Richard Garner

Propagation of ionization waves during ignition of fluorescent lamps

The propagation of the first ionization wave in a compact fluorescent lamp (T4 tube with standard electrodes) during ignition was investigated for various initial dc-voltages (both polarities measured against ground) and gas compositions (with and without mercury). In addition the effect of the presence of a fluorescent powder coating was studied. The propagation velocity of the initial wave was measured by an assembly of photomultipliers installed along the tube, which detected the light emitted by the wave head. The propagation was found to be faster for positive than for negative polarity. This effect is explained involving processes in the electrode region as well as in the wave head. Waves propagate faster in the presence of a fluorescent powder coating than without it and gases of lighter mass show a faster propagation than gases with higher mass.

Determination of absolute Ba densities during dimming operation of fluorescent lamps by laser-induced fluorescence measurements

Investigations of fluorescent lamps (FL) are often focused on the electrodes, since the lifetime of the lamps is typically limited by the electrode lifetime and durability. During steady state operation, the work function lowering emitter material, in particular, barium, is lost. Greater barium losses occur under dimming conditions, in which reduced discharge currents lead to increased cathode falls, the result of the otherwise diminished heating of the electrode by the bombarding plasma ions. In this work the barium density near the electrodes of (FL), operating in high frequency dimming mode is investigated using the high-sensitivity method of laser-induced fluorescence. From these measurements we infer barium loss for a range of discharge currents and auxiliary coil heating currents. We show that the Ba loss can very easily be reduced by moderate auxiliary coil heating.

Dynamics of plasma?electrode coupling in fluorescent lamp discharges

A time dependent model of a low pressure, mercury–rare gas discharge with thermionic electrode is presented. The model is applicable to ac-operated fluorescent lamps, which is the focus of this work. The model describes a one-dimensional negative glow plasma that is bounded on one side by a thermionic electrode and a sheath, and on the other by a positive column plasma. The electrode/sheath component of the model, together with the mutually interacting negative glow plasma, allows for self-consistent calculation of the electrode sheath potential. The model describes a smooth transition in the plasma parameters from electrode to positive column and thus reveals the spatial extent of the influence of the electrode and sheath processes. A detailed description of the model is presented, as well as results of calculations pertaining to a standard fluorescent lamp. Also shown are measurements from a 2 mm interferometer and an internal floating probe, both of which compare favourably with the calculations.

Interpretation of the external band technique for cathode fall measurements of fluorescent lamps

Cathode fall is a key parameter in assessing the performance of a fluorescent lamp since it is a fundamental part of a lamps operation and, at the same time, may be responsible indirectly for many undesirable effects, such as shortened life and severe end-darkening. This work addresses the familiar band diagnostic (BD), a technique often used to measure cathode fall. The diagnostic is an easily implemented, non-invasive procedure in which a metal foil is placed around the outer glass wall of a lamp in the electrode region, and the potential difference between the foil and electrode is measured. Unfortunately, there are severe limitations in interpreting the measurement. The goal of this work has been to properly conceptualize this diagnostic to improve understanding, to identify factors limiting interpretation and to identify features from experimental measurements. The result is a lumped parameter model, and a key component is a representation of the wall sheath at the plasma/wall boundary. Conceptually, the BD is identical to the internal floating probe, and therefore the model is applicable to that diagnostic as well. In addition to descriptions of the model and model calculations, comparisons are made between band and probe and between band and experimental measurements.

Comparison between millimeter wave interferometry and Langmuir probe measurements in low pressure discharges

Summary form only given. used to measure the electron density in a low pressure, rare gas plasma maintained in a cylindrical discharge tube. A comparison of measurements shows that the interferometer-derived density is greater than the probe-derived density. Initial measurements indicate that the ratio (interferometer/probe) increases with increasing gas pressure. Measurements were made over ranges of gas pressure and discharge current where the corresponding discharge was free of striations. Both diagnostics were expected to be valid in the parameter regime under investigation. Descriptions of the diagnostics, comparisons between the measurements, and suggestions for the discrepancies will be presented. The interferometer, which has a Mach-Zehnder configuration, employs a heterodyne detection system and has two free-running oscillators, both operating nominally at 75 GHz and separated in frequency by about 60 MHz. The radiation from one oscillator is split along two paths. In one path the radiation is directed to, and mixed with, radiation from a second (local) oscillator; this constitutes a reference signal. In the other path the radiation is frequency doubled to 150 GHz, directed through the discharge, and subsequently mixed with the local oscillator; this constitutes a test signal. The line-averaged density through the discharge is determined from the phase difference between the paths, which is given by the phase difference between the test and reference signals. Both probe and interferometer were scanned across the transverse direction of the cylindrical discharge tube. The 150 GHz (/spl lambda/=2mm) radiation that passes through the plasma has a fundamental Gaussian transverse mode with 6 min diameter beam waist that is located at the axis of the discharge tube (midway between transmitting and receiving antennae, which are 75 min apart). The probe and interferometer measurements were compared directly by integrating the probe-derived density over position to obtain line-averaged density. This was compared to the line-averaged density obtained by the interferometer for the same chord. In addition, density versus position was obtained from the interferometer data by assuming a cylindrically symmetric model for the electron density and finding free parameter(s) in the model which allow for the best fit, in the least squares sense, to the interferometer data. One assumed model was a constant multiplied by the density obtained from the probe. Other models included cylindrically symmetric polynomials containing terms of various order. Interferometer-derived densities were found to be fairly independent of these model choices. The probe and interferometer were compared by then considering the average density over the cross section of the discharge tube. These comparisons were quantitatively the same as those from the direct comparison cited earlier.

Accelerated optical polymer aging studies for LED luminaire applications

There is a need in the lighting industry to design and implement accelerated aging methods that accurately simulate the aging process of LED luminaire components. In response to this need, we have built a flexible and reliable system to study the aging characteristics of optical polymer materials, and we have employed it to study a commercially available LED luminaire diffuser made of PMMA. The experimental system consists of a “Blue LED Emitter” and a working surface. Both the temperatures of the samples and the optical powers of the LEDs are appropriately characterized in the system. Several accelerated aging experiments are carried out at different temperatures and optical powers over a 90 hour period and the measured transmission values are used as inputs to a degradation model derived using plausibility arguments. This model seems capable of predicting the behavior of the material as a function of time, temperature and optical power. The model satisfactorily predicts the measured transmission values of diffusers aged in luminaires at two different times and thus can be used to make application recommendations for this material. Specifically, at 35000 hours (the manufacturer’s stated life of the luminaire) and at the typical operational temperature of the diffuser, the model predicts a transmission loss of only a few percent over the original transmission of the material at 450 nm, which renders this material suitable for this application.

Time resolved measurements of cathode fall in high frequency fluorescent lamps

Measurements are presented of the time resolved cathode and anode falls of high frequency fluorescent lamps for a range of discharge currents typically encountered in dimming mode. Measurements were performed with the movable anode technique. Supporting spectroscopic emission measurements were made of key transitions (argon 420.1 nm and mercury 435.8 nm), whose onset coincide with cathode fall equalling the value associated with the energy, relative to the ground state, of the upper level of the respective transition. The measurements are in general agreement with the well-known understanding of dimmed lamp operation: peak cathode fall decreases with increasing lamp current and with increasing auxiliary coil heating. However, the time dependence of the measurements offers additional insight.

Quantification analysis of input/output current of interleaved power factor correction (PFC) boost converter

Quantification analysis of input/output current facilitates the optimal design of the interleaved PFC boost converter. This paper presents a method to calculate these currents both in switching cycle and mains half cycle for interleaved PFC AC/DC boost converter, which is valid for this converter with arbitrary number of phases operating at continuous conduction mode (CCM) and discontinuous conduction mode (DCM). The proposed method is straightforward and the time function can be derived with it in a very short calculation time. Moreover, it provides a foundation for further research. It can be applied to interleaved PFC converters with other topologies, such as flyback and Sepic. Experimental results validate the proposed method, as is shown.

Fabry–Perot measurements of barium temperature in fluorescent lamps

A scanning Fabry–Perot interferometer (FPI) is used to determine the temperature of barium atoms that are liberated from the electrodes of fluorescent lamps during their steady-state operation. Barium, a constituent of the work function lowering emitter material that is placed on the tungsten coil that forms the electrode, is liberated primarily by evaporation from the hot (~1300 K) thermionic electrode. However, there may be situations or modes of operation in which barium is, in addition, sputtered, a condition which may lead to increased end-darkening, shortened life and increased mercury consumption in the lamp. Using the FPI diagnostic, the occurrence of sputtering is inferred when barium temperatures are much greater than the electrode temperature. The FPI diagnostic senses resonance radiation (λ = 553 nm) emitted by barium atoms excited in the low pressure discharge environment, and infers temperature from the Doppler broadened linewidth. The diagnostic has proven to be successful in a number of situations. Measurements have been made on rare gas discharges and on Hg–argon discharges for different discharge currents, gas pressures and auxiliary coil currents. Measurements are phase resolved for ac-driven discharges.

Application of a 150 GHz interferometer to fluorescent lamps

A 150 GHz interferometer system has been developed, and is being used, to gain new insights into many aspects of fluorescent lamps discharges. This presentation describes results of the application of the interferometer to studies of the negative glow plasma, especially with regard to plasma-electrode coupling. In a related presentation, details are provided of the interferometers setup, function, and performance. Through measurements of the electron density in the vicinity of the electrode, the interferometer can sense effects of the beam electrons. These are a subset of the electrons which, after having been emitted from the electrode, acquire significant energy when traversing the electrode sheath, and emerge into the bulk plasma where they excite and ionize mercury atoms. Through this process, they create and sustain the near-electrode plasma. When lamp current is time-varying, the interferometer senses variations in bulk electron density that are, in part, related to variations in beam electron energy, a consequence of the variations in electrode sheath potential that are necessary for supporting the current changes. Thus, inferences can be drawn from interferometer measurements which provide insight into the this key plasma-electrode coupling mechanism. Augmentation by additional diagnostics (e.g., floating probes and electrode temperature), provides additional insight. In the course of these studies, electron density measurements were acquired for different modes of lamp operation, as characterized by variations in the driving current (e.g., waveform shape, frequency, amplitude) and by the presence or absence of auxiliary electrode heating. In addition, measurements were acquired over a range of positions in the lamp in order to see the contrasting behaviors of the near- electrode and positive column plasmas. Measurements were acquired during the plasma build-up time following ignition, the steady-state plasma, and the afterglow plasma. Many of these measurements are shown, and interpretations gleaned from these measurements, are discussed.