A NEW EFFECTIVE HIGH QUALITY LIGHTING SYSTEM: QL THE ELECTRODELESS COMPACT FLUORESCENT LAMP

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1 A NEW EFFECTIVE HIGH QUALITY LIGHTING SYSTEM: QL THE ELECTRODELESS COMPACT FLUORESCENT LAMP A. de Bijl, J. van Kemenade, J. Schlejen and H. Wessels Philips Lighting BV Central Development Laboratories P. O. Box JM Eindhoven The Netherlands The principle of light generation by induction has been applied in the development of a new electrodeless lighting system: QL. A short description of the main components of the system is given. The QL lighting system combines the benefits of HF electronic fluorescent lighting with a compact shape of the light source and offers a high luminous flux and an extreme long life time ( hrs). This combination of properties eliminates in many lighting designs the need to compromise and offers, due to the life, tremendous new possibilities in lighting solutions. An overview of the application potential of the QL lighting system is given together with the luminaires employed. INTRODUCTION To create the desired luminous environment the lighting designer needs freedom of choice regarding to luminous flux, light colour, colour rendition, size and shape of the luminaires. Compact light sources are highly preferred to allow freedom in design of the luminaires. Although a large variety of compact HID lamps is on today's market the applicability of these systems is limited, inherent to the properties of HID lamps regarding instant start, hot restrike and the limited choice of colour and colour rendition. Compact fluorescent lamps with all the advantages ( instant light, free choice of colour, free choice of colour rendition, easy switching and dimming) of fluorescent lamps are not available with a luminous flux over 3000 lumen. This makes the choice for a compact luminaire using a flux of about 5000 lumen (which is often used for an office or commercial general lighting scheme) a tedious one in which one has to compromise between HID or twin or multi lamp Compact Fluorescent Luminaires. Due to the lifetime of these lamps the design of compact luminaires has to allow for easy relamping of the system, putting some limitations on the design of the luminaires. This illustrates the need for a compact light source combining the features of fluorescent lighting (instant light, choice of colour and colour rendition) with those of compact HID sources (compactness and high luminous flux). Electrodeless light sources fulfil this demand by offering many attractive features over existing light sources, both to luminaire manufacturers, architects, light designers and end-users. Among others, the most important features due to the absence of electrodes, are: the exclusion of the main lifetime determining element of the lamp and, thus, a large extension of lamp life. a better lamp maintenance, due to lack of material sputtered-off from the electrode. freedom in lamp (and luminaire) design and the possibility to create new innovative light sources and lighting designs. Although the idea of electrodeless lamps by Tesla (1) is 100 years old, until now, no practical electrodeless light sources for general lighting purposes have been available. The main problems that prevented a commercial introduction were size (temperature), electronics, electro-magnetic interference (EMI) and price aspects. However, exactly 100 years from Tesla, due to large developments in electronics and modern phosphors, Philips Lighting has been able to develop a commercially available light source, based on the inductive electrodeless discharge: the QL induction lighting system. In this paper the principles of this inductive lighting system will be described. Then an overview will be given of the characteristics, features, benefits and applications of this new QL lighting system. THE PRINCIPLES OF INDUCTIVE DISCHARGE LIGHTING From a physical point of view, electrodeless discharges can be classified into four types: (1) capacitive E- discharges, (2) inductive H-discharges, (3) microwave discharges and (4) travelling wave discharges. A very

2 useful overview and short explanation on the four electrodeless discharge types has been given by Wharmby (2). Induction lighting systems do not contain electrodes; the energy transfer to the discharge is by means of a Power Coupler that is shown in Fig. 2. The power coupler contains an induction coil, wound around a ferrite core. The current through the coil induces a magnetic field H (Fig. 1). The ferrite core intensifies the field. The alternating magnetic field induces an alternating electrical field (at 2.65 MHz), perpendicular to the magnetic field lines. The electrons are accelerated in the electrical field to high energies and collide with the gas particles. This results in a circular electrical current in the plasma (Fig. 1). The phenomena which then occur in the plasma are similar to those in a common low pressure mercury (Hg) fluorescent lamp. The gas plasma contains Hgatoms (as active medium) and a noble gas as buffer gas. By collisions with electrons, the Hg-atoms are excited to a higher energy level (some are ionized, also), from which they fall-back by emission of an Ultra Violet (UV) photon. These UV-photons are converted into light by the phosphor on the bulb wall and stem. The electrical operation of the power coupler and lamp can be considered as a transformer. The induction coil in the power coupler acts as the primary coil. The secondary coil is formed by the circular current in the discharge; this 'plasma' coil has one turn only. DESIGN OF THE QL SYSTEM. The QL system is composed from four main components: The High Frequency (HF) generator supplies a HF current to the Power Coupler The Power Coupler which transfers energy into the QL lamp The QL lamp in which the plasma generates UV which is converted to light by the phosphors. The luminaire which directs the light into the desired directions and provides electrical and mechanical shielding By connecting the power coupler and the HF generator with a coaxial cable a physical separation is possible which allows for maximum flexibility in luminaire design and reduces the thermal load on the HF generator. Design of the QL-lamp and Power Coupler The description of the main parts of the system is given in Fig. 2. In the development of the QL-lamp several problems had to be solved. First, due to the high lamp wall temperature, using only pure Hg would result in a Hg-pressure in the lamp being higher than the pressure needed to generate an optimal luminous flux. Therefore the Hg-pressure is controlled by an amalgam, located in one of the exhaust tubes. Moreover, the use of an amalgam has the advantage of stabilizing the luminous flux over a broad temperature range. To achieve a fast run-up of luminous flux after ignition an auxiliary amalgam is placed on the lamp stem. Secondly, due to the compactness of the QL-lamp, special measures are taken to prevent over-heating of several parts of the lamp. The power balance of the QL-system is given in Fig. 3. From these data it follows that much of the energy, dissipated on the inner glass stem wall, needs to be transported through the power coupler towards the flange in order to maintain a sufficiently low temperature (T < 250 o C) of the glass parts. Therefore a high thermal conductivity towards the flange is necessary. To achieve this two measures have been taken: Filling the space between stem and power coupler with a silicon rubber material increases the thermal conductivity between stem and power coupler. Using a heat-pipe in the power coupler. The special design of the heat-pipe assures its functioning in any burning position of the lamp. Applying these measures, the stem temperature (and all other parts in the power coupler) are well below the limit, noted above (T stem = ca. 200 o C).

3 Thirdly, EMI interference from the system has to be minimized. This is done by locating the induction coil inside the lamp instead of outside. In this way, the inductive discharge itself has a screening effect for the interferences from the coil. Furthermore, the induction coil is bifilary wound: the (second) bifilary winding screens the (main) winding. The luminaire design aspects to minimize EMI will be further discussed in section 3.4. Fourthly, due to the high wall loads it was necessary to apply a protective layer between phosphor and glass bulb wall, to obtain a good maintenance during the long lifetime. The necessity of the protective layer is clearly demonstrated by Fig. 4, in which the maintenance of QL-lamps with and without protective layer are compared. To convert the UV into light, any of the modern narrow band phosphors (exhibiting a high maintenance over the life time of the system) can be chosen, thereby giving a large freedom of choice of lamp colour temperatures from 2700 K to 6000 K can be made. Design of HF-generator The HF-generator system consists of the following parts, which are indicated in the block diagram of Fig. 5. Radio Frequency Interference (RFI) filter preconditioner (including buffer capacitor) power oscillator 2.65 MHz The RFI filter is one of the measures to ensure that the system complies to the EMI standard (CISPR 15). The preconditioner, is designed similar to HF fluorescent ballasts and provides: - constant DC output voltage, independent of mains voltage variations - low mains current distortion (acc. IEC 555-2) Besides these functions the preconditioner blocks mains transients and reduces the influence of line interruptions. The HF-power oscillator generates and controls the HF power supplied to the power coupler hence to the discharge. In Fig. 6 the main parts of the power oscillator are shown. The circuit is a self-oscillating half bridge circuit. Both power MOS transistors (Q1,Q2) are alternately driven and generate a square wave voltage. This voltage supplies the tank circuit (La,C1,C2) with energy via the ballast choke Ls. The tank circuit consists of the antenna coil (La) and capacitors (C1,C2). The tank circuit is loaded by the gasdischarge lamp. Using the transformer model the combination of lamp and power coupler can be approximately simulated by the coil (primary of the transformer) and a series circuit of a resistor (Rd) and an inductor (Ld) representing the "transformed" discharge. The square wave voltage (VHB), lamp voltage (VPC), choke current (ILS) and gate drive voltage (VGS) are given in Fig. 7. In this circuit the efficiency of the HF power oscillator depends solely on the quality of the components used and on the timing for the switching of the transistors, so that no detectable switching losses occur. For Ls a coreless type is chosen in order to avoid ferrite losses. In order to keep ferrite losses in the power coupler La to a minimum its temperature must be kept as low as possible. So the ferrite core of La is heatsinked.(see 3.1) The efficiency of the 85 Watts QL generator is 90% Ignition: At starting the power coupler merely acts as a coil over which a limited high voltage is generated. This high voltage starts the ignition process in the gas discharge vessel. Lifetime of electrical circuit: The electrical design includes low component stress to achieve a low failure rate.(i.e. lower D.C. voltage, ripple current, lower ambient temperature) The specified systems lifetime is calculated and verified by testing at: - high ambient temperature of 55 o C and at - fast switching (cycle 1 s on / 1 s off).

4 SPECIFICATION OF QL INDUCTION LIGHTING-SYSTEMS. These design principles result in a QL-system as specified in Annexe I. This specification can be described in terms of following features and related benefits: Features Benefits No electrodes -ultra long life - low maintenance (cost) - low retrofit cost / long lamp replacement cycle - high reliability - new luminaire constructions possible -low lumen depreciation - maintained performance -no sparks - increased safety -low component temperatures - wide choice in material for luminaire constructions High efficacy - low operating costs HF-operation - no flicker - no stroboscopic effects - universal mains supply - light output independent from mains fluctuations Instant start and restart - suitable for security/safety lighting Colour of De luxe Fluorescent - freedom of choice in colour - easy matching with other light sources High colour rendering index - easy colour recognition - offers natural, appealing colour rendition Amalgam controlled operation - light output nearly independent of ambient temperatures / wider field of applications Universal burning position - wider choice of lamp / luminaire position Conventional bulb shape with - applicable in existing luminaires optics uniform luminous intensity - opportunity to create with luminaire optics distribution several kinds of light distributions - freedom in luminaire optics Separate HF generator - flexible positioning of components - freedom of luminaire construction DESIGN OF LUMINAIRES FOR QL-LIGHTING SYSTEM These benefits must be maintained in the final lighting system which comprises also the optical and mechanical components. To maintain proper functioning of the system two aspects should be considered. Thermal aspect As indicated in 3.1 the heat dissipated in the power coupler is transported by the heat pipe towards the flange. Therefore this flange must be accommodated by the luminaire in such a way that at the ambient temperature at which the system is likely to operate the amalgam and the stem temperature is in the proper temperature range. To assure the life time of the electronic components the HF-generator must be placed in the luminaire in such a way that the temperature of the electronics is as low as possible. In any case the temperature of a test point located at the housing of the electronics should not exceed 65 o C Electromagnetic compatibility In order to comply fully with the EMI requirements two solutions can be applied. The luminaire or the reflector is metallic and the centre of the QL-lamp remains within these metal parts which are to be earthed. If no metallic part is surrounding the lamp an earthed screen has to be installed around the lamp in the form of a metal wire mesh (Faraday cage). The performance as a function of temperature of a typical QL Lighting system (QL-lamp system and luminaire) is shown in Fig. 8: the almost constant luminous flux over a wide temperature range (achieved by way of the amalgam) is shown. The compact size of the QL, its life time, which is as long as a typical lighting installation combined with the constant luminous flux over a large temperature range (amalgam control) offers new horizons for luminaire

5 design. Luminaires can be designed in a compact form, sealed for life, providing an optimal maintenance of the optical components and be furnished with all kinds of functional and/or decorative add-ons such as louvres, grids, coloured parts etc. To illustrate the flexibility of the QL system a number of luminaire designs will be discussed Basic Unit: This is the assembly of QL-lamp, power coupler and HF generator with a provision to absorb the heat flow to the flange. To comply with the EMI regulations the lamp is shielded by a metal grid which is earthed. This unit is suitable for integration into the constructing elements of the application. Fig. 9a. Down light: In such luminaires the system is provided with a reflector which directs the light downward. The light distribution depends on the choice of reflector and/or diffusing/refracting elements. Moreover by adding asymmetric reflectors a wall lighting function can be incorporated. Depending on the application more functional and/or decorative elements such as louvres, cones or coloured parts can be added to embellish the luminaire. The very compact design of this luminaire (diameter 26 cm) is only possible by the amalgam control of the luminous flux. Fig. 9d & 9f. A example of a semi recessed version of a larger shape is given in Fig. 9e. Pendant Luminaire: Such luminaires are in a way similar to semi recessed down lights. The HF-generator can be located at the top of the luminaire. Fig. 9c. Up lighter: In such kind of luminaires the QL-lamp can be located in the top compartment of a free standing up lighter whereas the HF generator can be located in the lower part of the up lighter to allow for flexibility in design. Post tops: In these luminaires the QL-system is placed into a larger unit which directs and diffuses the light in such a way that it supplies the vertical and horizontal illumination in residential or shopping areas. Since no internal maintenance of the system is required a vandal proof design can be used.fig. 9b. Floor recessed: In this luminaire the QL-lamp is incorporated into a reflector and the light is directed into an upward direction. The housing, which is fully enclosed, can be recessed into the floor. A reflector on top directs the light to the surroundings providing a horizontal and vertical illumination. Fig. 9g. APPLICATIONS OF THE QL LIGHTING SYSTEM. Commercial lighting In applications such as shops, merchandising banks, public buildings, hotels, atriums etc. the lighting contributes in an important way to the ambience and atmosphere of the building. Up to now in general lighting schemes luminaires employing 4 and 5 feet fluorescent tubes limit the designer to very structured layouts of luminaires. The best available fluorescent luminaires in this respect are 2*2 feet square luminaires. Down lights with HID lamps offer more freedom but lack the flexibility of fluorescent lamps such as colour, colour rendition, instant start, hot restrike and lifetime. The QL-system integrates the compactness of HID but with the benefits of HF-fluorescent lighting. Moreover a large extension of the lifetime and a stable luminous flux is added. The range of QL-luminaires fulfil in a most elegant way most of the lighting functions needed in these application fields. For example where high vertical illuminance are needed such as in entrance zones or on gondolas the asymmetric down light is the appropriate choice. In higher bay applications such as shop markets, lobbies and atriums the more decorative pendant or suspended luminaires can add to the style and image of the building. Moreover the extended lifetime is advantageous in the applications where access to the luminaire is difficult, therefore costly. The very long life of the QL-system makes it very suitable for integration within the elements of shop design such as the shop's furniture, the displays and decorations. Office Lighting.

6 Here the traditional long fluorescent luminaires which have dominated the ceiling layout so long can be replaced by down lights from the QL type which not only allow more freedom in positioning but also offer the possibility of decorative add ons to the basic lighting function. The freedom of design offers the creation of dynamic lighting schemes depending on the task. Moreover due to the compact size the veiling reflections of luminaires in VDU screens is easier to solve by slight changes in the position of the screen. In up lighting schemes up to now halogen lamps (low efficacy) or HID-lamps ( no instant start, moderate colour rendering, limited life) are often used. The QL-system eliminates all these drawbacks and offers a perfect solution. The instant start makes it suitable for a dynamic use in task dependent switching systems. Public Lighting In these applications a reliable maintenance free lighting system is most needed, where access to the luminaires is difficult and the security and safety of the public depends on the proper functioning of the system. The QL post top luminaires fulfil these needs by offering a reliable maintenance free residential area lighting system with a constant light quality. Moreover it allows white light with good colour rendition in streets which adds comfort and ambience not only to the residents but also to local shops in a shopping centre. Tunnel lighting In tunnel lighting relamping is extremely complicated due to the blocking of the traffic to allow free access to the luminaires. Here QL lighting systems provide a reliable long life time which highly reduces the operation cost of a tunnel lighting system. The flexible positioning of components allow a design tailored to the specific tunnel arrangement. Floodlighting In walking areas such as parks or in the floodlighting of small objects such as trees, walls, statues, signs, advertisements etc. the QL system can be fully or partly integrated into the building, pavement or other outdoor elements due to its maintenance free operation. In these cases an esthetic integration within the surround can be achieved. The Floor recessed QL luminaire described will allow high quality lighting. Also in this case the additional benefits are no maintenance costs and free choice of lamp colour. Miscellaneous The very long life time makes the QL system ideal for all applications where access to the luminaire is difficult or expensive. Examples of these are: High positioned warning sign for aircraft Platform lighting in the petrochemical and chemical industry In these latter two cases the absences of sparks on ignition and because all components in the system are at relatively low temperatures allows for an easy construction of explosion proof luminaires by employing sealed for life constructions.

7 CONCLUSIONS The development of the electrodeless QL induction lighting system offers new opportunities in lighting design. It is the combination of the benefits of electronic HF fluorescent lighting regarding colour, colour rendition, instant start, high efficacy with a compact shape and a high luminous flux through a very long life time. This eliminates in many lighting designs the need to compromise and offers, due to the life, tremendous new possibilities in lighting solutions. REFERENCES 1. Tesla, N., 1891, Elec. Engineer 7, Wharmby, D.O., 1989, "Review of Electrodeless Discharges for Lighting", 5th Int.Symp. of Science and Techn. of light sources, York. Annexe I: Data for the QL-system (at 100 h.). Luminous flux 5500 lumen System power 85 W System efficacy > 65 lumen/w Colour point 3000 K or 4000 K Colour rendition (CRI) > 80 Start within 0.5 s Run-up (luminous flux) > 30% after ignition. > 80% after 60 sec. Maintenance > 85% at h. > 70% at h. Ambient temperature range -20 o C to + 65 o C Supply voltage 200 V V Power factor > 0.85 EMI according to EN 60555/2, EN and FCC-18 (class A). Lifetime h. (i.e. < 20% mortality) Size - lamp diameter 110 mm, length 190 mm - generator 130 x 130 x 45 mm

8 Fig. 1 Schematic view of Power coupler and gas discharge for QL lamps. The electric plasma current and the electric and magnetic fields are shown too. Fig. 2 Schematic view of the main parts of the QL-lamp. Fig. 3 Power balance of the QL-system. Fig. 4 Luminous maintenance as function of life time for QL-lamps with and without protective layer.(100 h.=100%)

9 Fig. 5 Block Diagram of HF-generator. Fig. 6 Main parts of HF-power oscillator. -VHB 50 V/div - VGS 5 V/div - VPC 200 V/div - ILS 0.4 A/div time ( 50 ns/div.) Fig. 7 Main waveforms of power oscillator. Fig. 8 Performance of a typical QL Lighting system: the luminous flux versus the ambient temperature.

10 Fig. 9 A collection of QL lighting luminaires a: basic unit b: post top c: pendant luminaire d: down light e: semi recessed downlight f: down light g: floor recessed