4.1 Since solar radiation, air temperature, relative humidity, and the amount and kind of atmospheric contaminants vary continuously, results from exposures based on elapsed time will sometimes differ. The variations in the results will usually be reduced by timing the exposures in terms of:

4.1.1 One or more environmental parameters such as solar radiant exposure, or

4.1.2 A predefined property change of a weathering reference specimen with known performance.

4.2 Variations in temperature, moisture, and atmospheric contaminants can have a significant effect on the degradation caused by solar radiation. In addition, exposures conducted at different times of the year can cause large differences in the rate of degradation. Different materials generally have different sensitivities to heat, moisture, and atmospheric contaminants, and this could explain differences in rankings of specimens exposed to equivalent solar radiant exposure when other environmental conditions vary.

4.3 Since the method of mounting has an influence on the temperature and other parameters during exposure of the specimen, there shall be agreement between contractual parties as to the method of mounting the specimen for the particular exposure test under consideration.

4.4 There are differences among various single strength window glasses in their transmittance in the 300 to 350 nm region. For example, at 320 nm, the percent transmittance for seven different lots of single strength window glass ranged from 8.4 to 26.8 %. At 380 nm, the percent transmittance ranged from 84.9 % to 88.1 %.5

4.5 Differences in UV transmittance between different lots of glass generally continue even after solarization. The largest differences among window glasses in UV transmittance are in the spectral range of 300 to 320 nm.

4.6 This practice is best used to compare the relative performance of materials tested at the same time behind the same lot of glass. Because of variability between lots of glass and between exposures conducted at different times of the year, comparing the amount of degradation in materials exposed for the same duration or radiant exposure at separate times, or in separate fixtures using different lots of glass, is not recommended.

4.7 It is recommended that at least one control material with known performance be exposed with each test. The control material should be of similar composition and construction as the test specimen, and be chosen so that its failure modes are the same as that of the material being tested. It is preferable to use two control materials, one with relatively good durability, and one with relatively poor durability. When control materials are included as part of the test, they shall be used for the purpose of comparing the performance of the test materials relative to the controls.

4.8 Because of the possibility that certain materials will outgas during exposure and potentially contaminate other specimens, it is recommended that only similar materials be exposed in the same under glass cabinet at the same time.