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A New Standard Sun CIE 241 & CIE H1

Atlas Weathering Blog CIE 241

Solar Spectrum at Earth’s Surface

The spectrum and intensity of the sun (or the spectral irradiance of solar radiation) is influenced by the atmosphere and depends on location and time. CIE Publication No. 85 Table 4, defines the spectral irradiance of global radiation (direct and diffuse radiation) at sea level, when the sun is perpendicular and therefore filtered by exactly “one atmosphere” (air mass AM 1.0). This, for example, is a model of the solar spectrum at the equator on the 21st of March at noon on a day with a cloudless sky (environmental parameters: Ozone: 0.34 atm-cm; Water vapor: 1.42 atm-cm; Albedo: 0.2; Aerosol Optical Depth - AOD at 500 nm: 0.10).

Atlas Weathering Blog no 24 CIE241 Air mass
Position of the sun when the radiation is filtered by one atmosphere (air mass AM 1.0)

CIE 85 Reference Sun

Since its publication back in 1989, CIE Publication No. 85, Table 4 has served as reference sun in weathering and is used as the basis to compare and qualify artificial light sources. For example, CIE Publication No. 85 was referenced in the ISO 4892 and ISO 16474 series of standards as the target for artificial light sources to match for daylight conditions. However, this reference spectrum has some limitations:
   1)  Spectral data starts at 305 nm, while solar radiation cuts on between 295 nm and 300 nm;
   2)  The data resolution is poor and not consistent: 5 nm from 305 nm to 350 nm; 10 nm from 350 nm to 550 nm and even bigger intervals above 550 nm;
   3)  The calculation parameters are known, but the programming code to perform the calculations is no longer available. The software used to create the data was discarded many years ago so there was no way to recreate the models.

Spectrum Recalculated: CIE 241

In 2014, the spectral irradiance distribution of CIE Publication No. 85 Table 4 was recalculated at a higher resolution (0.5 nm up to 400 nm and 1 nm steps up to 1,700 nm) and published as Technical Report ISO/TR 17801.

A complete recalculation using the same input parameters as the original CIE 85 and a more recent calculation model (SMARTS 2.9.5), was published in 2020 as CIE 241, defining intervals of 0.5 nm for wavelengths from 280 nm to 400 nm, intervals of 1.0 nm up to 1,700 nm, and intervals of 5 nm up to 4,000 nm.

Note: CIE No. 241 only includes data in 5 nm resolution but data in 1 nm resolution are downloadable from the CIE server.

CIE No. 241 contains several calculated spectral irradiance distributions. The one to be used for weathering purposes is designated “CIE-H1” and is now the official replacement of the previous reference sun in CIE 85:1989, Table 4. The resulting values are very similar, but not identical, due to the poor resolution of the original data. Upcoming revisions of common weathering standards such as ISO 4892-1 and ISO 4892-3 will refer to CIE 241 instead of CIE 85. Since the differences between the spectra are small, compared to the differences to artificial light sources, this will have no technical influence on the test methods and test conditions.

Comparison of CIE 85 and CIE 241 Spectra

The table below shows the irradiance comparison for CIE 85:1989 Table 4 solar spectrum and CIE 241, CIE-H1 solar spectrum (using rectangular integration).

Atlas Weathering Blog 24 CIE241 Table

Atlas Weathering Blog CIE241 Spectral Irradence
Comparison of spectral irradiance of CIE 241 CIE-H1 and ISO/TR 17801 (based on CIE 85 Table 4).

More Information

Learn more about solar radiation in our recorded seminar on Factors of Weathering.

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