Figure 4. Spectral energy distribution of global solar radiation according to publication CIENo. 85, Table 4 (1989)

Figure 5. Global solar radiation (CIE No. 85, Table 4/1989) behind window glass - radiation function ISO 4892 part 2 - method B (1994) and ISO 105 - B06 (proposal for revision - 1995).

regarded tolerance bands based on the reference spectrum for simulating sunlight behind window glass as well as the spectral distribution of appropriately filtered xenon radiation measured on sample area in an equipment for comparison. It is obvious that the latter matches the requirements rather close.

It may not always be necessary to simulate the reference spectrum over the whole wavelength range of light emission, especially in those cases where detailed information on the spectral sensitivity or/and activation spectra4 are available. But on the other hand, the most accurate simulation minimizes the risk of anomalous reactions that may not occur in actual use environment.

Comparing the described requirements on simulation of spectral distribution of radiation with the contents of the ICH-Guideline, it is obvious that in the latter, quantitative requirements are missing. It is only pointed out that the spectral output of the artificial sources should be similar to ID65 (defined in ISO 10977:1993, as the spectral distribution of indoor indirect daylight). Instead of specifying the spectral distribution of artificial radiation on specimen area, various quite different artificial radiation sources are listed as being acceptable for photostability testing with the only additional remark - and this one is really important - that the radiation below 320 nm must be eliminated by an appropriate filter system.

It may be of interest to note that ID65 is defined by the relative spectral distribution of D65 as specified in CIE No. 15.2:1986 (a predecessor of the mentioned CIE No.85), folded with the spectral transmittance of soda lime float glass (thickness 6 mm). Based on the latter the relative spectral distributions compared to each other show only slight deviations (Figure 6).

Focussing only on the various listed types of radiation sources in option 1 of the ICHGuideline, it is well known that their spectral distributions may be quite different. This may be true as well for specific light sources of the same general type, as shown in Figure 7 (UV-wavelength range) for filtered metal halide lamps. In consequence of the missing quantitative requirements there are no data given that could support a decision whether these differences may be acceptable or not. Nevertheless, in practice testing chambers with these different radiation sources are allowed to be used for conducting tests according to the Guideline and to compare test results.

The same situation as discussed concerning the spectral distribution of radiation is valid for other important test conditions, especially the level and uniformity of irradiance on sample area and the sample surface temperature under incident radiation.

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