Table of contents

This special issue ofJournal of Physics D: Applied Physics covers some of the applied physics currently being addressed by light source researchers. Most of these papers originate from presentations made at the 10th International Symposium on the Science and Technology of Light Sources, held in Toulouse, France, on 18--22 July 2004. The conference attracted about 400 participants from all over the world. Abstracts of all papers, including extended abstracts of invited papers, were published in the conference book Light Sources 2004 (Institute of Physics Conference Series 182) edited by G Zissis.

The full papers published in this issue reflect the wide-ranging nature of research on light sources. These collected full papers survived our rigorous journal review process and they report completed, previously unpublished, pieces of work. This is a research field that has seen many major contributions over the last hundred years; nevertheless it continues to produce new sources and important improvements to existing types.

It is evident from these papers just how crucial are the materials used for light sources. Numerous recent lamp developments have depended critically on new or improved materials. Ten or more of the papers are concerned explicitly with materials: for example, an environmentally important and challenging area of research is to find a viable alternative to the use of mercury, particularly in fluorescent lamps. This is difficult because a substitute for Hg in general lighting must match the remarkable efficiency of Hg, or risk doing harm to the environment through increased energy consumption.

Large-scale computation of the properties of both high- and low-pressure discharge lamps has become indispensable. A good example is in the paper by Derra et al, which is the first major review of an important lamp type. The lamp of interest is a mercury arc used in data projectors, operating with an arc gap of approximately 1 mm at more than 100 bar pressure. Most of the information needed for design is not accessible experimentally. Computer models were essential tools in many aspects of development, including finding out how the highly stressed materials could be used in ways that ensured acceptable life.

Short-arc metal-halide lamps are used for producing white light in commercial premises, but they present numerous challenges to developers. Exceedingly complex chemistry, the many emitters and absorbers, the lack of symmetry, and the increasing influence of electrodes as the arc is shortened—all these pose many design problems that are being tackled with increasing success with the help of computer models. This special issue has many papers that deal with electrode issues, with special emphasis on the experimental verification of model predictions. Radiation transport also continues to be a major issue in constructing realistic discharge models. A number of papers tackle this in a simplified manner, whilst a new ray-tracing scheme offers hope of a realistic calculation of radiation in the complicated short-arc metal-halide lamps mentioned above.

Light-emitting diodes are now having a major impact on some areas of lighting; two papers in this issue reflect that trend.

The group of papers in this special issue will surely provide an important resource for those researching new and improved lamps.

Projection systems have found widespread use in conference rooms and other professional applications during the last decade and are now entering the home TV market at a considerable pace. Projectors as small as about one litre are able to deliver several thousand screen lumens and are, with a system efficacy of over 10 lm W⁻¹, the most efficient display systems realized today. Short arc lamps are a key component for projection systems of the highest efficiency for small-size projection displays.

The introduction of the ultra high performance (UHP) lamp system by Philips in 1995 can be identified as one of the key enablers of the commercial success of projection systems. The UHP lamp concept features outstanding arc luminance, a well suited spectrum, long life and excellent lumen maintenance. For the first time it combines a very high pressure mercury discharge lamp with extremely short and stable arc gap with a regenerative chemical cycle keeping the discharge walls free from blackening, leading to lifetimes of over 10 000 h.

Since the introduction of the UHP lamp system, many important new technology improvements have been realized: burner designs for higher lamp power, advanced ignition systems, miniaturized electronic drivers and innovative reflector concepts. These achievements enabled the impressive increase of projector light output, a remarkable reduction in projector size and even higher optical efficiency in projection systems during the last years.

In this paper the concept of the UHP lamp system is described, followed by a discussion of the technological evolution the UHP lamp has undergone so far. Last, but not least, the important improvements of the UHP lamp system including the electronic driver and the reflector are discussed.

A solution method for three-dimensional radiation transport using finite-element and ray-tracing techniques is described. It solves the radiation transport equation for local thermodynamic equilibrium (LTE) and non-scattering gases in three-dimensional systems without system geometry constraints and with complex radiation spectra. The method considers finite volumes whose spectral emission intensities are transported by a set of isotropic rays. The pre-calculated distances travelled by a given ray within each traversed volume are stored and used later to calculate the optical path when the radiation transport equation is solved. As the rays travel, their spectral power distributions are attenuated and the energy absorbed by each finite volume is used to compute the divergence of the radiation flux vector. Adding the spectral power distribution that remains on the rays leaving the system gives the spatially integrated spectral power distribution.

This paper investigates the reliability of deducing the emissivity at the peaks of a self-reversed emission line from a simple empirical one-parameter approximation for the source function. The theory of spectral line shape formation owing to self-absorption in inhomogeneous axially symmetric plasma layers was reformulated and readily calculable expressions were obtained. In the case of self-reversed lines, the emissivity at the line maximum was calculated using different relative distributions for the densities of the absorbing and independently emitting atoms. The results were compared with those calculated using a source function described by a simple exponential law, the exponent of which is known as the inhomogeneity parameter. The obtained difference in the emissivity is less than 3%, which implies a difference in the (density-ratio) temperature between the line-levels better than 0.5%. Therefore, if the inhomogeneity parameter is known, the line emissivity can be deduced from the one-parameter approximation for the source function with reasonable accuracy. The effect of the structure of the plasma layer on the emissivity as well as the lateral dependence of the inhomogeneity parameter was also studied through numerical simulation.

In this paper the influence of electrode composition, buffer gas fill pressure and control gear on the performance of metal halide lamps is investigated. It is shown that pure tungsten electrodes improve lumen maintenance and reduce voltage rise over lamp life. An optimum buffer gas fill pressure condition is discovered which allows for reduced electrode erosion during lamp starting as well as under normal operating conditions. Use of electronic control gear is shown to improve the performance of metal halide lamps.

Polycrystalline alumina is an excellent ceramic material for use as the envelope for metal halide discharge lamps. Although this material was introduced in the mid-1960s, and is thus already known for several decades, recent years have seen considerable effort aimed at further development of these ceramic envelope materials.

Developments are not only in the field of ceramic shaping technologies, but are also concentrated on the material properties of the ceramic material itself. Optical, mechanical as well as the chemical properties of the ceramic envelope are strongly controlled by the shape as well as the microstructure of the ceramics used.

Several analytical techniques have been used to obtain a detailed mass balance of a test series of metal halide lamps, containing sodium and scandium iodide. The lamps were made to burn for 5000 h and showed appreciable sodium loss, up to 45% at the end of 5000 h.

In burners dosed with an excess of scandium metal the loss of sodium caused an increase in the amount of scandium iodide. Thermodynamic modelling showed that scandium, present as a silicate, is converted into scandium iodide. This implies that the loss of sodium will cause a significant shift in the composition of the salt pool in the burner and this will affect the light technical properties and lifetime determining phenomena. Sodium that has escaped from the arc tube has a certain propensity for being deposited on the metal frame in the outer bulb. Since it is a very effective electron emitter, the emission of electrons by photo- and thermionic emission will be strongly enhanced. It is argued that this must have a significant effect on the sodium loss process and on the interpretation of experiments done in the past to reveal the mechanism of sodium loss. A two-phase model for sodium loss is proposed.

Residual stresses in the seal material connecting the metallic feedthrough and the ceramic body of high pressure sodium lamps have been investigated. Anisotropy of the residual stresses were examined experimentally with Vickers indentation and were also modelled with the finite element modelling method. The two techniques resulted in good qualitative agreement. No microcrack formation or other microscale change happened during ageing, but relaxation of the stresses took place. Application of the technique based on counting the cracks initiated by Vickers indentation is described. The method is capable of showing relative changes in the stress level of samples after various ageing times. The method is also capable of testing systems with characteristic sizes of even less than 1 mm.

Transparent ceramic materials with optical qualities comparable to single crystals of similar compositions have been developed in recent years, as a result of the improved understanding of powder-processing-fabrication- sintering-property inter-relationships. These high-temperature materials with a range of thermal and mechanical properties are candidate envelopes for focused-beam, short-arc lamps containing various fills operating at temperatures higher than quartz. This paper reviews the composition, structure and properties of transparent ceramic lamp envelope materials including sapphire, small-grained polycrystalline alumina, aluminium oxynitride, yttrium aluminate garnet, magnesium aluminate spinel and yttria–lanthana. A satisfactory thermal shock resistance is required for the ceramic tube to withstand the rapid heating and cooling cycles encountered in lamps. Thermophysical properties, along with the geometry, size and thickness of a transparent ceramic tube, are important parameters in the assessment of its resistance to fracture arising from thermal stresses in lamps during service. The corrosive nature of lamp-fill liquid and vapour at high temperatures requires that all lamp components be carefully chosen to meet the target life. The wide range of new transparent ceramics represents flexibility in pushing the limit of envelope materials for improved beamer lamps.

In high intensity discharge (HID) lamps getters are generally used to remove gaseous impurities that otherwise may have negative effects on the lamp characteristics.

Hydrogen, in particular, is a dangerous impurity because it can induce ignition problems and, in lamps with a quartz burner, it can diffuse from the outer bulb into the arc tube and cause early corrosion of the quartz.

Recently a new getter, suitable to work in compact HID lamps operated at high temperatures, has been developed: this getter is an appropriate solution to sorb hydrogen at high working temperatures in lamps where the use of a conventional getter is inadequate because of its small capacity for gettering hydrogen, its relatively high hydrogen equilibrium pressure and problems associated with the evaporation of getter and container material.

The newly developed getter, named high temperature getter (HTG), is based on a special alloy enclosed in a niobium container. The new alloy is very effective in sorbing hydrogen in the temperature region of 500–900°C.

It has been proved that by introducing the HTG the processing of high-wattage compact HID lamps can be simplified and a better lamp performance can be obtained.

Infrared (IR) continuum radiation from the arc of high and ultra-high pressure (UHP) mercury lamps was measured and modelled. Three major contributions to the IR continuum are electron–atom bremsstrahlung, which is dominant, electron–ion bremsstrahlung and electron–ion recombination radiation. The line width of the resonance broadened Hg 7¹S₀ to 6¹P₁ transition at 1014 nm was used to determine the arc core Hg density, and the radiance of this line was used to determine the arc temperature as a function of radius. The temperature map for the UHP lamp was checked using the Bartels method on the 546 nm line of Hg. Model results based on recently published electron–atom bremsstrahlung coefficients were found to be in good agreement with measurements across the near IR.

A series of 150 W quartz metal-halide test lamps containing four different chemistries has been studied with optical emission spectroscopy and x-ray absorption imaging. The four chemistries are pure Hg, Hg–HgI₂, Hg–NaI and Hg–NaI–DyI₃. Core temperatures and comprehensive distributions of Hg vapour densities were measured and combined to obtain comprehensive gas temperature distributions. The concentrations of additives in these specially designed test lamps were found to be much smaller than is typical for a commercial metal-halide lamp. As a consequence, the core temperatures in all lamps are largely characteristic of a pure Hg discharge. The gas temperature distributions have been used to determine power losses resulting from thermal conduction through the Hg vapour. The fraction of total input power dissipated thermally was found to be 0.49 ± 0.01 in pure Hg, similar to published measurements for such lamps. In the Hg–NaI and Hg–NaI–DyI₃ lamps, thermal losses are 0.41 ± 0.01 and 0.42 ± 0.01, respectively. The Hg–HgI₂ lamp has thermal losses of 0.29 ± 0.03.

This is an effort to characterize the lowest possible order approximation to how HID lamps of different wattages should scale. We are looking for broad, simple relationships that connect a lamp's power to its dimensions for ceramic HID lamps of constant low-aspect ratio. We consider a class of lamps in which the arc-tube size and power vary together over an arbitrary range, but the operating pressure, temperature profiles and gas chemistries are all as identical as possible throughout the range. For this class of lamps, we characterize the scaling relationships between the rated power P and the arc-tube inner radius R. Corresponding wall-loading expressions are also derived.

High-intensity discharge (HID) lamps have widespread and modern areas of application including general lighting, video/movie projection (e.g. UHP lamp), street/industrial lighting, and automotive headlight lamps (D2/xenon lamp). Even though HID lamps have been known for several decades now, the important plasma–electrode interactions are still not well understood. Because HID lamps are usually operated on ac (electrodes switch alternately from anode to cathode phase), time-dependent simulations including realistic and verified anode and cathode models are essential. Therefore, a recently published investigation of external laser heating of an electrode during anode and cathode phase in an operating HID lamp [28] provided the basis for our present paper. These measurements revealed impressive influences of the external laser heating on electrode fall voltage and electrode temperature. Fortunately, the effects are very different during anode and cathode phase. Thus, by comparing the experimental findings with results from our numerical simulations we can learn much about the principles of electrode behaviour and explain in detail the differences between anode and cathode phase. Furthermore, we can verify our model (which includes plasma column, hot plasma spots in front of the electrodes, constriction zones and near-electrode non-local thermal equilibrium-plasma as well as anode and cathode) that accounts for all relevant physical processes concerning plasma, electrodes and interactions between them. Moreover, we investigate the influence of two different notions concerning ionization and recombination in the near electrode plasma on the numerical results. This improves our physical understanding of near-electrode plasma likewise and further increases the confidence in the model under consideration. These results are important for the understanding and the further development of HID lamps which, due to their small dimensions, are often experimentally inaccessible. Thus, modelling becomes more and more important.

From the experimental finding that the cathodic plasma boundary layer in front of a thermionically emitting cathode is independent of the bulk plasma, the conclusion is drawn that the power flux density and the current density from the boundary layer to the cathode can be reduced to functions which depend only on the cathode temperature and the cathode fall. To advance the calculation of these so called transfer functions an already existing model of the cathodic plasma boundary layer consisting of a space charge sheath and a pre-sheath is reconsidered. The latter is split into a zone in which the ion current is formed and into an ion acceleration zone. A closed expression is deduced for the ion current density with regard to the back diffusion of neutralized particles from the cathode but with disregard of mass inertia. The electron temperature in the boundary layer is related to the cathode temperature and the cathode fall by the power balance of the electrons in the boundary layer. Special properties of the cathodic boundary layer of an argon arc are given. They are calculated with rate coefficients which are evaluated with cross sections from the literature. Numerical results are presented showing the dependence of the transfer functions on the cathode fall, gas pressure, work function of the electrode material and on the properties of the rare gases neon, argon, krypton and xenon.

Based on a model for the plasma boundary layer of high intensity discharge cathodes, simulations are performed and compared with experimental results. To solve the power balance of the cathode body different methods are used, namely a 1D integral solution as well as 1D, 2D and 3D finite-element calculations. The simulations are done for cylindrical tungsten cathodes operated in different pure noble gas discharges (0.1–1.0 MPa) and with currents between 0.5 and 10 A. Under these conditions different modes of arc attachment are found, both in simulations and experiments.

For the diffuse mode of arc attachment an excellent quantitative agreement between measurements and the simulations is obtained, reflecting an improved accuracy of measurements and simulation. In addition, different spot modes are found. At least one of these modes is also observed in the experiment. Also for this spot mode the agreement between measurements and simulation for the integral quantities is good. There are still some open questions concerning the spot mode of cathodic arc attachment.

Varying the geometric dimensions of the cathode, the proper simulation of the heat conduction problem of the cathode body is shown. Variations of the plasma properties, like gas type and pressure, prove the conceptional capability of the boundary layer model for the simulation of different modes of arc attachment. Evaluating the cathode fall characteristics, regions of existence for the different modes are found, which are similar to the experiments.

To investigate the properties of the plasma boundary layers of an argon arc in front of high intensity discharge (HID) electrodes a special spectroscopic measuring system was set-up in front of a model lamp. It is characterized by a high spatial resolution and a high degree of automation of the measuring procedure. Measurements were performed at the boundary layers in front of anodes and cathodes made of pure and thoriated tungsten with diameters from 0.6 to 2 mm operated with currents from 0.5 to 10 A at pressures from 0.1 to 0.3 MPa. The electron temperatures T_e were deduced from Boltzmann-plots of population densities of a couple of excited states just below the ionization limit, which were obtained from measured line intensities. Electron densities n_e were determined from measured continuum intensities. In front of the anodes, maxima of T_e and n_e were found with T_e of the order of 12 000 K and n_e of the order of 5 × 10²¹ m⁻³, indicating strong deviations from local thermal equilibrium. In the diffuse mode of cathodic arc attachment, steep gradients are measured with values of T_e of the order of 12 000 K and of n_e of the order of 5 × 10²² m⁻³. The measured temperatures in the boundary of the spot mode of cathodic arc attachment exceed 22 000 K. The electron temperatures in front of the cathode are in accordance with the results of modelling.

This work is concerned with investigation of the main features of current transfer to cathodes under conditions characteristic of metal halide (MH) lamps. It is found that the presence of MHs in the gas phase results in a small decrease of the cathode surface temperature and of the near-cathode voltage drop in the diffuse mode of current transfer; the range of stability of the diffuse mode expands. Effects caused by a variation of the work function of the cathode surface owing to formation of a monolayer of alkali metal atoms on the surface are studied for particular cases where the monolayer is composed of sodium or caesium. It is found that the formation of the sodium monolayer affects the diffuse mode of current transfer only moderately and in the same direction that the presence of metal atoms in the gas phase affects it. Formation of the caesium monolayer produces a dramatic effect: the cathode surface temperature decreases very strongly, the diffuse-mode current–voltage characteristic becomes N–S-shaped.

Electrode temperature diagnostics and a two-dimensional electrode model have been developed to improve our understanding of electrode behaviour in ceramic metal halide lamps. Using transparent YAG arc tubes in dc and ac operation, anode and cathode characteristics, like the effective work function, anode fall and electrode input power, could be derived from the measured temperature profiles. It is found that Dy-iodide in the metal halide lamp filling has a strong so-called gas-phase emitter effect. In order to improve our understanding of the observed phenomena and to help design electrodes for future lamps, a rotational symmetric two-dimensional electrode model has been created. The model is completely phase resolved so that time-dependent effects can be studied (both ac and dc). Furthermore, it contains various options for calculating the power input distribution, including a complex cathode sheath model as well as a simple anode model. The model has been shown to predict spot/diffuse transitions (in Hg-lamps with heavy electrodes/low currents) very similar to the behaviour seen in real lamps.

A numerical model of the radiative transfer in high pressure metal halide discharge lamps is used to determine line broadening parameters for atomic scandium lines. The determined broadening constants are in qualitative agreement with theoretical estimates in many cases, but significant deviations exist. The data obtained from this paper can, therefore, be used to further improve modelling of radiative contributions to the energy balance in such types of discharge lamps.

The effect of an additional electrode heating by laser light exposure of one electrode has been studied in a high-pressure mercury discharge. The exposed electrode operates as cathode or anode during the corresponding half-cycles of a square-wave current which has been used for driving the discharge. The additional heating influences the discharge voltage and the electrode tip temperatures in different ways during the cathode and anode phases. Only during the cathode phase is there a noticeable decrease in the discharge voltage connected with a moderate increase in the cathode tip temperature. The maximum absorbed laser power of 6.2 W causes a voltage decrease of about 3.5 V for a discharge current of 1.8 A. The exposure during the anode phase is characterized by a strong increase in the electrode tip temperature and a nearly unchanged discharge voltage. Theoretically determined parameters of the cathode boundary layer are given. For a constant discharge current the power balance at the cathode surface suggests a nearly linear dependence of the discharge voltage lowering on the laser input power, which has been verified experimentally.

Based on perspectives of the development of semiconductor materials systems for high-power light-emitting diodes (LEDs), an illumination facility for greenhouse plant cultivation was designed with the dominating 640 nm photosynthetically active component delivered by AlGaInP LEDs and supplementary components from AlGaN (photothropic action, 455 nm) and AlGaAs (photosynthetic 660 nm and photomorphogenetic 735 nm) LEDs. Photosynthesis intensity, photosynthetic productivity and growth morphology as well as chlorophyll and phytohormone concentrations were investigated in radish and lettuce grown in phytotron chambers under the LED-based illuminators and under high-pressure sodium (HPS) lamps with an equivalent photon flux density. Advantages of the high-power LED-based illuminators over conventional HPS lamps, applicability of AlGaInP LEDs for photosynthesis and control of plant growth by circadian manipulation of a relatively weak far-red component were demonstrated.

In this paper, the typical characteristics of a planar excilamp based on KrCl^* and XeCl^* exciplex molecules are presented. The excitation of the working mixture Kr/Xe/Cl₂ is realized by means of the surface barrier discharge at pressures of 0.1–1 bar. The following properties are measured and discussed: spectra emitted by the plasma in the UV/VIS/NIR spectral range, intensity of emitted light versus total pressure in the discharge, the composition of the working mixture and the power of emitted light. The radiation power versus input electric power, and space distribution of the emitted light including basic electrical parameters of the discharge were also measured. It was shown that the characteristic power of UV radiation emitted in the spectral range 200–400 nm is about 6 mW cm⁻² while the efficiency could be about 8%.

Simple-geometry exciplex lamps of a novel type excited by a capacitive discharge (CD-excilamps) have been investigated. An efficient radiation has been obtained on KrBr^*, KrCl^*, XeBr^*, XeCl^* molecules and I^* atom. The highest values of efficiency of various working molecules are approximately 10–18%. The lifetime of the operating gas mixture in KrCl^*, XeCl^*, I^* and XeBr^* exciplex lamps excited by a CD is above 1000 h. Owing to the above-mentioned characteristics, the exciplex lamps excited by a CD are supposed to be very promising for various applications.

Colour-perception differences under illumination by two quadrichromatic solid-state sources of light have been studied with respect to a tungsten radiator with the same correlated colour temperature (2600 K). A virtual RYgCB source (illuminant), which contains red, yellow–green, cyan and blue components with the line width typical of AlGaInP and AlInGaN light-emitting diodes (LEDs), was fully optimized for the highest value of the general colour-rendering index (CRI) (R_a = 98.3). An implemented RAGB source (lamp) contained commercially available red, amber, green and blue LEDs (R_a = 79.4). Colorimetric calculations in the Commission Internationale de l'Eclairage 1976 (u',v') colour plane for 40 Munsell colour samples (value 6, chroma/6, hue increment 2.5) revealed the differences in hue discrimination and distortion for both sources in the yellow–green and blue–cyan ranges. These differences were not revealed by the standard analysis of the special CRIs and were lower for the RYgCB illuminant, which contained primary LEDs in the sensitive ranges. A psychophysical experiment on seven subjects was performed using the RAGB lamp stabilized against thermal and ageing drifts. Despite different colour-perception abilities of the subjects under investigation, the experiment confirmed the calculation results. Methods of obtaining composite white light with high subjective ratings are discussed, based on the obtained data.

In this paper, we demonstrate the possibilities that exist in developing a high brightness white light source. The lamp employs mercury at a few Torr and is operated with short pulses of the order of 1 µs at a frequency of 10 kHz. The emission spectrum is atomic in nature and the white light is the outcome of a relative enhancement of the mercury yellow lines at 577 and 579 nm with respect to the rest of the visible lines, which shifts the colour coordinates of the source towards the black body locus of the chromaticity diagram. The pulse operation of a lamp containing mercury at a vapour pressure of 20 Torr offers a greater near-UV and visible output compared to a phosphor-uncoated, low-pressure pulsed compact mercury discharge.

The small bulb of the tungsten halogen lamp often operates at several hundred degrees Celsius and an internal pressure higher than atmospheric pressure. The cold lamp contains a gas filling at about 3 bar; the pressure during operation is much higher than that. The objective of our project is to analyse the internal operating conditions (such as the pressure and the thermal behaviour) of certain halogen lamps. The actual measurement of the operating conditions is far from easy. Rather, a computer model has been developed to evaluate the operating conditions in these lamps, based on the substantial partial differential equations of the free convection problem. This model can help determine the temperature distribution, the velocity field and the pressure in these lamps.

The free convection has been simulated by the following two appropriate equations: the convection–conduction equation—it is the heat transfer equation for convecting materials—and the incompressible Navier–Stokes equation. We could get the temperature field from our results, which helped us to calculate the operating pressure of the lamps. The pressure dependence on dimensionless characteristic numbers was evaluated.

We describe an improved method and apparatus for making capacitive measurements of the cathode fall in fluorescent lamps employing known behaviour of anode oscillations to provide a zero-of-potential reference, placing the entire cathode and anode fall waveform on an absolute rather than relative scale. The improved method is applicable to any diameter of fluorescent lamp currently manufactured. We also describe a method and apparatus for making spectroscopic measurements of the cathode fall in fluorescent lamps. This uses the abrupt onset of emission of certain selected spectral lines of the rare gas filling as a signal that the cathode fall has exceeded the excitation potentials of the spectral lines in question.

This study is focused on the energy consumption parameters of large, higher-density lighting installations such as factory halls, commercial centres, offices and many other space-limited areas. Current oscillations are related only to installations with the discharge lamps supplied by standard magnetic ballasts. Those oscillations occur under normal operating conditions as a result of the presence of a good quality resonant circuit formed by the complex net impedance and the compensation capacitors of installed luminaries. Those oscillations cause power losses which can lead to the malfunction of the power grid.

Presented here are the studies of the four different types of luminary wirings: parallel compensation, DUO (serial) compensation, mixed wiring and group compensation. The objective is to choose the wiring arrangement with the best combination of serviceability and consumption parameters. Measurement of real systems were used for analysis and verification.

Electrically-fused quartz glass, flame-fused quartz glass and plasma-fused quartz glass as well as synthetic fused silica samples were irradiated stepwise with a high energy Xe barrier discharge excimer lamp at 172 nm. VUV spectra were measured before and after every irradiation step. The results show that the VUV transmittance and the resistance against high energy radiation strongly depend on the quartz glass type, as well as on the thermal pretreatment of the quartz glass samples. In electrically-fused and plasma-fused quartz glass the VUV transmission decreases by the formation of oxygen deficiency and E' centres with absorption bands at 163 nm and 215 nm. Best irradiation resistance is found in synthetic fused silica and in thermally treated flame-fused quartz glass.

Photoluminescence spectra measured under excitation with a KrF excimer laser before and after irradiation indicate fundamental differences in the SiO₂ network structure of the different quartz glass types. Whereas a poor radiation resistance correlates with a blue photoluminescence band at 390 nm, the photoluminescence of flame-fused quartz glass changes from blue to green by a thermal treatment which is correlated with a significant improvement of radiation resistance. A simplified model is presented referring to hydride and oxygen deficiency centres as precursors to colour centre formation in different types of quartz glass.

Luminescent Y₃Al₅O₁₂ : Eu³⁺ (YAG : Eu³⁺) and Y₃Al₅O₁₂ : Tb³⁺ (YAG : Tb³⁺) powders with nanometre and micrometre-sized particles and films have been successfully synthesized by sol–gel route from alkoxide precursors. Advanced YAG coatings were achieved by spray and dip-coating methods, giving rise to different structural and optical properties. High temperature x-ray diffraction, x-ray powder diffraction, scanning electron microscopy and atomic force microscopy were used to investigate the materials structure and morphology. Pure polycrystalline YAG powder was obtained from 900°C. Laser induced luminescence spectra as well as luminescence decays of Tb³⁺ and Eu³⁺ ions were recorded. This study showed, undoubtedly, the spectral features of an Eu³⁺ emitting centre embedded in a unique site of D₂ symmetry. Furthermore, the well-known cross-relaxation phenomenon was observed for Tb³⁺. Decay times were found to be exponential and shorter than 5 ms for both Eu³⁺ and Tb³⁺ activators. Finally, the spectroscopic properties of YAG : Tb³⁺ powders under vacuum ultra violet excitation were analysed and encouraging results were obtained regarding luminescence efficiency and time constants.

Y₂O₃ : Eu and Zn₂SiO₄ : Mn are the red and green phosphors actually used in plasma display panels. These phosphors have been prepared by spray pyrolysis synthesis and thermal post-treatments. Their optical properties in the vacuum ultraviolet (VUV) energy range have been investigated. The absorption coefficients have been extracted from electron energy loss spectroscopy. The luminescence properties of phosphors from spray pyrolysis have been characterized under UV or VUV excitations with different dedicated experimental set-ups: excitation spectroscopy, measurements of luminescence efficiencies under Hg lamp, pure Xe and Ne–Xe discharges, determinations of luminescence decay times under plasma excitation. The characteristics have been compared with those of corresponding commercial phosphors.

Radial emission properties of high-frequency electrodeless discharge lamps (HFEDLs) in argon–mercury mixtures are investigated both numerically and experimentally. The radial profile of the intensities of the mercury triplet lines 404.7, 435.8 and 546.1 nm are measured for two different values of the high frequency generator power. A model describing physical processes in an HFEDL, including the calculation of radial plasma parameters, is developed. Radial intensity dependences of the lines 404.7, 435.8 and 546.1 nm are calculated and are found to be in good agreement with the experimental measurements.

In this paper, numerical and experimental investigations of high-frequency (HF) electrodeless lamps in argon–mercury mixtures are performed. The intensities of the mercury spectral lines having wavelengths λ = 404.66, 435.83, 546.07 nm (7³S₁–6³P_0,1,2) and the resonance line λ = 253.7 nm (6³ P₁–6¹S₀) are measured at a wide range of mercury pressures, varying the HF generator current and argon filling pressure. A stationary self-consistent model of HF electrodeless discharge lamp is developed including kinetics of the excited mercury and argon atomic states. Based on the developed model, the radiation characteristics of the discharge plasma are calculated. Numerical simulation of the line intensities behaviour in dependence on the mercury pressure, HF generator current and argon filling pressure is performed. The model results are in qualitative agreement with the experimental data. The calculations of the relative intensities of the visible triplet lines 7³S₁–6³P_0,1,2 are presented for the first time in this paper.

The high energy ion bombardment during instant start of a fluorescent lamp (FL) leads to intense sputtering of the electrode material including tungsten and emitter. Thus, a cold started FL often suffers from early failures due to coil fracture. The main goal of this paper is to investigate tungsten erosion. We have employed the ultra-sensitive method of laser-induced fluorescence. This technique is particularly well-suited to determining absolute population densities of neutral and singly ionized atoms of liberated electrode material. In addition to FL, our investigations have been performed also on hollow cathode lamps (HCLs). These are useful because they provide a variable source of sputtered tungsten atoms and can serve as tuning tools for precise adjustment of the laser radiation.

We will present absolute atomic tungsten population densities in a commercial FL and in an HCL. Furthermore, the results of a theoretical investigation of the argon plasma and the tungsten density in the HCL are represented.

In this paper, the shift and splitting of the energy levels in the circular polarized alternating electric field were modelled for an Ar atom. Moreover, calculations of the transition probabilities needed for obtaining the spectral line intensities and lifetimes were carried out. An anisotropy of the probabilities for transitions between Stark levels was investigated.

The decay rate of Hg 6 ³P₁ atoms, due primarily to the escape of trapped 254 nm resonance radiation, was measured as a function of both Hg and Ar density in cylindrical, sealed fused silica cells. Time-resolved laser-induced fluorescence at 254 nm was used to obtain the decay rates for Hg densities from 5 × 10¹³ to 7 × 10¹⁵ cm⁻³ (22–101°C cold spot temperatures) and Ar densities from 0 to 9.7 × 10¹⁷ cm⁻³ in a cell of inner radius 1.05 cm. These new experimental data are compared to Monte Carlo results from a highly realistic code for simulating radiation trapping. This code includes Voigt profiles from a combination of Doppler, resonance, buffer gas and radiative line broadening, as well as hyperfine and isotopic structure with proper collisional redistribution. Upper limits on the rate constants for the quenching of Hg 6 ³ P₁ atoms by collisions with ground level Ar and Hg atoms were derived. Additional Monte Carlo simulations covering cell radii from 0.1 to 3.0 cm were performed. A broadly applicable engineering formula is derived from this study, which predicts the Hg 6 ³P₁ decay rate from 254 nm resonance radiation trapping as a function of Hg and Ar densities and cell radius.

Nitrogen is well known as a UV-emitter because of its 2nd positive band lying around 350 nm. However, it is a molecular gas and when energy is stored in rotational or vibrational levels, the gas temperature can become high enough to melt even the glass tube. To avoid such problems the authors tried to use the Penning-like effect between the argon metastables and nitrogen C ³Π_u states with very low partial pressure of nitrogen compared with that of the argon. By adding a very small amount of nitrogen to argon, the discharge changes drastically. Though pure argon discharge and pure nitrogen discharge are both unstable and easy to constrict, a small amount of nitrogen improves the stability of the discharge and emits the nitrogen 2nd positive band UV light by energy transfer from the argon metastable to the nitrogen molecule C ³Π_u state. Nitrogen-added argon fluorescent lamps operated at 30 kHz of alternative square voltage wave achieve about 4500 cd m⁻² at 3.75 W and at 9.3 kPa of pressure with good dimming characteristics. The maximum luminance should be higher than this because the input power is limited by the power supply. At this condition the luminous efficacy achieves 7.1 lm W⁻¹. These results show a possibility of argon–nitrogen fluorescent lamps as mercury-less fluorescent lamps.

The replacement of mercury in conventional fluorescent lamps by other components is highly desirable for environmental reasons. This paper gives a short review of new types of mercury-free plasma light sources operating at low pressure. In particular, the features of cylindrical glow discharges in rare-gas mixtures including xenon are discussed, focusing on the generation of the 147 nm resonance radiation of xenon and its transition into visible light by new phosphors with sufficient efficiency. Laser absorption and vacuum ultraviolet emission spectroscopy are applied for several rare-gas mixtures to reveal the contributions of the different gas components and their excited states to the power balance and radiation output. The experimental research is assisted by theoretical studies applying self-consistent hybrid models of the cylindrical column plasma. The good agreement between experimental and theoretical results obtained at selected discharge conditions makes it possible to predict optimum discharge parameters by means of extensive model calculations. It is demonstrated that about half of the efficacy of a mercury fluorescent lamp can be reached if the rare-gas discharge is operated at pressures below 100 Pa.

An experimental and theoretical study on ³He–Ne and ⁴He–Ne helical hollow cathode lasers is presented. Enhanced laser operation on the near IR NeI lines is observed when the natural isotope ⁴He is substituted by the lighter isotope ³He. A four-fold increase in the laser output power and a three-fold increase in the laser gain for the strongest NeI 1.1523 µm line is measured in the ³He–Ne gas mixture compared to the ⁴He–Ne gas mixture.

On the basis of the theoretical analysis done by means of a non-stationary kinetic model for the negative glow plasma of ³He–Ne and ⁴He–Ne hollow cathode lasers, a study on the changes in the particle kinetics is carried out and an explanation of the experimental results is proposed. In the ³He–Ne mixture the electron temperature is lower than in the ⁴He–Ne mixture, while the gas temperature is higher. As a result the helium triplet metastable density and the rate constant for excitation transfer to neon atoms are higher in the ³He–Ne mixture. The lower laser level de-excitation due to intra-multiplet mixing of 2p_1–10levels by ³He atoms is more efficient.