Stable high radiance in visible and near-ultraviolet wavelengths is desirable for radiometric calibration sources. In this work, newly available electrodeless radio-frequency (RF) driven plasma light sources were combined with research-grade, low-noise power supplies and coupled to an integrating sphere to produce a uniform radiance source. The stock light sources consist of a 28 VDC power supply, RF driver, and a resonant RF cavity. The RF cavity includes a small bulb with a fill gas that is ionized by the electric field and emits light. This assembly is known as the emitter. The RF driver supplies a source of RF energy to the emitter.

In commercial form, embedded electronics within the RF driver perform a continual optimization routine to maximize energy transfer to the emitter. This optimization routine continually varies the light output sinusoidally by approximately 2% over a several-second period. Modifying to eliminate this optimization eliminates the sinusoidal variation but allows the output to slowly drift over time. This drift can be minimized by allowing sufficient warm-up time to achieve thermal equilibrium. It was also found that supplying the RF driver with a low-noise source of DC electrical power improves the stability of the lamp output. Finally, coupling the light into an integrating sphere reduces the effect of spatial fluctuations, and decreases noise at the output port of the sphere.

The RF-driven lamps have several advantages over traditional calibration sources. Currently, accurate radiance measurements can be made at infrared and the red portion of the visible wavelengths using tungsten filament-style FEL lamps. However, the blackbody output of these lamps is limited to 3,000 K, and intensity falls exponentially at shorter wavelengths at the blue end of the spectrum. For reproduction of the solar spectrum, with an equivalent blackbody temperature of 6000 K, the blue and ultraviolet wavelengths have typically been produced using high-pressure xenon arc discharge lamps. These lamps achieve the high temperature necessary in a narrow filament of ionized gas between two electrodes. This ion channel suffers from instabilities produced by buoyancy-induced turbulence of the surrounding gas. There is also longer-term drift associated with the sputtering of electrode material through ion impact, which changes both the electrode spacing and surface profile. Due to the high electric field gradients, these small changes in geometry result in nonnegligible changes to the light output.

Additionally, much of the sputtered electrode material is deposited as a thin layer on the inner surface of the lamp. This decreases light transmission through the glass and ultimately limits the useful life of the lamp to no more than 1,000 hours, over the course of which the radiant flux may decrease by a factor of two. Additionally, the xenon lamps generate several undesirable sharp emission lines with large intensity variation over a small spectral range. The electrode-induced instabilities are eliminated in the RF lamp, and the lifetime is not limited by electrode erosion. The higher operating pressure of the RF-driven bulbs produces a smoother broadband spectrum. The RF lamps are also more efficient, and have more conducive geometry for coupling their light into an integrating sphere.

This work was done by Brendan McAndrew and John Cooper of Goddard Space Flight Center; and Angelo Arecchi, Greg McKee, and Christopher Durell of Labsphere, Inc. GSC-16399-1


NASA Tech Briefs Magazine

This article first appeared in the December, 2012 issue of NASA Tech Briefs Magazine.

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