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What is the Difference Between Type I and Type II Daylight Filters

Material Testing under the sun

Some years ago, Atlas was able to develop a daylight filter system for xenon-arc weathering with a much closer fit to natural solar radiation than any other filter system on the market. We called it the Right Light™ filter. This filter system was first implemented in standardization in the ASTM D7869 method for transportation coatings.

The solar UV cut-on is a critical factor for photochemical degradation. Even though there is a big difference between the traditionally used Boro-S/Boro-S type filter systems and the Right Light™ filters, both daylight filter combinations fulfill the same requirements of common standards, such as ISO 4892-2, ISO 16474-2 and ASTM G155.

The Problem

Standards ISO 4892-2, ISO 16474-2 and ASTM G155 do not differentiate between the daylight filter types. This often led to inconsistent test results for materials with sensitivity to short wavelength UV radiation below the solar UV cut-on when different daylight filters were used for the same type of testing.

For clarification, Atlas initiated the implementation of new types of daylight filters in international standardization, recognizing the potential inconsistencies. ISO/TC 61/SC 6 “Plastics - Ageing, chemical and environmental resistance” was the first committee, which adopted this and recently published:

ISO 4892-2:2013/Amd 1:2021(en) Plastics - Methods of exposure to laboratory light sources - Part 2: Xenon-arc lamps - AMENDMENT 1: Classification of daylight filters.

The Solution

Filters with a close match to natural solar radiation now fall under the category of Type I daylight filters, including filters fulfilling the requirements of ASTM D7869. The traditionally used Boro-S/Boro-S filters used in Ci instruments now fall under the category of Type II daylight filters.

Daylight Filters Graph

Committees ASTM G03 (General weathering, ASTM G155) and ISO TC 35/SC 9 (Test methods for paints and varnishes, ISO 16474-2) will soon follow up in a harmonized approach. Nobody will need to worry anymore about different daylight filter specifications in different committees.

With these revisions, the intent was not to infer that “type I filters are better than type II” (or vice-versa), even though it is obvious that type I is more realistic. The goal was to make users aware that the results may be different if different daylight filter types are used and reinforce that comparisons between different tests are best made using the same daylight filter type.

How Does This Affect Your Weathering Testing?

Not at all. You can still use the same filters as you did in the past, according to ISO 4892-2, ISO 16474-2 or ASTM G155. It is only recommended to mention in the test report, which daylight filter type has been used. Knowing about the daylight type used increases the comparability and repeatability of weathering testing.

Day Light Type 1 and 2

You find more information on the Atlas Right Light™ filter here.

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