1. Closed carbon arc (ASTM G 153)
Closed carbon arcs have been used since 1918 as solar simulators for accelerated weathering and photostability testing. Many test methods still specify its use. Some flaws become apparent when the light output from this device is compared to sunlight. Figure 12.4 compares the UV spectral energy distribution of summer sunlight (solar maximum) with that of a closed carbon arc. The UV output of the closed carbon arc consists mainly of two very large energy peaks, with little output below 350 nm. Because the shortest UV wavelengths are the most damaging, closed carbon arcs are very slow to test on most materials and correlate poorly with materials sensitive to short-wave UV.
2. Sunshine carbon arc (open flame carbon arc: ASTM G 152)
The Sunshine Carbon Arc, introduced in 1933, had advantages over the closed carbon arc. Figure 12.5 plots the UV SED for summer sunlight compared to the solar carbon arc SED (with a Corex D filter). While the match to sunlight is better than that of a closed carbon arc, there is still a very large energy peak, much higher than sunlight, around 390 nanometers.
A more serious problem with sunlight carbon-arc spectroscopy is in the short wavelengths. To illustrate this, it is necessary to exaggerate the lower end of the chart. Figure 12.6 shows the solar maximum versus insolation carbon arc between 260 and 320 nm. Carbon arcs emit a lot of energy in the UV-C part of the spectrum, well below the normal solar cutoff of 295 nm. This type of radiation is realistic for outer space, but has never been found on the Earth's surface. These short wavelengths cause unrealistic degradation compared to natural exposure.
Figure 12.5 Solar carbon arc and sunlight: 260 to 400 nm
Figure 12.6 Solar carbon arc and sunlight: 260 to 320 nm
3. Xenon arc lamp (ASTM G 155)
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 (IR) light. Xenon arcs use filters to get the proper spectrum (for example, sunlight outside or sunlight filtered through window glass).
1) Effect of xenon filter
Xenon arcs require combination filters to reduce harmful radiation. A common filter combination is the Daylight filter. Figure 12.7 shows the spectral power distribution (SPD) of summer noon sunlight compared to a xenon arc with a daylight filter.
Figure 12.7 Xenon Arc Lamp with Daylight Filter and Sunlight
Figure 12.8 Xenon Arc Lamp with Window Glass Filter vs. Sunlight Through Window Glass
Another type of xenon arc filter designed to simulate sunlight passing through window glass is the window glass filter. It is often used to test products whose primary service life is indoors. Figure 12.8 shows a surge arrester with midday sunlight behind glass, compared to a xenon arc with a window glass filter.
2) Xenon arc humidity
The Xenon Arc uses an intermittent water spray system to simulate the effects of rain and dew. Water mist circulation is especially useful for introducing thermal shock and mechanical attack.
3) Influence of irradiance setting
Modern xenon arc models, including the Q-Sun, have a light monitoring system to compensate for the inevitable light output decay due to lamp aging. The operator pre-sets the desired irradiance or brightness. When the light output drops, the system compensates by increasing the wattage of the xenon burners. Common irradiance settings are 0.35 or 0.55 W/m-/nm at 340 nm. Figure 12.9 shows how these two irradiance settings compare to midday summer sunlight.
Several different sensors to measure and control irradiance (depending on the manufacturer): 340 nm, 420 nm, TUV (total ultraviolet), or total irradiance. The difference between these sensors is the wavelength or band at which they control irradiance, and the wavelength or band at which they are calibrated (via nist-traceable calibration radiometers).
Figure 12.9 Effect of Irradiance Settings
The 340nm sensor measures a narrowband wavelength centered at 340nm with a half-bandwidth of 10nm and should be used for testing materials primarily damaged by short-wave UV. This is because the 340nm setting will remain the same even as the lamp ages and the spectrum shifts. In general, this is a good control point for paints, plastics, roofing, and other durable products.
420nm measures a narrowband wavelength centered at 420nm with a half bandwidth of 10nm, it should be used for testing materials primarily damaged by visible light such as dyes and pigments in textiles, paper and inks. In general, broadband TUV and total irradiance sensors are not recommended.
Several factors complicate irradiance control for xenon burners: solarization of the filters and aging of the burner. Any of these factors can cause a non-uniform change in the xenon SPD, with short wavelength output dropping faster than long wavelength output.
Figure 12.10 shows the measured SPD of a burner at four different times in its life. Irradiance monitoring and control is only at 340 nm. The increase in wattage is enough to maintain irradiance at 340 nm, but not enough to compensate for the attenuation below 340 nm. At the same time, higher wattage increases the visible light output of the burner.
This changes the spectral power distribution of the lamp. As can be seen from the graph, while the irradiance controller does a good job at 340 nm, there is a drop in irradiance in the short-wave UV portion of the spectrum. This spectral change due to aging is an inherent characteristic of xenon lamps. However, this can be compensated by replacing the lamps on a regular basis.
