Filters

One item frequently neglected, when researchers report on the "room light" used for testing, is whether the fixture did or did not have a diffuser. This one item can affect the SPD of the radiation source very dramatically as illustrated in Figure 2, taken from a publication of Cole and associates.34 Many have probably personally experienced this same phenomena. The absorption of UV rays by the plastic sandwiched between the glass of windshields, accounts for why photochromic lenses do not work in automobiles. A similar problem might be found in using windowsill or more properly called "energy-efficient window-glass filtered daylight" for testing in some modern laboratory buildings.

Filters are prescribed in the ICH38 guideline, Option 1 for use with only Xenon (more correctly called long-arc Xenon [LAX]) and metal halide lamps. Cole and associates34 clearly demonstrated that filters should also be used for fluorescent lamps. In their study they detected UV-C radiation coming from bare "cool white" fluorescent lamps. The importance of this problem is well demonstrated in the adoption of the previously cited ANSI/IESNA RP 27-127 and 27-328 standards.

Filters are used to isolate unwanted radiation from the system. They are generally defined by various parameters such as, narrow or wide band, sharp cut-off, narrow band pass and cut-off wavelength. ISO 10977,39 ISO 4892-240 and the currently proposed ASTM G 3.Û341 standards currently define the glass to use relative to the SPD of a Xenon lamp.

Figure 2. Spectral irrctdiance ofF40T12 WWRS WMwith and without K-12 acrylic diffuser (Cole and associates).34

WAVELENGTH nm

Figure 2. Spectral irrctdiance ofF40T12 WWRS WMwith and without K-12 acrylic diffuser (Cole and associates).34

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