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The very first devices designed to test light-fastness durability of materials used the carbon arc light source to simulate sunlight. The first Atlas Colour Fade-Ometer®, introduced in 1919, used an enclosed carbon arc (ECA) light source. The spectral emission in the UV bears little resemblance to sunlight. For the enclosed carbon arc, two strong emission bands, peaking at 358 nm and 386 nm, are much more intense than natural sunlight. This type of light source can be expected to have a weaker effect than solar radiation on materials that absorb only short wavelength UV radiation, because there is very little irradiance below 310 nm. But because of the strong emission bands, ECA exposures will have a stronger effect on materials that also absorb long wavelength UV and visible light.

The first open-flame carbon arc device, the Atlas Sunshine Carbon Arc Weather-Ometer® with Corex® filters, was introduced in the 1930s. The light produced by the Sunshine Carbon Arc provides more UV at wavelengths below 300 nm than natural sunlight but gives a better match than the Enclosed Carbon Arc at longer wavelengths. When used without filters for faster testing, stability rankings of some materials may be distorted when compared with outdoor testing. Therefore, in trying to evaluate the relative light stability of materials, some of which absorb only short wavelength UV and others that also absorb long wavelength UV, the carbon arc light source could distort the rankings when compared with samples exposed to solar radiation.

There is a vast amount of historical data on the use of carbon arcs, and a number of test methods still specify their use. While good correlation with outdoor exposures has been reported for some materials whose weathering mechanisms are appropriate for these limited spectrum sources, this technology has largely been replaced with fluorescent UV or xenon arc systems. However, even though these newer artificial weathering instruments have been developed to better simulate natural sunlight, the historical data collected, specifications, and the desire to avoid comparing "apples to oranges" has resulted in the limited, but continued, use of carbon arc devices for material durability testing.

Contact info@davidson.com.au for Atlas Carbon Arc supply packages.