NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by 

Results of first outdoor comparison 

Absolute Cavity Pyrgeometer (ACP) a

Integrating Sphere (IRIS) Radiomete

Atmospheric System Research

Science Team Meeting (March 18‐21

by

Ibrahim Reda*, Julian Gröbner**, Stefan Wacker**

* = NREL , ** = PMOD/WRC

NREL/PR‐3B10‐58121
OutlineThe ACP and IRIS are developed to establish a wor

for calibrating pyrgeometers with traceability to SI 

two radiometers are unwindowed with negligible 

dependence, and traceable to SI units through the 

scale (ITS‐90).

The first outdoor comparison between the two de

held from January 28 to February 8, 2013 at the Ph

Metorologisches Observatorium Davos (PMOD). Tbetween the irradiance measured by ACP and that 

within 1 W/m2.

A difference of 5 W/m2 was observed between the 

measured by ACP&IRIS and that of the interim WoStandard Group (WISG).
Absolute Cavity Pyrgeometer (ACP

‐ ACP Net irradiance:‐ By cooling the ACP case

temperature, and since Watm

is stable, then,‐ Then the atmospheric longwave irradiance isWhere,K1, Vtp, ε , K2, Wr , Wc and τ are the reciprocal of ACP’s responsivity, thermopile voltage, gold 

emittance, receiver irradiance, CPC irradiance, and throughput (NIST characterization), cons

KV W W KW 1 2 tp atm c r * * ( )* ( )* *     1 2   

W

KV KW W

atm

tp r c



    1 2 * ( )* * ( )* 2 1  



K

W KW

V

c r

tp

1

2 1 2



( )* ( )* *      



Reference: Reda, I.; Zeng, J.; Schulch, J.; Hanssen, L.; Wilthen, B.; Myers, D.; Stoffel, T. Dec. 2011. “An absolute cavitabsolute outdoor longwave irradiance with traceability to International System of Units, SI”. Journal of Atmospheric a(2012) 132-143. http://dx.doi.org/10.1016/j.jastp.2011.12.011
The Infrared Integrating Sphere (IRIS) 

Key features of the 

• Windowless• Irradiance measurem

gold‐plated integratin

optic

• High sensitivity from 

pyroelectric detector

• Flat spectral response• Measurement freque• Automatic unattende• Nighttime measurem

Reference surface

Detector Aperture

Shutter

IRIS UncertaintyU95%= 1.8 Wm‐2 

2.4 Wm‐2 

Reference: Gröbner, J., A Transfer Standard Radiometer for atmospheric longwave irradiance measurement
ACP versus PMOD‐BB on Jan 29 to Fe

ACP Net irradinace v

‐350 ‐300 ‐250 ‐200 

Vtp (uV

WACP

0123

1.00 

W/m2

Residuals of fitting N

‐0.4‐0.2

0

0.20.4

1.00 

W/m2

ACP Net irradinace vs Vtp during cooling

y = 0.0834x ‐ 254.62

R2 = 1 ‐400

‐350‐300‐250

‐1400 ‐1200 ‐1000 ‐800 ‐600 ‐400 ‐200 0

Vtp (uV)

W/m2

WACP‐WBB

0123

13.16 13.24 13.33 13.41 13.49

UTC

W/m2

Residuals of fitting Net irradiance vs Vtp

‐0.3‐0.1

0.10.3

13.16 13.24 13.33 13.41 13.49

UTC

W/m2

ACP Net irradinace vs Vtp during cooling

y = 0.0827x ‐ 254.7

R2 = 0.9992

‐360‐340‐320‐300‐280

‐1400 ‐1200 ‐1000 ‐800 ‐600 ‐400 ‐200 0

Vtp (uV)

W/m

2

WACP‐WBB

0123

11.62 11.71 11.79 11.87

UTC

W/m

2

Residuals of fitting Net irradiance vs Vtp

‐1.5‐0.5

0.51.5

11.62 11.71 11.79 11.87

UTC

W/m

2

ACP Net irradinace v

‐1200 ‐1000 ‐800 ‐600 

Vtp (

WACP

0

0.5

1

1.5

17.66 17.7

W/m2

Residuals of fitting N

‐1.5‐0.5

0.51.5

17.66 17.7

W/m2
Transient vs steady state in BB, Jan 29‐F

WACP‐WBB

0

0.5

1

1.5

2

2.5

3

3.5

12.85 12.94 13.02 13.10 13.19 13.27 13.35 13.44

UTC

W /m

2

Steady State before cooling Transient during cooling

WACP

0

0.5

1

1.5

2

2.5

3

3.5

23.38 23.47 23.55 

W /m

2

Steady State before cooling 

WACP‐WBB

0

0.5

1

1.5

2

2.5

3

3.5

11.32 11.41 11.49 11.57 11.66 11.74 11.82 11.91

UTC

W /m

2

Steady State before cooling Transient during cooling

WACP

0

0.20.40.60.8

1

1.21.41.61.8

2

17.35 17.43 17.51 17.60 

W /m

2

Steady State before cooling 
Outdoor ACP&IRIS at night Feb. 5, 
Outdoor ACP&IRIS at night Feb. 5, 
Outdoor ACP, IRIS & WISG at night F
Irradiance difference (WISG minus IRIS

ResultsAverage Offset (IWV>10) ‐4.Gradient (IWV<10)‐0.45 ± 0.1 Wm

WISG 1 WISG 2 WISG 4

Night averages

IRIS u95

From Julian’s presentation, IRS2012-G

(Data from 180 nights)
Irradiance difference (ACP minus WISG

Three cooling cycles on November 18 and 21, 2012 with 

Consistent with Julian’s observation with high w

* Algebra is reversed for consistency with NREL’s historical files
Preliminary Conclusions

• Special set‐up of ACP in BB due to unknown g

• Outdoor agreement between ACP & IRIS to w

• Irradiance measured by WISG is ~4 W/m2 low

measured by ACP&IRIS. Is Consistent with a W

Column of 8 mm. This was also observed at N

RH = 40% (on November 18, 2012 at NREL/SR

Vapor Column from 7 mm to 9 mm during coo

• Future comparison with higher/lower water v

resolve observed spectral effect on outdoor p

calibrations

• A 3rd design might increase confidence in esta

consensus reference with traceability to SI un