Simulating Solar Radiation in Weathering Instruments

The primary weathering factors are known to be heat, water and solar radiation. There are two key parameters when looking to simulate solar radiation in weathering instruments through properly filtered light sources: spectral power distribution and irradiance (E).

Simulating Solar Radiation in Weathering Instruments

Image Credit: ATLAS Material Testing Technology

It is necessary to accurately and reproducibly simulate the spectrum of global (direct and diffuse) solar radiation on the earth’s surface. A well-defined reference sun is required to reflect this ‘daylight spectrum.’

The International Commission on Illumination (CIE) standardized the first reference sun in 1972, publishing this in its publication No. 20. CIE No. 20 states that to achieve worst-case solar radiation for maximum acceleration, the equatorial sun should be simulated.

This should reflect conditions at noon in the time of the equinox, in a cloudless sky, and with well-defined ozone, water vapor and aerosol contents in the atmosphere.

The development of advanced modeling techniques fed into CIE publication No. 85 in 1989, updating the reference sun with smaller wavelength steps and improving the accuracy of data.

Virtually all ISO and national weathering standards still refer to the solar benchmark spectrum provided in CIE No. 85 (Table 4). The following parameters are used in this process.

  • Relative air mass: 1
  • Water vapor content: 1.42 cm precipitable water
  • Ozone content: 0.34 cm at standard temperature and pressure (STP), 0 °C, 1 atmosphere
  • Spectral optical depth of aerosol extinction (at 500 nm): 0.1
  • Sea level
  • Ground reflectance: 0.2

Solar Spectrum at Earth’s Surface.

Solar Spectrum at Earth’s Surface. Image Credit: ATLAS Material Testing Technology

Reference Irradiance Level

CIE No. 20 states that the total global irradiance in the wavelength region reaching the earth’s surface (290 nm - 3000 nm) is 1120 W/m2. An estimated 10% radiation contribution is subtracted by the surroundings, so CIE No. 20 makes a number of recommendations for material testing in weathering instruments.

It highlights that simulation of the relative solar spectral power distribution should be undertaken as robustly as technically possible and that an irradiance level of 1000 W/m2 should be applied to the sample surface in the total wavelength region of solar radiation from 290 nm to 3000 nm.

It is important to note that CIE No. 85 does not include a recommended irradiance level for testing purposes - the recommendation in CIE No. 20 for a total irradiance of 1000 W/m2 is still considered the standard.

Table 4 shows E levels for daylight conditions (xenon with daylight filters) with a cut-on wavelength at approximately 300 nm calculated using this irradiance value. These have been calculated from the spectrum in CIE No. 85.

  • 550 W/m2 in the UV + VIS wavelength region (wide band, 300 nm - 800 nm)
  • 60 W/m2 in the UV (broad band, 300 nm – 400 nm)

The majority of ISO and related national standards also employ this irradiance level (550 W/m2) for daylight behind window glass conditions. The longer-wavelength cut-on at approximately 320 nm means that the UV irradiance is approximately 10 W/m2 lower, however.

  • 550 W/m2 in the UV + VIS wavelength region (wide band, 300 nm - 800 nm)
  • 50 W/m2 in the UV (broad band, 300 nm – 400 nm).

Narrow Band, Broad Band, and Wide Band, as defined in ISO 9370.

Narrow Band, Broad Band, and Wide Band, as defined in ISO 9370. Image Credit: ATLAS Material Testing Technology

Narrow Band Irradiance

As almost all photochemical degradation occurs in the UV, modern standards such ISO 16474-2 and ISO 4892-2 specify irradiance levels at either broad band (300 nm - 400 nm), narrow band at 340 nm (daylight) or 420 nm (daylight behind window glass). These include:

  • 0.51 W/(m2nm) at 340 nm – this is the typical ratio used in most xenon-arc standards.
  • 1.10 to 1.25 W/(m2nm) at 420 nm – these are the typical ratios utilized in most xenon-arc standards, though this does depend on the type of window glass filters in use.

It is difficult to calculate narrow band irradiance from CIE No. 85 because it specifies data in 5 nm steps around 340 nm and 10 nm steps around 420 nm.

Only a few standards currently specify wide band irradiance control (300 nm – 800 nm), and this continues to be replaced by narrow or broad band control.

New Developments

CIE Publication No. 85 Table 4 was recalculated in 2014 to include a higher resolution. This was later published as Technical Report ISO/TR 17801. A further complete recalculation was published in 2020 as CIE No. 241.

Resources

This information has been sourced, reviewed and adapted from materials provided by Atlas Material Testing Technology LLC.

For more information on this source, please visit Atlas Material Testing Technology LLC.

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