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Does the Cooling of the Xenon Lamp Influence Test Results?

Atlas Water air cooled what light is right cover
Xenon spectrum (red) compared to sun (yellow) and other light sources

Xenon-arc Lamps

For weathering applications, properly filtered xenon-arc lamps deliver state-of-the-art simulation of worst-case solar radiation in the ultraviolet (UV) and visible (VIS) wavelength range, i.e. 290 – 800 nm.

A xenon lamp, similar to the sun, not only emits UV and VIS but also produces a large amount of heat energy. While heating up the samples may be intended, the filters, the lamp itself, and surrounding components have to be cooled to avoid over-heating, damage, and aging. Like the motor of a car can be cooled by water or by air, both techniques can also be applied for weathering instruments and are used according to the requirements of the different types of xenon lamps.

Spectrum at Sample Surface

Instruments using water (like the Ci-series Weather-Ometers) are generally called “water-cooled xenon instruments.” The SUNTEST family and Xenotest instruments are examples of “air-cooled instruments.”

Atlas Water air cooled instrument comparison
Ci3000 water-cooled (left) and Xenotest 220+ air cooled (right) xenon-arc instruments

The technology of lamp cooling does not affect the spectral irradiance distribution in the UV and VIS, or the cut-on of radiant energy. The spectrum, especially in the critical UV/VIS wavelength ranges is entirely controlled by the optical filter system in use. If the spectral irradiance distribution - by using appropriate optical filter systems - and the irradiance level on the sample surface are identical, the type of lamp cooling has no influence on the test results. This has been proven many times over by Atlas’ spectroradiometer measurements.

Lamp Cooling With Water or Air?

But why do some types of xenon weathering instruments use water, others air for lamp cooling? The answer is easy: The heat conductivity of water is 10-times higher than air. To experience this yourself, just compare swimming in a 15°C pool with walking in 15°C air.

The Ci4400 Weather-Ometer uses a xenon lamp with a nominal maximum power of 6.5 kW; the Ci5000 uses a 12 kW xenon-arc lamp. These high-powered lamps are required to run at standard irradiance levels since the exposed samples are further away from the lamp than in smaller air-cooled instruments. Air is just not effective enough to cool these high-wattage lamps and maintain the ideal operating temperature throughout the entire lifetime of the lamp. Therefore, they need water cooling. Since they have relatively smaller capacity and samples are closer to the xenon lamp, air-cooled instruments like SUNTEST and Xenotest use xenon lamps between 1.5 and 2.2 kW nominal power – so air is just fine as a cooling agent.

More Information

To learn more about xenon instruments and technology, check out our recorded seminar on xenon-arc-technology and weathering applications.

For more information and a deeper dive into weathering and lightfastness technology and applications, have a look into our online library.

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