Time resolved measurements of HID lamp acoustic frequency spectra

As more electronic ballasts for HID lamps continue to be designed, built, and marketed, more power electronics experts are faced with the difficulties of powering a vexing nonlinear load. Power electronic devices can be reduced in size and cost by operating at higher than line frequencies, but the HID lamp suffers front acoustic instabilities at these frequencies. Designers of electronic ballasts have been interested in determining the frequencies of destabilizing resonances to avoid in the lamps, or conversely, the allowed frequency bands and threshold levels for modulation components in the lamp power. In addition to full operation at rated power, designers need to be concerned with avoiding resonances while the lamp state is changing. Two examples of interest are the state of the lamp as it changes through warm-up and the state of the fully warmed lamp as the power changes through a full cycle at line frequency. Acoustic resonance frequencies have previously been measured by a number of techniques, including external sound measurement and visual observation of arc instabilities. Refinement of a recently-developed, unobtrusive, acoustic frequency measurement technique is reviewed in this paper. This technique is used to compare the changes in the acoustic spectra under different modes of operation. The small signal frequency response of fluctuations in light intensity, as measured with a broadband photodiode, to lamp power with intentionally introduced bandlimited noise is used to probe the acoustic frequencies without major disruption of the operating lamp. Externally gating the capture of the time signals can track the changing lamp state and phase resolve slower waveforms. The experimental setup and procedure are described. Measured frequency responses are presented for experimental metal halide lamps. The summary establishes the context of this work with past experimental and theoretical efforts.