Optical measurement of acoustic resonance frequencies in HID lamps

The operation of high-pressure discharge lamps at higher than line frequencies can be hindered by acoustic resonance. 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. Acoustic resonance frequencies have previously been measured by a number of techniques, including external sound measurement and visual observation of arc instabilities. An unobtrusive acoustic frequency measurement technique is presented in this paper. The small-signal frequency response of fluctuations in light intensity, as measured with a broadband photodiode, to lamp power with intentionally introduced bandlimited noise can be used to probe the acoustic frequencies without major disruption of the operating lamp. The purpose of this paper is to explain, in detail, the experimental technique and its usefulness. The experimental setup and procedure are described. Measured frequency responses, from 10 kHz to 60 kHz, are presented for selected metal halide lamps. This work is put in context with past experimental and theoretical efforts