The first thing to say is that there is no standard spectrum to replicate indoor lighting conditions. However, a recent study by Kodak concluded that indirect window-filtered daylight is the dominant condition for indoor lighting in homes. "Let's review printed images from photostability tests of some widely used laboratory light sources.
Figure 16.1 Cool white fluorescent lighting compared to sunlight streaming through window glass.
1. Fluorescent lamp
Historically, photostability testing using high output cool white fluorescent lamps has been used for color photographs. For example, standard photographic test conditions (450 lux/12 hours per day of low-watt cool white light, 60% relative humidity, 70°F ambient room temperature) don't even come close to the variety of end-use environments in which inkjet-printed computer-generated images are used. While the output of cool white fluorescent lights may reproduce low light or office environments to some extent, the light spectrum of these lights is limited. That is, the output power of such lamps does not match the spectral power distribution of other commercial light sources or sunlight through window glass (see Figure 16.1).
Cool white fluorescent lamps are used to test products for primary end uses, such as in illuminated display cases or retail environments. (See Figure 16.2). However, lifespan predictions for the images such lamps display in a typical indoor environment (i.e., a home or office) are inaccurate. For example, on a clear morning, images displayed near windows, sliding glass doors, skylights, etc. can receive up to 50,000 lux of full-spectrum sunlight (i.e., ultraviolet, visible, and infrared [IR]).
Figure 16.2 Accelerated cool white fluorescence testing in the QUV accelerated weathering Tester is quick and easy
2. Xenon arc lamp
In Germany in 1954, a xenon arc was used to accelerate weathering. Xenon arc Testers, such as the Q-Sun Xenon Test Chamber, are suitable for the photostability of materials because they provide a better simulation of the full spectrum of sunlight: ultraviolet, visible, and infrared (see Figure 16.3).
Figure 16.3 The Q-Sun Xenon Arc Test Chamber is ideal for exposing printing inks and substrates to full spectrum sunlight
Xenon arcs require a combination of filters to reduce unwanted radiation and achieve the proper spectrum (for example, outdoor sunlight or sunlight filtered through window glass). The Windowpane filter simulates sunlight through windowpane. It is often used to test products whose primary service life is indoors. Figure 16.4 shows the spectral power distribution (SPD) of midday summer sunlight behind glass, comparing a Q-Sun Xenon Arc with a window glass filter. Remember: Fluorescent lights produce a very different spectrum than daylight or xenon arc lights. The choice of laboratory light source should best match the actual use environment of the product. To help illustrate the importance of lamp choice, consider the following example.
An ink that is expected to last 35 years under cool white fluorescent lighting (450 lux/12 hrs per day) will last only 1 year under 3 hrs/50,000 lux per day (equivalent to morning sun shining through a window).
Figure 16.4 Q-Sun Xenon Arc with window glass filter and sunlight through window glass
Figure 16.5 Xenon Arc Lamp with Window Glass Filter and Cool White Fluorescent Lamp vs. Sunlight Through a Window Glass
The explanation for this difference is that the spectral output of cool white fluorescent light sources is very different from the spectral power distribution of window-glass filtered sunlight.
This example clearly illustrates the dangers of using low-intensity light sources that do not account for high-intensity, full-spectrum sunlight for lifetime prediction.
Figure 16.5 compares the spd of a cool-white fluorescent lamp and a Q-Sun xenon-arc lamp with a window glass filter to sunlight filtered through a window glass.
