Infrared Integrating Sphere (IRIS) Radiometer

Infrared RadiometryInfrared Integrating Sphere (IRIS) Radiometer

Overview

The Infrared Integrated Sphere Radiometer (IRIS) was developed and built by PMOD/WRC, and has been designed as a new reference radiometer to measure downward longwave irradiance. IRIS is calibrated relative to the well-characterised PMOD/WRC blackbody cavity, thus providing traceability to SI units.

Introduction

Two independent radiometers have recently been constructed to demonstrate traceability of atmospheric longwave radiation measurements to SI units: IRIS developed by PMOD/WRC (Gröbner, 2012) and the Absolute Cavity Pyrgeometer (ACP) from NREL (Reda et al., 2012). While IRIS is calibrated relative to the well-characterised PMOD/WRC blackbody cavity, thus providing traceability to SI units through temperature measurements (Gröbner, 2008), the ACP uses a self-calibrating technique for in-situ calibration of the instrument. Both radiometers are operated as windowless devices in order to minimize spectral inhomogeneities of their spectral responsivities, which has been shown to be the main cause for the observed discrepancies of commercial pyrgeometers (Gröbner and Los, 2007; Gröbner and Wacker, 2013).

Figure 1. Several IRIS on the PMOD/WRC roof platform during the IPgC-II pyrgeometer international comparison campaign.

Figure 2. IRIS cross-section, illustrating: 1) Aperture for incoming longwave radiation, 2) rotating shutters, 3) pyroelectric detector, 4) blackened reference cavity and thermistor, and 5) shutter motors.

IRIS Characteristics

The IRIS radiometer (Figures 1 and 2) consists of a gold-plated integrating sphere, with a blackened pyroelectric sensor as thermal detector. By virtue of its windowless design its spectral responsivity is expected to be nearly uniform over the whole spectral range of interest. The average of 10 individual measurements is stored every 10 s. The sensitivity in V.W−1.m2 is obtained by measuring the radiant exitance from the PMOD/WRC blackbody cavity (Gröbner, 2008). The resulting uncertainty of atmospheric downwelling longwave irradiance measurements with the IRIS radiometer is estimated at ±2.0 W.m−2 (Gröbner, 2012). IRIS radiometers in operation at PMOD/WRC are calibrated several times a year and deployed alongside the WISG during night-time clear-sky conditions.

Specifications

Measured parameter Downwelling longwave radiation
Main features Automatic unattended operation, windowless design, flat spectral response, night-time operation during cloudless conditions
Cavity design 60 mm diameter gold-plated integrating sphere
Detector SPH-40 series windowless pyroelectric detector (Spectrum Detector Inc.)
Measurement frequency 0.1 Hz
Expanded uncertainty ±2.0 W.m−2
IRIS dimensions 10 x 10 x 10 cm

References

Gröbner, J., (2008), Operation and investigation of a tilted bottom cavity for pyrgeometer characterizations, Appl. Opt., 47, 4441–4447, https://doi.org/10.1364/ao.47.004441
Gröbner, J., (2012), A transfer standard radiometer for atmospheric longwave irradiance measurements, Metrologia, 49, S105–S111, https://doi.org/10.1088/0026-1394/49/2/S105
Gröbner, J., and A. Los, (2007), Laboratory calibration of pyrgeometers with known spectral responsivities, Appl. Opt., 46, 7419–7425, https://doi.org/10.1364/ao.46.007419
Gröbner, J., and S. Wacker, (2013), Longwave irradiance measurements using IRIS radiometers at the PMOD/WRC-IRS, AIP Conf. Proc., 1531, 488, https://doi.org/10.1063/1.4804813
Gröbner, J., I. Reda, S. Wacker, S. Nyeki, K. Behrens, and J. Gorman, (2014), A new absolute reference for atmospheric longwave irradiance measurements with traceability to SI units, J. Geophys. Res. Atmos., 119, https://doi.org/10.1002/2014JD021630
Reda, I., J. Zeng, J. Scheuch, L. Hanssen, B. Wilthan, D. Myers, and T. Stoffel, (2012), An absolute cavity pyrgeometer to measure the absolute outdoor longwave irradiance with traceability to International System of Units, SI, J. Atmos. Sol. Terr. Phys., 77, 132–143, https://doi.org/10.1016/j.jastp.2011.12.011
For further information please contact: Dr. Julian Gröbner