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

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

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

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

Contract No. DE-AC36-08GO28308

Technical Report

NREL/TP-1900-72633

October 2018

NREL Comparison Between Absolute 

Cavity Pyrgeometers and Pyrgeometers 

Traceable to World Infrared Standard 

Group and the InfraRed Integrating 

Sphere 

Ibrahim Reda,1 Mike Dooraghi,1 Afshin Andreas,1

Julian Gröbner,2 and Christian Thomann2

1 National Renewable Energy Laboratory 

2 Physikalisch-Meteorologisches Observatorium Davos/World 

Radiation Center (PMOD/WRC)
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

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

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

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Technical Report

NREL/TP-1900-72633

October 2018

NREL Comparison Between Absolute 

Cavity Pyrgeometers and Pyrgeometers 

Traceable to World Infrared Standard 

Group and the InfraRed Integrating 

Sphere 

Ibrahim Reda,1 Mike Dooraghi,1 Afshin Andreas,1

Julian Gröbner,2 and Christian Thomann2

1 National Renewable Energy Laboratory 

2 Physikalisch-Meteorologisches Observatorium Davos/World 

Radiation Center (PMOD/WRC)

Suggested Citation 

Ibrahim, Mike Dooraghi, Afshin Andreas, Julian Gröbner, and Christian Thomann. 2018. 

Comparison Between Absolute Cavity Pyrgeometers and Pyrgeometers Traceable to 

World Infrared Standard Group and the InfraRed Integrating Sphere. Golden, CO: 

National Renewable Energy Laboratory. NREL/TP-1900-72633.

https://www.nrel.gov/docs/fy19osti/72633.pdf.
NOTICE

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

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

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

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

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

This report is available at no cost from the National Renewable 

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

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

and a growing number of pre-1991 documents are available 

free via www.OSTI.gov.

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

NREL 46526.

NREL prints on paper that contains recycled content.
iv

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

Acknowledgments

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

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

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

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

Environment, Safety, Health, & Quality center (ESH&Q).
v

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

Table of Contents

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

2 Instrument List...................................................................................................................................... 2

3 Measurement Equations...................................................................................................................... 3

4 Results................................................................................................................................................... 4
vi

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List of Figures

Figure 1. ACPs’ irradiance ........................................................................................................................... 4

Figure 2. Average ACPs’ irradiance minus the irradiance measured by each ACP ..................................... 5

Figure 3. Average ACPs’ irradiance vs irradiance measured by PIR and KZ.............................................. 6

Figure 4. Average of ACPs’ irradiance minus irradiance measured by PIR and KZ vs PWV ..................... 7

List of Tables

Table 1. Average ACPs’ irradiance minus the irradiance measured by each ACP....................................... 5

Table 2. Average ACPs’ irradiance minus the irradiance measured by the PIR and KZ ............................. 7
1

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

1 Introduction

The comparison of the absolute cavity pyrgeometers (ACPs) with a precision infrared radiometer

(PIR) pyrgeometer traceable to the World Infrared Standard Group (WISG) and a Kipp & Zonen 

(KZ) pyrgeometer traceable to the InfraRed Integrating Sphere Radiometer (IRIS) was held 

during the National Renewable Energy Laboratory (NREL) Pyrheliometer Comparison (NPC2018) from September 24 to October 5, 2018. Data from all instruments was collected during 

nighttime clear sky conditions only. The irradiance measured by the ACPs and the PIR 

pyrgeometer traceable to the WISG were collected every 10 seconds during the ACPs’ selfcalibration and every 30 seconds during the measurement runs. The measurement runs lasted for 

two hours, while the calibration runs lasted for 6 minutes. The KZ pyrgeometer collected 

irradiance measurements every 30 seconds. During the comparison, the average (av) irradiance 

difference measured by three ACPs varied from 0.2 W/m2 to -0.5 W/m2, with a standard 

deviation (sd) from 0.5 W/m2 to 0.8 W/m2. The average irradiance difference measured by the 

three ACPs minus the irradiance measured by the PIR was 4.3 W/m2 and sd 4.2 W/m2

, and the 

average irradiance difference measured by the KZ was 2.3 W/m2 and sd 3.5 W/m2.
2

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2 Instrument List

• Absolute Cavity Pyrgeometer (ACP): 57F3, 95F3, and 96F3.

• PIR pyrgeometer: 31197F3

• KZ pyrgeometer: CGR4 110390
3

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3 Measurement Equations

ACP 

WW = KK1 ∗ VVtttt + (2−∈) ∗ KK2 ∗ WWrr − (1+∈) ∗ WWcc

ττ

Where,

• W is the atmospheric longwave irradiance (W.m-2).

• K1 is the reciprocal of the ACP’s responsivity (W.m-2.uV-1).

• Vtp is the thermopile output voltage (uV).

• ε is the gold emittance.

• K2 is the emittance of the black receiver surface.

• Wr is the receiver irradiance (W.m-2).

• Wc is the concentrator irradiance (W.m-2).

• τ is the ACP’s throughput.

PIR

WW = KK0 + KK1 ∗ VVtttt + KK2 ∗ WWrr + KK3 ∗ (WWdd − WWrr)

Where,

• K0, K1 , K2 , and K3 are the calibration coefficients.

• Wd is the dome irradiance, in W/m2.

KZ

WW = VVtttt

CC + KK2 ∗ WWcc

Where C and K2 are the calibration coefficients and Wc is the case irradiance.
4

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4 Results

Figure 1 shows the irradiance of ACPs 57F3, 95F3, and 96F3; and Figure 2 shows the difference 

between the three ACPs’ average irradiance and the irradiance measured by each ACP. Table 1 

shows that the difference varied from 1.0 W/m2 to 1.7 W/m2, with a 95% confidence level.

Figure 3 shows the ACPs’ average irradiance vs that of the PIR and KZ, as well as the PIR 

output thermopile voltage (Vtp). The Vtp is correlated to the net longwave irradiance, i.e., Vtp

approaches zero when the sky gets cloudier. As is illustrated in the figure, the average irradiance 

of the ACPs is larger than that of the pyrgeometers when Vtp is less than -300 μV (i.e., during 

clear sky conditions), and smaller than irradiance measured by the pyrgeometers when Vtp is 

more than -300 μV (i.e. thin clouds). This behavior might be correlated to the spectral response 

of the pyrgeometers and the estimated precipitable water vapor (PWV), yet more comparisons 

and data might help in resolving this issue. 

Figure 4 shows the estimated PWV vs the difference between the ACPs’ average irradiance and 

the irradiance measured by the PIR pyrgeometer with traceability to WISG and the KZ 

pyrgeometer with traceability to the IRIS. As is shown in Table 2, the difference is 9.3 W/m2 for 

the PIR and 7.3 W/m2 for the KZ, with a 95% confidence level. 

Figure 1. ACPs’ irradiance

260

265

270

275

280

285

290

295

300

305

310

19:5721:0221:4922:4323:30

0:171:121:592:463:414:285:23

18:4319:3020:1718:5719:4423:44

0:311:292:163:17

W/m2

Mountain Standard Time (MST)

ACPs' Irradiance from September 24 to 28, 2018

Win 95F3 Win57F3 Win 96F3
5

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Figure 2. Average ACPs’ irradiance minus the irradiance measured by each ACP

Table 1. Average ACPs’ irradiance minus the irradiance measured by each ACP

WavACPs - W(95F3) WavACPs - W(57F3) WavACPs - W(96F3)

av 0.3 0.2 -0.5

sd 0.6 0.5 0.8

U95 1.2 1.0 1.7

nrdg 1754 1754 1754

-3

-2

-1

0

1

2

3

20:3121:1121:5122:3223:1923:59

0:391:262:062:463:344:144:545:43

18:5519:3520:15

0:121:031:432:233:17

W/m2

MST

ACP Average Irradiance Minus Each ACP Irradiance 

from September 24 to 28, 2018

Wav-W95F3 Wav-W57F3 Wav-W96F3
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This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 3. Average ACPs’ irradiance vs irradiance measured by PIR and KZ

-500

-450

-400

-350

-300

-250

-200

-150

-100

-50

0

230.000

235.000

240.000

245.000

250.000

255.000

260.000

265.000

270.000

275.000

280.000

285.000

290.000

295.000

300.000

305.000

20:31:3121:23:3122:15:3123:15:16

0:07:161:06:501:58:502:50:503:50:304:42:305:43:22

19:07:3919:59:39

0:08:371:11:232:03:232:55:23

μV

W/m2

MST

ACPs' Average Irradiance vs PIR and KZ 

Irradiance from September 24 to 28, 2018

Wav ACPs W(PIR) W(KZ) Vtp-PIR
7

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

Figure 4. Average of ACPs’ irradiance minus irradiance measured by PIR and KZ vs PWV

Table 2. Average ACPs’ irradiance minus the irradiance measured by the PIR and KZ

WavACPs - W(PIR) WavACPs - W(KZ)

av 4.3 2.3

sd 4.2 3.5

U95 9.3 7.3

nrdg 1754 1754

0

2

4

6

8

10

12

-8

-6

-4

-2

0

2

4

6

8

10

20:3121:2022:0923:0623:55

0:451:402:293:264:155:04

18:3419:2320:12

0:181:182:073:10

mm

W/m2

MST

ACPs' Average Irradiance Minus PIR and KZ 

Irradiance vs PWV from September 24 to 28, 2018

Wav-W(PIR) Wav-W(KZ) PWV (mm)