbalance between the thermopile radiometer 

and the sky. This situation produces a thermal energy exchange in which the thermopile emits 

energy to the sky (Myers 2013), and this was evident in the data as a negative output of 

irradiance by the thermopile radiometers in the absence of solar radiation. For this reason, some 

of the data from thermopile radiometers have been corrected for this infrared-induced thermal 

imbalance using coincident measurements from a pyrgeometer. The use of pyranometer 

ventilators can affect the amount of thermal offset in solar irradiance measurements. 

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19 

Figure 10. Understanding nighttime thermal offset (top) under all sky conditions and (bottom) 

normalized differences (UUT minus reference) versus temperature for (left) ventilated and (right) 

unventilated Eppley Laboratory, Inc., PSP radiometers under clear-sky conditions (1-minute data)

The purpose of the ventilators is to remove any accumulation of dust, snow, frost, dew, etc., from 

the dome of the radiometers and to discourage the presence of insects. Although both the Eppley 

ventilated and unventilated radiometers were corrected for thermal offset, the ventilator on the 

ventilated PSP tended to affect the irradiance by creating an additional temperature imbalance 

between the case and dome of the instrument. This scenario was more apparent during winter or 

cold ambient temperatures (as shown in bottom left plot of Figure 10). Usually during overcast 

skies, the ambient and sky temperature are near equilibrium. However, the ventilator was 

generating additional heat to the radiometer, which we postulated created an additional exchange 

of energy between the radiometer and the sky. This situation ultimately increased the negative 

bias within the radiometer. Further, this condition could be exacerbated if the ventilator filter of 

the PSP is uncleaned for some time and airflow is restricted. The ventilator effect appeared to be 

particularly noticeable during nighttime (as shown in the top plot of Figure 10). However, a 

couple important observations are worth mentioning here. The thermal offset correction for the 

ventilated PSP was performed using a ventilated Eppley Laboratory, Inc., model PIR (precision 

infrared radiometer), but if the flow of the air that is caused by the ventilators in both the 

precision infrared radiometer and the PSP is not equivalent—for example, because of a 

difference in the amount of dirt or other obstruction in the PSP and precision infrared 

radiometer—then the results as shown in the bottom left plot of Figure 10 could be justified. On 

the other hand, we used a ventilated Eppley precision infrared radiometer (PIR) to correct the 

unventilated PSP (as shown in bottom right plot of Figure 10). This correction method stipulates 

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20 

that like ventilation be used to correct both the precision infrared radiometer and the PSP, and 

this could be the reason for an overestimation of irradiance for the unventilated Eppley PSP.

As mentioned above, in addition to cleaning, routine maintenance of the radiometers is critical 

for accurate solar irradiance measurements. Imprecise leveling in either the instrument bubble or 

the precision of mounting the sensor within the body could cause asymetric responsivity in a 

radiometer’s output throughout the day. This situation is noticeable in the ATI GHI data (Figure 

11). Here, the instrument bubble level had been maintained during routine maintenance, so the 

effect could have been caused by an imprecise mounting of the sensor surface within the 

radiometer.

Figure 11. Asymmetric conditions caused by an unleveled radiometer

3.2 DNI Radiometers (Pyrheliometers)

As mentioned above, the reference CH1 is traceable to the World Radiometric Reference by 

means of the NREL Transfer Standard Group and the Broadband Outdoor Radiometer 

Calibration. As shown in previous studies, this reference instrument has less calibration and 

measurement uncertainty than the pyrheliometers described in this report. For this reason alone, 

this instrument was chosen as the DNI reference for the study. The data quality assessment was 

applied to the DNI data set as stated in previous sections by comparing it to other reference 

diffuse and global measurements. Appendix B contains result plots of individual DNI 

radiometers included in the report. These plots show differences mainly as a function of solar 

zenith angle and temperature.

Nineteen DNI data sets were included in this analysis. Some of the instruments had a calculated 

DNI value and others had a direct measurement of DNI. The UUT radiometers, such the silicon 

photodiode instruments, have a calculated DNI. These instruments measure irradiance 

components, GHI, and DHI. The calculation for DNI was obtained from these two data sets 

using Equation 1 and solving for DNI. However, Myers (2011) described that the computed DNI 

from these instruments produces an approximate 7% error because these instruments are 

susceptible to spectral sensitivity and the spectral distribution from GHI and DHI. These 

conditions carry forth the greater errors in GHI and DHI to provide higher errors for the 

calculated DNI. 

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21 

Table 7. DNI Instrument List

Instrument 

Number

Instrument 

Type Model Manufacturer Comment

1 Thermopile NIP2 Eppley Laboratory, Inc. ---

2 Thermopile NIP1 Eppley Laboratory, Inc. ---

3 Semiconductor LI-201 LI-COR Experimental sensor (not available)

- Calculated using global and diffuse 

measurement

4 Semiconductor ATI/ LI-COR Ascension Technology 

Inc. /LI-COR

- Calculated using global and diffuse 

measurement

5 Semiconductor RSR2/LI-COR Irradiance Inc./LI-COR Experimental instrument. Not for sale.

- Calculated using global and diffuse 

measurement

http://www.irradiance.com/

6 Thermopile TSR-590 YES, Inc. TSR

- Calculated using global and diffuse 

measurement

7 Thermopile TSR-591 YES, Inc. TSR

- Calculated using global and diffuse 

measurement

8 Thermopile TSR-592 YES, Inc. TSR

- Calculated using global and diffuse 

measurement

9 Thermopile TSR-590LH YES, Inc. TSR

- Calculated using global and diffuse 

measurement

10 Thermopile TSR-591LH YES, Inc. TSR

- Calculated using global and diffuse 

measurement

11 Thermopile TSR-592LH YES, Inc. TSR

- Calculated using global and diffuse 

measurement

12 Thermopile TSR-590JM YES, Inc. TSR

- Calculated using global and diffuse 

measurement

13 Thermopile TSR-591JM YES, Inc. TSR

- Calculated using global and diffuse 

measurement

14 Thermopile TSR-592JM YES, Inc. TSR

- Calculated using global and diffuse 

measurement

15 Semiconductor NREL-CRADARSR2/LI-CORIrradiance Inc./LI-COR - Calculated using global and diffuse 

measurement

http://www.irradiance.com/

16 Thermopile SPN1 Delta-T - Calculated using global and diffuse 

measurement

17 Thermopile DR018066 Hukseflux ---

18 Thermopile DR018068 Hukseflux ---

19 Semiconductor Solar-Mil.-

CRADA-RSR/

LI-COR

Solar Millennium 

AG./LI-COR

Not available (Solar Millennium, Inc., is out of business)

- Calculated using global and diffuse 

measurement

Figure 12 and Figure 13 show the (top four plots) 10-minute and (bottom four plots) hourly 

average comparison results using MBE for the DNI instruments. The MBE differences tended to 

decrease with the increase of the averaging time for the same reasons as those described for the 

GHI instruments. The hourly average data had relatively less MBE than the 10-minute average 

under various sky conditions. 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
22 

The clear-sky conditions demonstrated tighter differences among the instruments than the rest of 

the sky conditions. The two Eppley Normal Incidence Pyrheliometers (NIP) and the two 

Hukseflux model “DR” types had less MBE than others under clear-sky conditions. However, 

the sensitivity of the DNI was much higher than the GHI under partly- or mostly-cloudy sky 

conditions. 

Figure 12. Bias error in percent for the 10-minute average for all DNI data under study. The red line 

signifies the mean value of the MBE for the 95% confidence level. See Table 7 to obtain the 

instrument name and model by using the instrument number (x-axis).

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
23 

Figure 13. MBE in percent for the hourly average for all DNI data under study. The red line 

signifies the mean value of the MBE for the 95% confidence level. See Table 7 to obtain the 

instrument name and model by using the instrument number (x-axis).

The DNI instruments also have temperature dependency. For instance, the Hukseflux radiometer 

model number DR108068 had relatively more evident temperature dependency than model 

number DR108066 (Figure 14). The two Eppley NIP radiometers tended to have less 

temperature dependency, especially at temperatures below 20°C. The two NIPs did not have 

temperature correction, and the two Hukseflux radiometers did not have the temperature 

correction either. However, a previous study that included the Hukseflux radiometers showed 

that the manufacturer’s temperature correction curve for these instruments reduced temperature 

error to a much lower magnitude (Michalsky et al. 2011). Correction for temperature dependency 

for pyrheliometers assists in acquiring less overall uncertainty (Michalsky et al. 2011). 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
24 

Figure 14. Effect of temperature on DNI measurement

Table 8 demonstrates the MBE for all the DNI instruments under clear and mostly-cloudy sky 

conditions. The differences were divided in various solar zenith angle ranges. As described in the 

GHI section, some of the outliers that were close to the +100% MBE limit might have skewed

the outcome in Table 8, especially in the high zenith angle and partly- or mostly-cloudy sky 

conditions. These higher deviations could have been a result of fast-moving clouds accompanied 

by differing fields of view (Vignola, Stoffel, and Michalsky 2012), for example calculated DNI 

for the RSR and the reference instrument. Therefore, temperature dependency, differing fields of 

view, differing instrument response characteristics, and solar zenith angle dependency can cause 

disagreements between the UUT and the reference instrument.

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25 

Table 8. Hourly Average MBE in Percent Under Various Solar Zenith Angle Ranges for DNI Data 

Inst. # Instrument 

Model Sky Condition

Solar Zenith Angle Ranges in Degrees

17 to 20 20 to 30 30 to 40 40 to 50 50 to 60 60 to 70 70 to 80 80 to 85

1 NIP2

Clear 0.12 0.27 -1.83 0.03 -0.26 0.96 0.92 2.12

Mostly cloudy 8.75 15.33 13.85 10.92 12.37 7.07 12.10 -1.07

2 NIP1

Clear 0.05 0.18 0.52 0.04 0.24 0.99 0.80 1.92

Mostly cloudy 6.82 11.59 12.85 10.73 10.28 18.88 11.10 11.81

3 LI-201

Clear 4.29 -0.82 -1.99 -0.01 0.35 1.82 6.00 12.11

Mostly cloudy 5.17 0.28 6.61 2.31 -2.18 -0.06 -8.25 2.75

4 ATI

Clear -1.40 -5.22 -1.93 -0.73 -0.34 0.39 3.91 8.59

Mostly cloudy -6.11 -16.98 -7.76 -2.54 -10.28 -11.91 -14.01 -20.49

5 RSR2

Clear -0.15 -0.34 -0.62 -0.23 0.32 1.55 4.07 6.94

Mostly cloudy -26.63 -34.79 -37.65 -35.08 -36.70 -41.62 -47.64 -50.91

6 TSR-590

Clear -0.12 -0.03 0.34 0.60 1.00 1.24 1.56 0.45

Mostly cloudy -4.91 -20.16 -13.38 -7.38 -8.87 4.24 -0.09 16.59

7 TSR-591

Clear -0.42 -0.23 0.46 0.47 0.95 0.64 2.39 2.78

Mostly cloudy -7.69 -21.27 -14.30 -6.41 -8.55 5.26 1.20 23.82

8 TSR-592

Clear 0.42 0.53 0.86 1.12 1.61 1.39 2.88 4.59

Mostly cloudy -5.32 -19.84 -14.16 -8.76 -7.78 4.44 2.03 21.07

9 TSR-590LH

Clear 0.52 0.51 0.24 0.73 0.67 0.23 -1.90 -8.32

Mostly cloudy -9.65 -5.04 -4.50 -0.37 -0.96 -0.27 -0.15 -4.13

10 TSR-591LH

Clear 0.35 0.44 0.06 0.77 0.89 0.40 -0.84 -5.83

Mostly cloudy -12.13 -10.25 -4.02 -11.34 -8.86 -11.28 -11.22 -13.90

11 TSR-592LH

Clear 0.84 0.87 0.95 1.08 1.17 0.28 -0.38 -3.32

Mostly cloudy -8.75 -15.16 -10.56 -2.54 -1.55 5.10 -0.88 7.06

12 TSR-590JM

Clear 0.49 0.44 0.76 1.09 1.31 1.34 0.35 -2.44

Mostly cloudy -7.41 -23.73 -17.75 -12.07 -13.25 0.06 -5.38 11.64

13 TSR-591JM

Clear 0.17 0.23 0.89 0.96 1.29 0.76 1.20 -0.11

Mostly cloudy -12.03 -25.34 -17.77 -10.47 -11.86 0.10 -0.78 16.36

14 TSR-592JM

Clear 1.04 1.01 1.30 1.62 1.93 1.46 1.69 1.71

Mostly cloudy -9.83 -24.53 -18.40 -12.03 -11.61 -0.22 -1.58 15.69

15

NRELCRADARSR2

Clear 0.05 0.05 -0.52 -0.36 -0.37 0.53 0.85 2.92

Mostly cloudy -23.68 -36.21 -37.09 -33.69 -39.54 -41.04 -49.59 -50.76

16 SPN1

Clear -0.77 -2.30 -1.83 -1.49 0.44 2.22 8.43 13.84

Mostly cloudy 32.59 30.01 27.13 27.48 35.48 39.37 40.73 35.67

17 DR018066

Clear 2.03 -0.92 -0.89 0.43 0.30 1.37 0.74 1.02

Mostly cloudy -2.91 7.69 7.96 7.61 5.30 9.97 -2.65 -8.71

18 DR018068 Clear 2.43 -0.47 -0.82 0.34 -0.04 0.61 -0.59 -0.01

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
26 

Table 8. Hourly Average MBE in Percent Under Various Solar Zenith Angle Ranges for DNI Data 

Inst. # Instrument 

Model Sky Condition

Solar Zenith Angle Ranges in Degrees

17 to 20 20 to 30 30 to 40 40 to 50 50 to 60 60 to 70 70 to 80 80 to 85

Mostly cloudy -1.32 5.98 11.80 8.73 4.43 7.29 -7.09 -11.15

19 Solar-Mil.-

CRADA-RSR

Clear -1.37 -1.05 -1.09 -1.20 -1.46 -1.49 -1.91 -3.09

Mostly cloudy -24.48 -32.11 -28.59 -28.65 -29.49 -38.17 -45.85 -42.71

Note: Highlighted values are MBE greater than +5% and the data exclusion threshold was +100% MBE.

The RMSE result showed that under clear-sky conditions, all the DNI data from the UUT 

radiometers agreed relatively well with the reference instruments (Figure 15). However, under 

partly- and mostly-cloudy sky conditions, some factors, such as temporal responsivity and field 

of view of the radiometers, become an influence for the higher RMSE in percent. Also consider 

that much of the data in the mostly-cloudy sky conditions had a small magnitude, and the ratio 

among the small numbers can present unrealistically alarming ratios in which the actual 

difference in solar irradiance is quite small. Users should take this into account when considering 

the mostly-cloudy sky conditions presented in Table 8.

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
27 

Figure 15. RMSE in percent for the hourly average for all DNI data under study. The red line 

signifies the mean value of the RMSE for the 95% confidence level. See Table 7 to obtain the 

instrument name and model by using the instrument number (x-axis).

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
28 

4 Summary 

Accurate solar resource measurements are essential for understanding solar energy conversion 

system performance, developing improved satellite-based models for estimating solar resources 

at the Earth’s surface, validating solar radiation forecasts, and advancing climate science. The 

pyranometers for measuring GHI and pyrheliometer/rotating shadowband/fixed shadowband 

radiometers for measuring DNI demonstrated a range of performance differences when 

compared to the reference irradiances developed for this study. The measured irradiance data for 

this study were normalized to angular responsivities at 45-degree solar zenith angles. Differences 

in calibration methods and the specific calibration factors used for each instrument were not 

affected significantly by the normalization. Under clear-sky conditions when the solar zenith 

angle was less than 60 degrees, differences of less than 5% were observed for both GHI and DNI 

measurements compared to the reference radiometers. For data during periods when the solar 

zenith angle was greater than 60 degrees, differences in GHI under mostly- and clear-sky 

conditions increased to 17%. Differences of up to 40% in DNI measurements on a few 

instruments were found for high solar zenith angles under mostly-cloudy sky conditions. Some 

of these differences are expected from the various instrument design characteristics for time 

response, spectral response, angular (cosine) response, and temperature response. Further, the 

ventilator used on some of the thermopile-based pyranometers exacerbated the differences 

between the reference and test instruments. Available correction methods for some 

instruments—e.g., solar zenith angle and temperature corrections—appeared to lessen the 

differences with respect to the reference irradiance measurements. We also investigated the 

effects of selected environmental parameters on the results. There were noticeable temperature 

correlation events for some of the radiometers. Correlations of radiometer performance 

differences with observed wind speed near the installed height of the radiometers were not 

significant. The estimated measurement uncertainties of the reference radiometers and the data 

logger system were not included in the reported values. Therefore, the interpretation of the MBE 

and RSME differences observed for the normalized data in this study must exceed the estimated 

expanded uncertainties of the reference and test instruments to be statistically significant. 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
29 

5 References 

Augustyn, J.; Geer, T.; Kern, E.; Little, R.; Stoffel, T.; Vignola, F.; Kessler, R.; Boyson, B. 

(2004). “Update of Algorithm to Correct Direct Normal Irradiance Measurements Made With a 

Rotating Shadowband Pyranometer.” Campbell-Howe, R. ed. American Solar Energy Society 

Solar Conference Proceedings, Including Proceedings of the 33rd American Solar Energy 

Society Annual Conference and Proceedings of the 29th National Passive Solar Conference; July 

11–14, 2004, Portland, Oregon. Boulder, CO: American Solar Energy Society; pp. 295–300. 

NREL/CP-560-38328. Golden, CO: National Renewable Energy Laboratory. 

King, D.L.; Myers, D.R. (1997). “Silicon-Photodiode Pyranometers: Operational Characteristics, 

Historical Experiences, and New Calibration Procedures.” 26th Institute of Electrical and 

Electronics Engineers Photovoltaic Specialists Conference Proceedings; Sept. 29–Oct. 3, 

Anaheim, California. 

Maxwell, E.; Wilcox, S.; Rymes, M. (1993). Users Manual for SERI_QC Software: Assessing 

the Quality of Solar Radiation Data. NREL/TP-463-5608. Golden, CO: National Renewable 

Energy Laboratory.

Michalsky, J.J., et al. (2011). “An Extensive Comparison of Commercial Pyrheliometers Under a 

Wide Range of Routine Observing Conditions.” Journal of Atmospheric and Oceanic 

Technology (28); pp. 752–766. 

Myers, D.R. (2011). “Quantitative Analysis of Spectral Impacts on Silicon Photodiode 

Radiometers.” American Solar Energy Society Solar Conference Proceedings; May 17–20, 

Raleigh, North Carolina. Boulder, CO: American Solar Energy Society. 

Myers, D.R. (2013). Solar Radiation: Practical Modeling for Renewable Energy Applications.

Boca Raton, FL: CRC Press, Taylor and Francis Group. 

Myers, D.R.; Andreas, A.; Stoffel, T.; Reda, I.; Wilcox, S.; Gotseff, P.; Kay, B. (2001). 

“Radiometric Calibrations, Measurements, and Standards Development at NREL.” NCPV 

Program Review Meeting Proceedings; Oct. 14–17, Lakewood, Colorado. NREL/CP-560-30964. 

Golden, CO: National Renewable Energy Laboratory. 

Reda, I. (2011). Method to Calculate Uncertainties in Measuring Shortwave Solar Irradiance 

Using Thermopile and Semiconductor Solar Radiometers. NREL/TP-3B10-52194. Golden, CO: 

National Renewable Energy Laboratory.

Stoffel, T.; Reda, I. (2013). National Renewable Energy Laboratory Pyrheliometer 

Comparisons: 24 September–5 October 2012 (NPC-2012). Golden, CO: National Renewable 

Energy Laboratory. 

Stoffel, T.; Renne, D.; Myers, D., Wilcox, S.; Sengupta, M.; George, R.; Turchi, C. (2010). 

Concentrating Solar Power: Best Practices Handbook for the Collection and Use of Solar 

Resource Data (CSP). NREL/TP-550-47465. 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
30 

Vignola, F. “Removing Systematic Errors From Rotating Shadowband Pyranometer Data.” 

(2006). American Solar Energy Society Solar Conference Proceedings; July 9–13, Denver, 

Colorado. Boulder, CO: American Solar Energy Society. 

Vignola, F.; Stoffel, T.; Michalsky, J.J. (2012). Solar and Infrared Radiation Measurements.

Boca Raton, FL: CRC Press, Taylor and Francis Group. 

Wilcox, S.M.; McCormack, P. (2011). “Implementing Best Practices for Data Quality 

Assessment of the National Renewable Energy Laboratory’s Solar Resource and Meteorological 

Assessment Project.” American Solar Energy Society Solar Conference Proceedings; May 17–

20, Raleigh, North Carolina. Boulder, CO: American Solar Energy Society. 

Wilcox, S.M.; Myers, D. (2008). Evaluation of Radiometers in Full-Time Use at the National 

Renewable Energy Laboratory Solar Radiation Research Laboratory. NREL/TP-550-44627. 

Golden, CO: National Renewable Energy Laboratory.

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
31 

6 Appendix A: GHI Comparisons

GHI plots: CM22

Data resolution: Minute data (bias error in percent) 

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
32 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
33 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
34 

GHI plots: CM6b

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
35 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
36 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
37 

GHI plots: CM3-CNR1

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
38 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
39 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
40 

GHI plots: PSP ventilated, thermal, and zenith corrected

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
41 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
42 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
43 

GHI plots: PSP thermal and zenith corrected

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
44 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
45 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
46 

GHI plots: PSP ventilated

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
47 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
48 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
49 

GHI plots: PSP

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
50 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
51 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
52 

GHI plots: TSP-700 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
53 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
54 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
55 

GHI plots: TSP-1 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
56 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
57 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
58 

GHI plots: SPN1

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
59 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
60 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
61 

GHI plots: SPLite

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
62 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
63 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
64 

GHI plots: SP-110 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
65 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
66 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
67 

GHI plots: LI-200 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
68 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
69 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
70 

GHI plots: P007

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
71 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
72 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
73 

GHI plots: ATI

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
74 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
75 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
76 

GHI plots: RSR2

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
77 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
78 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
79 

GHI plots: RSR2-Secondary

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
80 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
81 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
82 

GHI plots: TSR-590 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
83 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
84 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
85 

GHI plots: TSR-591 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
86 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
87 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
88 

GHI plots: TSR-592 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
89 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
90 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
91 

GHI plots: TSR-590LH 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
92 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
93 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
94 

GHI plots: TSR-591LH

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
95 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
96 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
97 

GHI plots: TSR-592LH

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
98 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
99 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
100 

GHI plots: TSR-590JM 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
101 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
102 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
103 

GHI plots: TSR-591JM 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
104 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
105 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
106 

GHI plots: TSR-592JM 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
107 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
108 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
109 

GHI plots: SR11-7196 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
110 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
111 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
112 

GHI plots: SR11-7242 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
113 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
114 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
115 

GHI plots: LP02-41120 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
116 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
117 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
118 

GHI plots: LP02-41272 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
119 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
120 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
121 

GHI plots: NREL-CRADA-RSR2

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
122 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
123 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
124 

7 Appendix B: DNI Comparisons

DNI plots: NIP #2

Data resolution: Minute data (bias error in percent) 

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
125 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
126 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
127 

DNI plots: NIP #1

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
128 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
129 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
130 

DNI plots: LI-201 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
131 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
132 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
133 

DNI plots: ATI

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
134 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
135 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
136 

DNI plots: RSR2

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
137 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
138 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
139 

DNI plots: TSR-590 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
140 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
141 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
142 

DNI plots: TSR-591 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
143 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
144 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
145 

DNI plots: TSR-592 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
146 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
147 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
148 

DNI plots: TSR-590LH 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
149 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
150 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
151 

DNI plots: TSR-591LH 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
152 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
153 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
154 

DNI plots: TSR-592LH 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
155 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
156 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
157 

DNI plots: TSR-590JM 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
158 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
159 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
160 

DNI plots: TSR-591JM 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
161 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
162 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
163 

DNI plots: TSR-592JM 

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
164 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
165 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
166 

DNI plots: NREL_CRADA_RSR2

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
167 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
168 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
169 

DNI plots: SPN1

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
170 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
171 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
172 

DNI plots: DR018066

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
173 

Ten-minute average eata (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
174 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
175 

DNI plots: DR01068

Data resolution: Minute data (bias error in percent)

Minute data (bias error in W/m2)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
176 

Ten-minute average data (MBE in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
177 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
178 

DNI plots: Solar-Mil.-CRADA-RSR

Data resolution: 10-minute average data (bias error in percent)

Hourly data (MBE in percent)

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
179 

Daily data (MBE in percent)

Monthly mean daily total data (MBE in percent)

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