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Behind the Window – Spectral Requirements for Xenon Instruments

Spectral Irradiance Distribution in Xenon Instruments

Xenon arc weathering instruments require optical filter systems to adjust the radiation for either outdoor sunlight with daylight filters or indoor environments with window glass filters.

The definition of daylight or window glass filtered radiation can differ from standard to standard. Some standards are vague and allow a broad range of spectral irradiance distributions. However, the spectral power distribution is critical, especially in the UV where the “cut-on” (the short wavelength region where the spectrum actually “begins”) can have a noticeable influence on material degradation and needs to be defined as tightly as possible.

This requirement was recently implemented for daylight filters by dividing the existing specification into two categories:

Transmission of Window Glass

For the transmission of window glass, two different sets of data exist:
• ISO 16474-1 specifies the transmission of 3 mm thick window glass in a poor resolution (40 nm).
• In an ASTM G03 ring study the average transmittance of 2.5 mm think window glass was determined in 10 nm resolution.

Window Glass Specification in Standards

Not all standards specify window glass filters in the same way. Here are the most frequently used test methods.

ISO 105-B02 defines the color temperature of window glass filtered xenon-arc radiation from 5500 K to 6500 K. It requires that the transmission of the filter system used shall be at least 90% between 380 nm and 750 nm and falls to zero between 310 nm and 320 nm.

ISO 105-B06 defines a spectral irradiance distribution in 40 nm ranges in the UV. It includes filter examples which do not fall under the definition of ISO 105-B02.

AATCC TM16.3 requires window glass filters to reduce the irradiance at wavelengths shorter than 310 nm.

ISO 4892-2 and ISO 16474-2 define the spectral irradiance in the UV in 40 nm steps, allowing a wide range of minimum and maximum irradiance values, which basically covers all window glass filter requirements listed above.

Due to the varying definitions of window glass filters, different spectral requirements have a different UV cut-on. The UV cut-on is one critical parameter which can influence material degradation. Therefore, using filters with different definitions might result in different test results.

Optical window glass filter combinations of Atlas instruments

Comparison of xenon-arc radiation with optical filters according to ISO 105-B02 and AATCC TM16.3

Irradiance Control

To measure and control the irradiance of window glass filtered xenon-arc radiation, either narrowband radiometers (at 420 nm) or broadband radiometers (300 nm to 400 nm) are used. The ratio between narrowband irradiance (at 420 nm) and broadband irradiance control (300 nm to 400 nm) depends on the spectral irradiance distribution, and therefore varies from standard to standard:

Ratios between narrowband and broadband controlled irradiance in different standards

Most standards allow both narrowband and broadband control. However, looking at the different ratios in the methods raise doubts that the results will be directly comparable.

Clarification Required

The confusing definitions and standard requirements make it almost impossible for the users of weathering test instruments to select the right optical filter system for a specific test method without guidance.

This needs to be addressed and harmonized in standardization in close cooperation with instrument manufacturers. A similar categorization as for daylight filters (type I and II) would also be recommended for window glass filters in standards such as ISO 4892-2, ISO 16474-2, and ASTM G155.

More Information

Learn more about window glass filters in textile testing in our recorded seminar on lightfastness testing of textiles.

For further information check out our online library, listen to further recorded online seminars or review upcoming educational classes.