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High pressure Cs and Na light sources

Spectrum of high pressure Cs and Na light sources were compared and the advantages and disadvantages of both are discussed. It is well known that high pressure sodium lamps are covering almost entire globe, giving it a nice yellow color. This means that the blue portion of the sodium plasma is not well filled by either atomic or molecular features and therefore the color reproduction is not satisfactory. However, a high pressure cesium pulsed discharge plasma offers much better spectrum giving almost perfect white light appearance that gives excellent color rendering index. It would be already in general use, but the efficacy of cesium lamps does not exceed 50 lm/W, whereas sodium lamps reach 100-150 lm/W range. In order to investigate the differences in efficacy between Na and Cs lamps we compared their visible and their infrared spectra for the case of 70 W nominal powers but using the voltage from 180 V to 240 V. We found that although the Cs lamp is an excellent white light source it is in the same time an excellent emitter in the near infrared spectral region, which actually causes relatively low efficacy. We also measured the time evolution of spectral features of both light sources, starting from the ignition up to the stable burning condition. Namely, we measured the spectrum 10 times in each second and very precisely investigate the spectral phenomena and processes in plasma. This is very important for the better understanding atomic and molecular processes in plasmas. These will be used for the optimization of the future high pressure light sources. At present the new light sources are in the very rapid phase of development. These are White Light Emitting Diodes, ESL lamps and High Intensity discharges (HID). We used Cs HIDs with burners of pure sapphire (5 mm in diameter) or alumina (3 mm in diameter). The narrower alumina burner gave almost entirely continuum visible spectrum from the cesium plasma having temperature of almost 4000 K. In that case all atomic lines have been self absorbed, and the continuum in the visible originated from the electron-ion recombination emission. We used digital spectrometer OceanOptics (3600 pixels) for the visible and StellarNet (512 pixels) for the infrared spectral regions. We also used several different pulsed power supplies to observe the resulting spectra. In addition to this we surrounded the lamps with aluminum foils and various stainless steel meshes in order to see the effect on visible and infrared spectra. The spectra have been taken from the ignition to the final stable state of burning in order to follow how xenon spectral lines emerge and then disappear, and how the cesium spectra lines first appear, attain maxima and then disappear in the form of self-reversed shape. We also followed the time evolution of the recombination spectra and some interesting molecular features that were used for particle number diagnostics.

Cs; Na; spectrum; light

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