Xenon[edit]
Xenon, operated as a 'neon light,' consists of a collection of mostly spectral lines, missing much of the continuum radiation needed for good
color rendering.
Spectral line radiation from a xenon flashlamp. Although invisible to the naked eye, the digital camera is able to image the strong, IR spectral-lines, which appear as the blue light reflected off the table.
As with all ionized gases, xenon flashtubes emit light in various
spectral lines. This is the same phenomenon that gives
neon signs their characteristic color. However, neon signs emit red light because of extremely low current-densities when compared to those seen in flashtubes, which favors spectral lines of longer wavelengths. Higher current-densities tend to favor shorter wavelengths.
[13] The light from xenon, in a neon sign, likewise is rather violet. The spectrum emitted by flashtubes is far more dependent on current density than on the fill pressure or gas type. Low current-densities produce narrow spectral-line emission, against a faint background of continuous radiation. Xenon has many spectral lines in the UV, blue, green, red, and IR portions of the spectrum. Low current densities produce a greenish-blue flash, indicating the absence of significant yellow or orange lines. At low current-densities, most of xenon's output will be directed into the invisible IR spectral lines around 820, 900, and 1000 nm.
[14]Low current-densities for flashtubes are generally less than 1000 A/cm2.
Higher current-densities begin to produce
continuum emission. Spectral lines broaden and become less dominant as light is produced across the spectrum, usually peaking, or "centered", on a certain wavelength. Optimum output-efficiency in the visual range is obtained at a density that favors "greybody radiation" (an arc that produces mostly continuum emission, but is still mostly translucent to its own light; an effect similar to sunlight when it passes through a cloud). For xenon, greybody radiation is centered near green, and produces the right combination for
white light.
[9][11]Greybody radiation is produced at densities above 2400 A/cm2.
Current densities that are very high, approaching 4000 A/cm2, tend to favor
black-body radiation. Spectral lines all but disappear as the continuum radiation dominates, and output center shifts toward the ultraviolet. As current densities become even higher, visually, xenon's output spectrum will begin to settle on that of a blackbody radiator with a
color temperature of 9800 kelvins (a rather sky-blue shade of white).
[1] Except in cases where intense UV light is needed, such as water decontamination, blackbody radiation is usually not desired because the arc becomes opaque, and much of the radiation from within the arc can be absorbed before reaching the surface, impairing output efficiency.
[11][14][15]
Due to its high-efficient, white output, xenon is used extensively for photographic applications, despite its great expense. In lasers, spectral-line emission is usually favored, as these lines tend to better match
absorption linesof the lasing media. Krypton is also occasionally used, although it is even more expensive. At low current-densities, krypton's spectral-line output in the near-IR range is better matched to the absorption profile of
neodymium-based laser media than xenon emission, and very closely matches the narrow absorption-profile of Nd:YAG.
[16][17] None of xenon's spectral lines match Nd:YAG's absorption lines so, when pumping Nd:YAG with xenon, the continuum radiation must be used
