PORTFOLIO

We developed through this project the first high-pressure discharge mercury-free lamp of high color rendering index, which is fitted with a static electrodeless bulb, so of very long lifespan (lifespan > 10 years @ 50%, neglectable fading). Generally speaking, the higher the pressure is, the more continuous is the spectrum of emission and, as a result, the better the color rendering. In most high-pressure discharge lamps of today, like in HID lamps (High Intensity Discharge) for instance, the light is produced by means of an electric arc between electrodes housed inside the bulb; so, the plasma is maintained at a fixed position, between the electrodes. But on the other hand, the use of electrodes declines the efficiency and reduces the bulb lifespan, for the condensation of their erosion makes the bulb less transparent. Moreover, issues of chemical compatibility with the electrodes limit the choice of active substance. These plasmas have mechanical behaviors strongly influenced by the free convection in absence of electrode. The hotter core rises up till touching the bulb upper wall and may make it melt. Indeed, luminous efficiency depends on temperature; for usual discharge and incandescent bulbs, the higher the temperature, the better the efficient. The solution adopted in sulfur lamps commercialized today to prevent the bulb from melting is to maintain it in rotation, which allows breaking thermal avalanche in the plasma through the effect of mixing [1]. But our innovative solution allows avoiding any mechanical movement, which is greatly advantageous in terms of costs and reliability (patent EP1876633A1, which has been priced by the World Intellectual Property Organization in 2011). The LG industry has launched on the market different versions of the sulfur lamp. This technology first appeared actually in the USA, in the nineties, under the label Solar-1000. In all these designs, the bulb is rotating. The former design reaches an outstanding efficiency, of 115 Lm/W at 1.4 kW, with full spectrum fitting the eyes sensitivity.

Because our sensibility of vision varies with wavelength, the choice of a light source requires a trade-off between color rendering and luminous efficiency; improvements in one are generally detrimental to the other. As sustainable development has become a priority, the manufacturing industry of light sources is putting high effort into improving the efficiency of lamps, in order to reduce our electricity consumption. Indeed, only a few decades after the massive launch of fluo-compact sources, which are much more efficient than incandescence sources, the global lighting market has been witnessing an increasing penetration of solid-state sources (LEDs), although their spectral qualities may be low. The white LEDs of the highest efficiency are subject to the so-called ‘green gap’ [2, 3]. The sulfur lamp allows outstanding efficiency and, unlike white LEDs, provides a continuous spectrum that contains all the colors. Moreover, it does not contain any toxic materials: no mercury unlike all discharge lamps of good color rendering and no arsenic nor gallium unlike most LEDs (gallium arsenide, gallium nitride). The plasma is of the inductively coupled type; the bulb is placed in the emission field of a magnetron that supplies 2.45 GHz microwaves. A metallic grid surrounds the bulb so to confine the field and insure compliance with regulations covering electromagnetic compatibility. Our innovation consists in modulating the magnetron in the radio frequency domain in order to put the plasma in ultrasonic resonance, which allows better performances [4]. In 2012, we have discovered an amazing phenomenon by pulsing the magnetron at a duty-cycle below 20%: a mode of resonance that gathers the plasma into a ball [5]. In this special mode of resonance, the plasma takes the shape of a ball of about half the bulb radius and settles down in the center of it as long as the resonance is kept. This phenomenon had never been observed before and is actually very furtive. The present project allowed to investigate this special acoustic resonance, said spherical, and its applications.

With a Correlated Color Temperature (CCT) of 5,3 kK, the light of our lamp is pleasing at typical indoor office illuminance levels of about 400 lux, but not below (Kruithof effect). The examination of the spectrum reveals a great correlation in the visible range between the light of the lamp and that of the sky in sunny condition. The second being the ultimate reference in terms of lighting, this result has oriented our search for commercial opportunities. Nevertheless, daylight is marked by large fluctuations. For this reason, we have considered two spectra of daylight: on the one hand, the CIE daylight standard illuminant of same CCT, the D53 for our plasma lamp for example, and on the other hand, the horizontal illumination by a clear sky with the sun at 30° above the horizon. We also calculated the luminous efficiency of the radiation to compare the ideal energy performance, without taking into account the performance of the power supply, which is not yet optimized in our lamp. The three color-indexes recommended by the CIE - Ra, Ra14 [6] and Rf [7] - were also calculated. The market prospection was achieved by comparing these characteristics with those of the most commonly used bulbs, including various white LEDs, warm white and cold white, as well as with a solar simulation plasma lamp provided by Lumartix. Our lamp arrives in the leading pack in terms of luminous efficiency of the radiation. The same goes for color rendering, with a ranking above the cold white LEDs and HID lamps for all the three indexes. Its Rf index also rivals that of warm white LEDs, while the latter are largely outside the "daylight" category. The solar simulation plasma lamp naturally has higher color indexes, but the efficiency of its radiation is less good and its CCT drops below 5 kK. The correlation study shows that the light from our lamp is the closest to daylight, whether in comparison with the solar spectrum or that of the standard illuminant; the correlation coefficient reaches 0.99 in both cases! It is therefore almost impossible for a user to distinguish the lighting of our lamp from that of a daylight in sunny weather, regardless of the colors of the objects in view. In comparison, LEDs do not offer such a good correlation with daylight in sunny weather, which is explained by the so-called 'green gap' - a lack of light around 480 nm, clearly noticeable in their spectra [8]. Thus, it is clear that our lamp is particularly well suited to produce a daylight type of lighting. Its nominal power of 1 kW is also suitable for this market segment since lights of high CCT are perceived as pleasing only at high illumination level (Kruithof effect). It would be ideal for places where strong lighting is desired to stimulate awakening, such as recreational areas for example, or to make ‘sunny' patches in landscaped offices, atriums or shopping centers in order to break their uniformity. Our lamp offers an optimal spectrum for the illumination of all environments where it is essential to supply a true full spectrum, ideal for human beings, as in professional lighting (studios, high definition filming and broadcasting, art galleries…) and in commercial malls, hospitals, schools as well as in stadiums and sport fields, especially if some events are to be broadcasted on television. Furthermore, our lamp would meet an ideal outlet in the lighting of workstations without daylight. According to the Labor Act, companies must take countervailing measures to meet health protection requirements for posts situated in blind parts of a building. Permanent workstations without daylighting may be tolerated if the requirement for natural lighting is disproportionate. And in this case, a compensation system must be installed, with a lighting of the workstation similar to daylight.

This applied research project was enabled thanks to the financial support of the Gebert Rüf Stiftung foundation and the efforts of a number of researchers from the University of Applied Sciences and Arts Western Switzerland (HEIG-VD). The project was followed by the company Lumartix, who has provided specific components of the prototype, like the light bulbs and the microwave applicator. Created in 2010, Lumartix is one of the startup companies that HEIG-VD has supported as a way to ease technology transfers to the economy stakeholders (http://www.lumartix.com/). In addition, this project provided an opportunity of collaborating with the University of California (UCLA), Department of Physics and Astronomy, allowing technical and scientific exchanges in the fields of microwave plasmas and sonoluminescence.

The project is completed, all of its modules being achieved. The demonstrator we built is fitted up with a fast photodiode for the modulation of the released signal measures the plasma vibration, which is much too fast for the eye at the resonance frequency, around 30 kHz. From the measurements of this frequency, we have assessed the average temperature in the bulb [5]. In addition, we have shown by modeling that ball formation results from acoustic resonance resulting in compression-expansion cycles with a spherical distribution of pressure having an anti-node in the center, a characteristic necessary for the plasma ball formation. The wrong modes of close frequencies were furthermore identified as well, for catching the spherical resonance is a matter of selectively coupling this particular mode. This is the role of the phase locking regulation that we have developed, which allows to synchronize the microwave pulses to the plasma oscillation with the right phase; the energy needs to be delivered to the plasma when the pressure in the center of the bulb peaks. Moreover, we found the conditions in which the spherical resonance comes up. Hence, we have been able to develop an algorithm of signal processing that detects the resonance in real time in order to trigger the phase locking regulation as soon as the resonance appears. The project was continued by prototyping the pulse generator. A new digital input imports the resonance detection, in order to activate automatically the phase regulation. This regulation synchronizes the pulses to the input signal with the optimal phase shift that we obtained from tries. A major difficulty has arisen because a considerable amount of noise appears in some signals. Though the voltage comparator make use of a Schmitt trigger, we found that the parasites caused sudden outbursts of requests to interrupt the Digital Signal Processor (DSP) used to generate the pulses. To overcome this problem, we make the output signal of the photodiode pass first through a lock-in amplifier HF2LI, which is fitted with a Phase Lock Loop (PLL). This amplifier outputs a sinusoid of adjustable amplitude, free of parasite, of same frequency as the plasma vibration once its phase is locked to the signal of the photodiode. Hence, we had to achieve some related software developments. The new control allows automatic activation of the phase regulation, which is triggered in that case as soon as the spherical resonance is detected if the HF2LI has lock-in. Thus, the resonance can be hung even if its passage is too stealthy for a manual triggering, a point that proved decisive.
The project finishes with the final tests and adjustments. Especially, the lock-in parameters were tuned. Without phase regulation, the plasma oscillation in resonance (compression-expansion cycles) desynchronizes from the microwave pulses. This free oscillation means that the production of acoustic energy is no longer governed by electromagnetic induction but by acoustic resonance in the bulb [5]. As a result, the pulse frequency can no longer be used as an external reference by the lock-in amplifier to demodulate the signal of the photodiode. However, this problem has been solved through the automatic reference mode provided by the HF2LI. Through its PLL module, the HF2LI can indeed synchronize itself to a noisy signal without external reference frequency even if the signal-to-noise ratio is weak, but on the condition that the parameters of the PLL are well adjusted. In our application, it was difficult to obtain a satisfactory compromise between stability and speed, or precision, especially because the modulation of the input signal crushes at the very moment the resonance comes up. A decisive solution has been obtained by adding a band-pass filter with amplification between the photodiode and the HF2LI, and a phase shift compensation in the latter. We took furthermore advantage of the control module of its PLL to take into account the aperiodic intermissions of the plasma vibration, managing to stabilize the PLL by optimizing its proportional and integral gains (P I). Finally, the tests showed that the two new desired functions are successfully implemented:
- Automatic launch of the phase regulation of the pulses at the spherical resonance appearance, then
- Synchronization of the pulses to the plasma oscillation with the desired phase.
Final results showed that the demonstrator allows to make appear the plasma ball and hold it on the long term, but only intermittently. The jumps that occur sporadically could probably be avoided by adjusting the P I gains in real time. Such an upgrade would indeed allow to increase the PLL accuracy as soon as the regulation is on, without any steadiness counterpart for the lock-in as the plasma does not show any chaotic passage in resonance.

[1] G. Courret, L. Calame, M. Croci, P. Egolf, A. Meyer, Environmental Friendly High Efficient Light Source, 14 Schweizerisches Status-Seminar 2006 Energie und Umweltforschung im Bauwesen, ETH Zürich, Switzerland.
[2] Nicolas Grandjean, Le principe de fonctionnement des LED blanches et les développements futurs, Revue spécialisée et informations des associations Electrosuisse et AES, bulletin 1/2014;
[3] Huaizhou Jin, Shangzhong Jin, Kun Yuan, Songyuan Cen, Color rendering and luminous efficacy analysis in YAG PC-LED by improved Gauss simulation, Optik 125 (2014) 4898–4902;
[4] G. Courret, L. Calame, J. Croisier, M. Croci, A. Meyer, Sustentation of a thermal plasma by acoustic resonance as a light source of high efficiency, Schweizerisches Status-seminar 2008 Energie und Umweltforschung im Bauwesen, Zürich;
[5] G. Courret, P. Nikkola, S. Wasterlain, O. Gudozhnik, M. Girardin, J. Braun, S. Gavin, M. Croci and P. W. Egolf, On the plasma confinement by acoustic resonance — An innovation for electrodeless high-pressure discharge lamps, Eur. Phys. J. D (2017) 71: 214;
[6] Commission internationale de l'éclairage, Technical Report, CIE 13.3 - 1995;
[7] Commission internationale de l'éclairage, Technical Report, CIE 224 : 2017;
[8] G. Courret, J. Failleìtaz, O. Gudozhnik, D. Almeida Dias, P. Nikkola, S. Wasterlain, Projet Sulfur Lamp for High-Quality Efficient Lighting, Rapport WP4&5&6, Haute Ecole d'Ingénierie et de Gestion du Canton de Vaud Yverdon-les-Bains, Switzerland, Février 2018

IRO Magazine n°21, February 2010, Lumière Solaire sur commande.
AGEFI, Jeudi 14 février 2013, N°31, Differentiation indispensable
Emission Biosphère on Radio Télévision Suisse, 21 mai 2010