Designation: G207 − 11

Standard Test Method for

Indoor Transfer of Calibration from Reference to Field
Pyranometers1
This standard is issued under the fixed designation G207; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.

INTRODUCTION

Accurate and precise measurements of total solar and solar ultraviolet irradiance are required in: (1)
the determination of the energy incident on surfaces and specimens during exposure outdoors to
various climatic factors that characterize a test site, ( 2) the determination of solar irradiance and
radiant exposure to ascertain the energy available to solar collection devices such as flat-plate
collectors, and (3) the assessment of the irradiance and radiant exposure in various wavelength bands
for meteorological, climatic and earth energy-budget purposes. The solar components of principal
interest include total solar radiant exposure (all wavelengths) and various ultraviolet components of
natural sunlight that may be of interest, including both total and narrow-band ultraviolet radiant
exposure.
This test method for indoor transfer of calibration from reference to field instruments is only
applicable to pyranometers and radiometers whose field angles closely approach 180° ... instruments
which therefore may be said to measure hemispherical radiation, or all radiation incident on a flat
surface. Hemispherical radiation includes both the direct and sky (diffuse) geometrical components of
sunlight, while global solar irradiance refers only to hemispherical irradiance on a horizontal surface
such that the field of view includes the entire hemispherical sky dome.
For the purposes of this test method, the terms pyranometer and radiometer are used interchangeably.

1. Scope 1.5 The primary reference instrument shall not be used as a

1.1 The method described in this standard applies to the field instrument and its exposure to sunlight shall be limited to
indoor transfer of calibration from reference to field radiom- outdoor calibration or intercomparisons.
eters to be used for measuring and monitoring outdoor radiant NOTE 1—At a laboratory where calibrations are performed regularly it
exposure levels. is advisable to maintain a group of two or three reference radiometers that
are included in every calibration. These serve as controls to detect any
1.2 This test method is applicable to field radiometers instability or irregularity in the standard reference instrument.
regardless of the radiation receptor employed, but is limited to
1.6 Reference standard instruments shall be stored in a
radiometers having approximately 180° (2π Steradian), field
manner as to not degrade their calibration.
angles.
1.7 The method of calibration specified for total solar
1.3 The calibration covered by this test method employs the
pyranometers shall be traceable to the World Radiometric
use of artificial light sources (lamps).
Reference (WRR) through the calibration methods of the
1.4 Calibrations of field radiometers are performed with reference standard instruments (Method G167 and Test Method
sensors horizontal (at 0° tilt from the horizontal to the earth). E816), and the method of calibration specified for narrow- and
The essential requirement is that the reference radiometer shall broad-band ultraviolet radiometers shall be traceable to the
have been calibrated at horizontal tilt as employed in the National Institute of Standards and Technology (NIST), or
transfer of calibration. other internationally recognized national standards laboratories
(Standard G138).
1.8 This standard does not purport to address all of the
1
This test method is under the jurisdiction of ASTM Committee G03 on safety concerns, if any, associated with its use. It is the
Weathering and Durability and is the direct responsibility of Subcommittee G03.09
responsibility of the user of this standard to establish appro-
on Radiometry.
Current edition approved July 1, 2011. Published August 2011. DOI: 10.1520/ priate safety and health practices and determine the applica-
G0207–11 bility of regulatory limitations prior to use.

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States

1
 G207 − 11
2. Referenced Documents response (for example, black thermopile), or has been calibrated outdoors,
the difference between calibration and source spectral distributions is less
2.1 ASTM Standards:2 important, however should be taken into consideration.
E772 Terminology of Solar Energy Conversion
4.5 Monitor the output signal of the reference radiometer at
E816 Test Method for Calibration of Pyrheliometers by
the selected data collection interval.
Comparison to Reference Pyrheliometers
E824 Test Method for Transfer of Calibration From Refer- 4.6 Ensure the temporal stability of the source, as indicated
ence to Field Radiometers by the reference radiometer output, has stabilized at reasonable
G113 Terminology Relating to Natural and Artificial Weath- amplitude. Recommended source amplitude for broadband
ering Tests of Nonmetallic Materials solar radiometers is in the range 500 Wm-2 to 1000 Wm-2. For
G138 Test Method for Calibration of a Spectroradiometer narrowband radiometers, a source amplitude (spectral irradi-
Using a Standard Source of Irradiance ance distribution integrated over with respect to wavelength
G167 Test Method for Calibration of a Pyranometer Using a over the pass band of the radiometers) of 50% to 125% of the
Pyrheliometer peak amplitude to be expected in the source monitored by the
2.2 Other Standards:3 test instruments is recommended.
ISO 9847 Solar Energy Calibration of Field Pyranometers 4.7 The analog voltage signal from each radiometer is
by Comparison to a Reference Pyranometer measured, digitized, and stored using a calibrated data-
3. Terminology acquisition instrument, or system. A minimum of 30 data
readings is required.
3.1 Definitions:
3.1.1 See Terminology E772 and G113 for terminology 4.8 The test data are divided by the reference radiometer
relating to this test method. data, employing the instrument constant of the reference
instrument to determine the instrument constant of the radiom-
4. Summary of Test Method eter being calibrated. The mean value, the standard deviation,
4.1 Mount the reference pyranometer, and the field (or test) and coefficient of variation are determined.
radiometers, or pyranometers, on a common calibration table
for horizontal calibration. Adjust the height of the radiation 5. Significance and Use
receptor of all instruments to a common elevation. 5.1 The methods described represent a means for calibration
4.2 Connect the signal cables from the reference and test of field radiometers employing standard reference radiometers
sensors to a data acquisition system. indoors. Other methods involve the natural sunlight outdoors
under clear skies, and various combinations of reference
4.3 Adjust the data acquisition system to record data at the radiometers. Outdoor these methods are useful for cosine and
selected data collection interval. azimuth correction analyses, but may suffer from a lack of
NOTE 2—Data collection interval should be function of the time available clear skies, foreground view factor and directionality
constant of the sensor. Sensor time constant is the period of time required problems. Outdoor transfer of calibrations is covered by
for a sensor to reach 1 – 1/e = 63% of the maximum minus the minimum standards G167, E816, and E824.
amplitude of a step change in input stimulus. (e is base of natural
logarithms, 2.718282...). Often, “one over e” (1/e) time constants are 5.2 Several configurations of artificial sources are possible,
reported for radiation sensors, for example “1/e response time = 3 including:
seconds”. This represents the time for the sensor signal to reach 37% of
the full range step change representing the step change in the stimulus. 5.2.1 Point sources (lamps) at a distance, to which the
Four times the 1/e time constant can be considered the time for the sensor sensors are exposed
to fully respond to a step change in stimulus. 5.2.2 Extended sources (banks of lamps, or lamp(s) behind
4.4 Energize the source to be used for the transfer of diffusing or “homogenizing” screens) to which the sensors are
calibration. exposed
5.2.3 Various configurations of enclosures (usually spheri-
NOTE 3—It is mandatory that the spectral distribution of the source be
known or well characterized. Indoor calibration transfers between narrow cal or hemispherical) with the interior walls illuminated
band radiometers such as Ultraviolet and Photopic detectors shall be indirectly with lamps. The sensors are exposed to the radiation
accomplished using sources with spectral irradiance distributions as emanating from the enclosure walls.
similar as possible to the spectral distribution of the sources to be
monitored. This will reduce spectral mismatch errors arising from 5.3 Traceability of calibration for pyranometers is accom-
differences in the spectral response of sensors and dissimilar calibration plished when employing the method using a reference global
and ‘test’ source spectral distributions. In the special case of pyranometers pyranometer that has been calibrated, and is traceable to the
for solar radiation measurements, as long as the reference radiometer has
World Radiometric Reference (WRR)4. For the purposes of
a relatively flat and broad (greater than 700 nm passband) spectral
this test method, traceability shall have been established if a
parent instrument in the calibration chain can be traced to a
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
4
the ASTM website. WMO—No. 8, “Guide to Meteorological Instruments and Methods of
3
Available from Available from International Standards Organization (ISO), 1 Observation,” Fifth Ed., World Meteorological Organization, Geneva, Switzerland,
Rue De Varembre, Geneva, Switzerland CH-1211 20 1983

2
 G207 − 11
reference pyrheliometer which has participated in an Interna- spectroradiometer, or by direct calibration against standard sources of
tional Pyrheliometric Comparison (IPC) conducted at the spectral irradiance (for example, deuterium or 1000 W tungsten-halogen
lamps) is the subject of Standard G138.
World Radiation Center, (WRC), Davos, Switzerland.
5.3.1 The reference global pyranometer (for example, one 5.5 The calibration method employed assumes that the
measuring hemispherical solar radiation at all wavelengths) accuracy of the values obtained with respect to the calibration
shall have been calibrated by the shading-disk, component source used are applicable to the deployed environment, with
summation, or outdoor comparison method against one of the additional sources of uncertainty due to logging equipment and
following instruments: environmental effects above and beyond the calibration uncer-
5.3.1.1 An absolute cavity pyrheliometer that participated in tainty.
a World Meteorological Organization (WMO) sanctioned
IPC’s (and therefore possesses a WRR reduction factor). 5.6 The principal advantages of indoor calibration of radi-
5.3.1.2 An absolute cavity radiometer that has been inter- ometers are user convenience, lack of dependence on weather,
compared (in a local or regional comparison) with an absolute and user control of test conditions.
cavity pyrheliometer meeting 5.3.1.1. 5.7 The principal disadvantages of the indoor calibrations
5.3.1.3 Alternatively, the reference pyranometer may have are the possible differences between natural environmental
been calibrated by direct transfer from a World Meteorological influences and the laboratory calibration conditions with re-
Organization (WMO) First Class pyranometer that was cali- spect to the spectral and spatial distribution of the source
brated by the shading-disk method against an absolute cavity radiation (sun and sky versus lamps or enclosure walls).
pyrheliometer possessing a WRR reduction factor, or by direct
transfer from a WMO Standard Pyranometer (see WMO’s 5.8 It is recommended that the reference radiometer be of
Guide WMO—No. 8 for a discussion of the classification of the same type as the test radiometer, since any difference in
solar radiometers). See Zerlaut5 for a discussion of the WRR, spectral sensitivity between instruments will result in errone-
the IPC’s and their results. ous calibrations. However, The calibration of sufficiently
broadband detectors (approximately 700 nm or more), such a
NOTE 4—Any of the absolute radiometers participating in the above silicon photodiode detectors with respect to extremely broad-
intercomparisons and being within 60.5 % of the mean of all similar
instruments compared in any of those intercomparisons, shall be consid- band (more than 2000 nm) thermopile radiometers is
ered suitable as the primary reference instrument. acceptable, as long as the additional increased uncertainty in
5.4 Traceability of calibration of narrow band (for example,, the field measurements, due to spectral response and spectral
Ultraviolet) radiometers is accomplished when employing the mismatch limitations, is acceptable. The reader is referred to
method using a reference narrow band radiometer that has been ISO TR 96737 and ISO TR 99018 for discussions of the types
calibrated and is traceable to the National Institute of Standards of instruments available and their use.
and Technology (NIST), or other national standards organiza-
tions. 6. Interferences
5.4.1 The reference narrow band radiometer, regardless of 6.1 In order to minimize systematic errors the reference and
whether it measures total ultraviolet solar radiation, or narrow- test radiometers must be as nearly alike in all respects as
band UV-A or UV-B radiation, or a defined narrow band possible.
segment of ultraviolet radiation, shall have been calibrated by 6.1.1 The spectral response of both the reference and test
one of the following: radiometers should be as nearly identical as possible.
5.4.1.1 By comparison to a standard source of spectral
irradiance that is traceable to NIST or to the appropriate 6.1.2 The spectral content (spectral power distribution) of
national standards organizations of other countries using ap- the calibration source and the source to be monitored in the
propriate filters and filter correction factors [for example, field experiment should be matched to greatest extent possible.
Drummond 6]. If not, the relative spectral differences should be characterized,
5.4.1.2 By comparison of the radiometer output to the reported, and the spectral mismatch characterized.
integrated spectral irradiance in the appropriate wavelength 6.2 Source stability. The measurements selected in deter-
band of a spectroradiometer that has itself been calibrated mining the instrument constant shall be made during periods of
against such a standard source of spectral irradiance, essentially uniform levels or slow (less than 0.5% of full scale
5.4.1.3 By comparison to a spectroradiometer that has per minute) rates of change of radiation (as measured by the
participated in a regional or national Intercomparison of reference radiometer). Measurements selected under varying
Spectroradiometers, the results of which are of reference source amplitudes may result in erroneous calibrations if the
quality. reference and test radiometers possess significantly different
NOTE 5—The calibration of reference ultraviolet radiometers using a response times.

5 7
Zerlaut, G. A., “Solar Radiation Instrumentation,” Chapter 5 in Solar ISO Technical Report TR 9673, “Solar Radiation and Its Measurement for
Resources, The MIT Press, Cambridge, MA, 1989, pp. 173–308. Determining Outdoor Weathering Exposure Levels,” International Standards
6
Drummond, A.J, and A.K. Ǻngström, “Derivation of the Photometric Flux of Organization, Geneva, Switzerland.
8
Daylight from Filtered Measurements of Global (Sun and Sky) Radiant Energy”, ISO/TR 9901:1990, “Solar Energy—Field Pyranometers—Recommended
Applied Optics Vol 10 # 9, September 1971. Practice for Use.”

3
 G207 − 11
6.3 Spatial non-uniformity in the test plane with respect to reached). The maximum difference between the signal at the
the location of reference and test detectors will lead to starting placement and any other placement should not exceed
erroneous results, on the order of magnitude of the non- 1% of the expected (full scale) amplitude.
uniformity. 7.3.2.2 If the test instrument and the reference instrument
are replaced in the same location, record the maximum
7. Apparatus difference between an initial (first placement) of the reference
7.1 Data Acquisition Instrument—A digital voltmeter or radiometer signal and subsequent removal and re-placements
data logger capable of repeatability to 0.1 % of average of the reference radiometer in the calibration position. A
reading, and an uncertainty of 60.2 % with input impedance of sample size of at least 20 (removal and replacements) of the
at least 1 MΩ may be employed. Data loggers having printout reference radiometer are needed. The maximum difference
must be capable of a measurement frequency of at least two per between the signal at the starting placement and any other
minute. A data logger having three-channel capacity may be placement should not exceed 1% of the expected (full scale)
useful. amplitude.
7.2 Fixed-Angle Calibration Table—A calibration table/ 7.3.3 The spectral distribution of incandescent, xenon arc,
mounting fixture required for all horizontal calibrations. metal halide, light emitting diodes, and other lamp technolo-
gies are quite different from each other, and can be more or less
7.3 Stable Optical Radiation Source—A temporally (less representative of the spectral distribution of natural sunlight, or
than +/- 0.5% of full scale amplitude variation at a sample rate the source to be monitored by the radiometers under calibra-
of the 1/e time constant of the reference sensor, or a selected tion. It is required that the spectral distribution of the calibra-
data integration period) and spatially uniform (over the area of tion source be known, measured, or characterized so that it may
sensor(s) exposure) source of optical radiation such as a lamp, be compared with the spectral distribution of the source to be
bank of lamps, or illuminated enclosure, as described in section measured with the radiometers.
5.2. The spectral distribution of the source must be known over
the pass band of the instruments under test. The source 7.3.3.1 The absolute spectral distribution of the calibration
characterizations described here need not be accomplished source may be measured in the test plane of the radiometers by
before every calibration, but should be repeated periodically, use of a spectroradiometer system, calibrated in accordance
and especially if calibration data or reference radiometer data with Standard G138, with the input optic in the test plane.
show large deviations (more than 2%) from previous or 7.3.3.2 Relative spectral distribution data for lamp sources
historical results. may be provided by lamp manufacturers; but is suitable only if
7.3.1 Temporal stability is dependent upon the nature of the the range of the spectral response of the sensors under test is
illumination source; and type of power (uniform direct current, encompassed by the spectral data.
or alternating current) energizing the source. 7.3.3.3 If the test plane of the radiometers is illuminated
7.3.1.1 Direct current (DC) powered sources are more indirectly, either by reflection off enclosure (sphere or hemi-
stable, but generally of lower power and the required power sphere walls), or by radiation transmitted through other optical
supply stability requirements at recommended power may components, such as mirrors, diffusers, or lenses, the spectral
increase the source cost significantly optical properties of the intermediate materials must be known,
7.3.1.2 Alternating current (AC) powered sources will have and the product of the spectral optical properties and spectral
inherent fluctuations driven by the alternating current on the distribution data computed to arrive at the radiation spectral
order of the period of alternating current. Data captured on an distribution at the test plane.
“instantaneous” basis may reflect these fluctuations, especially 7.3.3.4 The recommended spectral resolution (step size in
if fast time response (silicon photodiode) detectors are used. spectral distribution or spectral optical properties, or both) is
Large (greater than 2%) standard deviations or coefficient of 10 nanometers. Digitization or interpolation, or both, of manu-
variation (ratio of the standard deviation of the mean to the facturer supplied spectral data to this resolution is recognized
mean value, expressed as a percentage) in data results may as a valid means of arriving at suitable data for computing the
reflect this problem. Integration of data over a large number of final spectral distribution at the test plane.
AC cycles (“line cycles”), up to several seconds, is recom-
mended to mitigate this problem when using AC powered 8. Procedure
sources.
8.1 Mount reference and test radiometers on a common
7.3.2 Spatial uniformity in the test plane of the sensors is
calibration table/fixture in the test plane. Adjust all instruments
required to assure all sensors are exposed to the same ampli-
to a common sensor elevation.
tude of radiation during the comparison process. In the
following, “signal reference value” means the instantaneous or 8.2 Connect both the reference and test instruments to their
integrated reference radiometer data as would be recorded respective, or common, data acquisition instrument, using low
during a calibration. capacitance, shielded cable of at least 20 gauge and of identical
7.3.2.1 Spatial uniformity in the “test plane” of the sensors length for each instrument. Check the instruments for electrical
may be evaluated by recording the maximum difference continuity, sign of the signal, and the nominal signal strength
between an initial (first placement) reference radiometer signal and stability. Clean the radiometer’s outermost photoreceptive
and subsequent placements of the reference radiometer in each surface (glass dome, filter, window, diffuser, etc.) in accor-
test instrument position (until a sample size of at least 20 is dance with the manufacturer’s instructions.

4
 G207 − 11
8.3 Adjust the data acquisition system to record data at the 9.2.1 Determine the series of calibration factors of the field
selected data collection interval or integration period, or both. radiometer from n readings of a measurement series j, (if more
than one measurement series is recorded) using the following
8.4 Measure zero off-sets. Check the off-set signals of both
equations:
the reference and field radiometers at the start and the end of
i51
each measurement series by measuring dark signals before and
after use of calibration source by recording simultaneous
FR
n
(V R ~ ij!
F~j! 5 i51 (3)
instantaneous or integrated (as appropriate, consistent with 7.3)
with the data logger. Sample sizes of 30 readings are recom- ( n
V F ~ ij!

mended. or
8.5 Energize the calibration source and allow the source to F R @ V R ~ j ! # integ
F~j! 5 (4)
stabilize so variations or fluctuations of no more than 0.5% of @ V F ~ j ! # integ
the operational amplitude of the source, as monitored at the where:
data sample integration period and sample rate from the [V(j)]integ = integrated values.
reference radiometer, occur.
9.3 Data Rejection:
8.6 After the source has stabilized, record instantaneous or
9.3.1 Reject any data that have been subject to operational
integrated voltage readings on both instruments for a minimum problems for either the reference or field pyranometer, or
of 30 readings. If the reference and test instruments are within radiometer. Also, reject those data for which F(ij) (see Eq 1)
the area of +/- 1% spatially uniform radiation, simultaneous deviates by more than 62 % from F(j) (see Eq 3 or Eq 4).
recording of reference and test signals is recommended. If Repeat the calculation of F (j) on the basis of the “clean” data.
instrument position exchange, replacement, or repositioning is Compute the final calibration factor in accordance with Eq 5 or
used, the time between the position exchanges, and recording Eq 6.
of data, should be minimized to the greatest extent possible.
9.4 Statistical Analysis:
9. Calculations 9.4.1 Determine the stability of the calibration conditions
during a measurements series by calculating the standard
9.1 First Step (Instantaneous Readings):
deviation of F(ij) about their mean for values of the set. For
9.1.1 From each reading i within a measurement series j,(if well controlled indoor laboratory sources, coefficient of varia-
more than one measurement series is recorderd) calculate the tion for a series should be less than 1.0% of the mean value.
ratio:
9.5 Determination of the Temperature-Corrected Final
V R ~ ij!
F ~ ij! 5 F (1) Calibration Factor:
V F ~ ij! R
9.5.1 If during a measurement series j the temperature T
where: deviates markedly (that is, by more than 610°C) from the
VR (ij) and VF (ij) = the voltages (for example, millivolts) desired typical value TN , and if the temperature response of the
measured using the reference and the field pyranometer is known to deviate markedly from that of
field radiometers, respectively the reference pyranometer, then calculate the final temperature-
FR = the calibration factor, for example, corrected calibration factor Fcorr at TN as follows: First correct
watts per square meter per microvolt, the F(j) data using the following equations:
of the reference radiometer, which has RT @T~j!#
been adjusted for the typical field F corr ~ i, T N ! 5 F ~ j ! (5)
R T ~ T N!
conditions, in the case where the field
and reference radiometer are of the and calculate Fcorr as
same type and have the type inherent 1 m
measurement specification (for Fcorr 5 ( F ~ j, T N !
m J51 corr
(6)
instance, in the temperature response).
Any other correction functions, such as where:
for cosine response, for the reference T ( j) = the mean air temperature during
radiometer may be used, but the form the measuring series j, in degrees
of the correction must be reported. Celsius;
RT [T(j)] and RT (TN ) = the responsivities of the field radi-
9.1.2 When FR as just defined is not applicable, it is ometer at T(j) and TN ,
replaced, for each measurement series, by a value of FR (j) that respectively,
is fitted to the calibration conditions (for instance, mean R = 1/F.
temperature) and that gives the most accurate value of irradi-
ance E (ij) according to the following equation: 9.5.2 For pyranometers and ultraviolet radiometers where
the temperature coefficients α of the instrument’s responsivity
F R ~ j ! V R ~ ij! 5 E ~ ij! (2)
are known, adjust the responsivities in accordance with the
9.2 Second Step: following:

5
 G207 − 11
R @ T ~ j ! # 5 @ 11α ~ T ~ j ! 2 T N ! # R ~ T N ! (7) eter) used, the precision of the data logging equipment, and
9.6 Determination of the Final Calibration Factor Without environmental conditions over the series of measurement
Temperature Correction of the Data: sessions. This is the transfer precision.
9.6.1 In cases where it is not necessary or not possible to 11.1.2 Within-laboratory transfer precision of derived cali-
correct the data relative to the temperature response, derive the bration values will vary depending on the stability of the
final calibration factor of the field pyranometer, or radiometer, reference standard, range of environmental conditions, source/
from the total number m of measurement series from the detector geometry, data selection/exclusion criteria, and sample
following equation: size for the calibration data set. For instance, the standard
m
deviation of the calibration value (WRR factor) for a reference
1 pyranometer exemplifies the precision for the standard radiom-
F5
m ( F~j!
j51
(8)
eter.
10. Report 11.1.3 Data for repeated calibrations of radiometers with
respect to a reference radiometer or spectroradiometer show
10.1 The report shall state as a minimum the following within-laboratory precision less than 2.0%, is achievable.
information:
10.1.1 Radiation source type (Incandescent, Metal Halide, 11.2 Bias—Bias with respect to WRR or NIST standards
Xenon, etc. lamp) will be determined by a combination of the estimated bias in
10.1.2 Source/Sensor geometrical configuration (for the reference radiometer or spectroradiometer (integrated) data
example, direct illumination, spherical/hemispherical enclo- and bias estimates for the data logging equipment. See Section
sure without direct illumination, etc.) 12 on Uncertainty.
10.1.3 Characterization of calibration source spectral distri- 11.3 Between-laboratory bias and precision will be a func-
bution relative to the expected spectral distribution for the tion of the precision and bias inherent in the respective
source to be monitored (for example, Relative spectral distri- laboratory reference radiometer or spectroradiometers, com-
bution plot of source and typical “field” spectrum). bined with the precision and bias estimates for the respective
10.1.4 Means of spectral distribution characterization (for data logging equipment.
example, Spectral measurements, manufacturer specifications) 11.4 Uncertainties of 62.0 % can be expected when cali-
10.1.5 Test Instrument type (UV-A radiometer, total solar brating radiometers at 0° horizontal based on a reference
pyranometer, etc.) instrument.
10.1.6 Manufacturer and serial number
10.1.7 Reference Instrument Type 12. Measurement Uncertainty
10.1.7.1 Reference instrument manufacturer and serial num- 12.1 Measurement uncertainty is an estimate of the magni-
ber tude of systematic and random measurement errors that may be
10.1.7.2 Reference instrument calibration date and calibra- reported along with the measurement errors and measurement
tion due date results. An uncertainty estimate relates to a particular result
10.1.7.3 Uncertainty statement for reference radiometer re- obtained by a laboratory carrying out this test method, as
sponsivity opposed to precision and bias statements in Section 11, which
10.1.8 Date of calibration(s), were derived from an engineering judgment based on experi-
10.1.9 Angle(s) of exposure: ences with interlaboratory calibrations.
10.1.9.1 Angle, (typically, horizontal)
10.1.10 Derived instrument responsivity, V W-1 m-2 ,or 12.2 It is neither appropriate for, nor the responsibility of
calibration factor, W-1 m-2 V-1 this test method to provide explicit values that a user of the
10.1.11 Temperature mean, °C, method would quote as their estimate of uncertainty. Uncer-
10.1.12 Scale: WRR, NIST spectral irradiance sale, etc., tainty values must be based on data generated by a laboratory
10.1.13 Traceability, a concise statement of the hierarchy of reporting results using the method. Measurement uncertainties
traceability including serial numbers of secondary and primary should be evaluated and expressed according to the NIST
reference instruments guidelines9 and the ISO Guide to Estimating the Uncertainty in
10.1.14 Reference and test instrument wavelength sensitiv- Measurements10, or “GUM”.
ity band (for example, 300 to 385 nm; or 285 nm to 2500 nm). 12.3 Sources of uncertainty in radiometer calibrations can
be divided into broad categories: voltage measurements, refer-
11. Precision and Bias ence radiometer performance, solar tracker performance, envi-
11.1 Precision—The precision in determining the instru- ronmental conditions, and test instrument performance.
ment constant of a field radiometer is influenced the indoor
calibration source character as described in 4.4 and 7.3. 9
B.N. Taylor and C.E. Kuyatt, Guidelines for Evaluating and Expressing the
Repeatability within any test series performed under stable Uncertainty of NIST Measurement Results. NIST Technical Mote 1297, U.S.
irradiance conditions should be such that the standard deviation Government Printing Office, Washington D.C. http://physics.nist.gov/Pubs/
is less than 6 1.0 % of the calibration value of the instrument. guidelines/TN1297/tn1297s.pdf
10
BIPM, Guide to the expression of uncertainty in measurement. Published by
11.1.1 The precision of the derived calibration factor of the
ISO TAG 4, 1993 (corrected and Reprinted 1995) in the name of the BIPM. It is now
test radiometer is influenced by the precision in the calibration referred to as the GUM. Its ISBN # is 92-67-10188-9 1995. http://www.bipm.org/
factor of the reference standard (radiometer or spectroradiom- utils/common/documents/jcgm/JCGM_100_2008_E.pdf

6
 G207 − 11
12.4 Uncertainty in calibration results obtained using this transfer of calibration from an absolute cavity pyrheliometer to
method depend on the calibration uncertainties for the refer- a secondary standard pyranometer is about 61.0 %, (2σ) at a
ence instruments used, test instrument performance, and the specific zenith angle. The total basic uncertainty in the transfer
signal noise encountered during the calibrations. of calibration values between comparable model radiometers is
12.4.1 For reference radiometer data based on spectroradio- approximately 62.0 % (2σ) for stable experimental indoor or
metric measurements, the uncertainty in the integrated refer- outdoor conditions with good sky conditions. Transfer uncer-
ence irradiance should be reported, based on spectroradiometer tainties depend particularly on the relative radiometer cosine
uncertainties estimated in accordance with Standard G138. responses, thermal offsets, sky conditions, and data logger
12.5 One can gather information describing the random uncertainty.
uncertainty of a measurement result by repeating the measure- 12.7.1 According to the GUM, the 2.0% basic uncertainty
ments several times and reporting the number of quoted above is an "expanded uncertainty" (represented by
measurements, and their range or standard deviation. multiplying the "standard" uncertainty of 1.0% by a "coverage
factor, k=2), assuming a normal distribution of random errors
12.6 Averaging over all data will result in larger uncertain- associated with the calibration and transfer process.
ties than averaging over selected subsets (such as limited zenith 12.7.2 If the calibration factors derived are plotted in a time
angle, irradiance, or ambient temperature ranges). Therefore a series, significant bias errors may be discerned. The calibration
description of the sample subsets used to derive the calibration report should include a statement of the estimated uncertainty
values and the reported uncertainty estimate is essential. based on a combination of reference radiometer uncertainty,
12.7 Example Uncertainty: standard deviation of the mean calibration value, estimated
The uncertainty in a primary standard pyrheliometer is bias in the data collection process.
approximately 60.3 % (representing 1σ) based on the results
of the WMO International Pyrheliometer Comparison since 13. Keywords
1980, and seven New River Intercomparisons of Absolute 13.1 calibration; field radiometers; pyranometer; Solar ra-
Cavity Pyrheliometers (NRIP’s). The mean uncertainty in the diation; solar radiometer; transfer

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