RF light sources follow the same principles of converting electrical power into visible radiation as conventional gas discharge lamps. The fundamental difference between RF lamps and conventional lamps is that RF lamps operate without electrodes .the presence of electrodes in conventional florescent and High Intensity Discharge lamps has put many restrictions on lamp design and performance and is a major factor limiting lamp life. Recent progress in semiconductor power switching electronics, which is revolutionizing many factors of the electrical industry, and a better understanding of RF plasma characteristics, making it possible to drive lamps at high frequencies.
The very first proposal for RF lighting, as well as the first patent on RF lamps, appeared about 100years ago, a half century before the basic principles lighting technology based on gas discharge had been developed.
1.1 DISCHARGE TUBES
Discharge Tube is the device in which a gas conducting an electric current emits visible light. It is usually a glass tube from which virtually all the air has been removed (producing a near vacuum), with electrodes at each end. When a high-voltage current is passed between the electrodes, the few remaining gas atoms (or some deliberately introduced ones) ionize and emit coloured light as they conduct the current along the tube. The light originates as electrons change energy levels in the ionized atoms. By coating the inside of the tube with a phosphor, invisible emitted radiation (such as ultraviolet light) can produce visible light; this is the principle of the fluorescent lamp.
We will consider different kinds of RF discharges and their advantages and restrictions for lighting applications.
There are three practical ways to energize RF light sources, though there are more ways to create RF plasma
2.1 CAPACITIVE RF DISCHHARGE
Capacitive RF discharge may be energised by RF electrodes placed inside or outside the discharge vessel .The current path in a capacitive RF discharge plasma is closed by displacement currents in the RF electrode sheaths (whether the electrodes are inside or outside the discharge vessel). Capacitive RF discharges operate at gas pressure considerably lower than atmospheric pressure and are exited by an RF electric field E with frequency lower than 1GHz wavelength l much larger than the discharge size L,(l >> L).
Due to electron depletion in the sheaths, the sheath impedance is much larger than the plasma impedance. Therefore, the voltage applied to the lamp is mainly dropped in the sheaths, and sheath impedance controls the discharge current of a capacitive RF discharge. A significant dc voltage is developed in the RF sheaths as a result of RF voltage rectification, and this leads to an acceleration of the plasma ions into the electrodes (or wall). This has important consequences that limit the application of the CRFD for RF lighting. The additional power loss of ion acceleration reduces RF lamp efficiency.
The CRFD power P is proportional to wVrf.,therefore, to achieve a significant lamp power; one must use a high RF voltage and/or a high frequency w. To reduce ion sputtering, the RF voltage, Vrf applied to the lamp electrodes should be lower than 100-200V. Since large RF voltages are prohibited because of ion sputtering, the practical application of a CRFD for lighting is limited to low power and/or relatively high frequency operation.
Example of a CRFD –based lamp is shown in figure 2. This is an RF –driven sub miniature (a few millimetres in diameter) fluorescent lamp with a cold cathode. Increasing its driving frequency from 50kHz in standard application to 40 MHz results in a significant reduction in power loss at the electrode sheaths. Sheath power loss decreases with frequency.
2.2 INDUCTIVE RF DISCHARGE
In an inductive RF discharge, the plasma RF current is closed within the plasma without forming RF sheaths. The electric field that maintains the discharge is induced by an RF current flowing through an induction coil outside or inside the plasma. Inductive RF discharges (IRFD) or inductively coupled plasmas (ICP) operate over a wide range of gas pressure and frequency for which l>>L.
The utility of an IRFD as a light source is defined by its power transfer efficiency h =Pp/(Pp+Pc), where Pp and Pc are the power delivered to the plasma and that dissipated in the inductor. To obtain an RF lamp efficiency equal to, or better than electroded discharge lamps, h should be no less than about 90%. Power transfer efficiency depends upon many factors, such as filling gas, gas pressure, discharge topology and geometry, driving frequency, and inductor construction. Lamp power also has a significant influence on power –transfer efficiency. Contrary to capacitive RF discharges, where the fraction of RF power transferred to the plasma falls with increasing discharge power,h usually increases with power in an IRFD.
Typical example of IRFD realisation is shown in figure 3.the closed ferrite cores increase the coupling between the coil and plasma, thus enhancing IRFD efficiency.
2.3 WAVE –SUSTAINED DISCHARGES
Wave-sustained RF discharges (WRFD) are maintained by electromagnetic waves that are incident on the plasma surface or propagate along a plasma boundary .the wavelength in a wave-sustained RF discharge is comparable to the plasma size (l @ L), which implies a relatively high RF driving frequency. Wave discharges are usually maintained by microwave power sources at frequency in the GHz range. However, in some surface wave discharges with a long plasma column working as a slow wave structure, the length of the propagating waves is much shorter than in a vacuum, and the driving frequency may be much smaller (10-100 MHz.). The application of microwaves is advantageous for the excitation of high –pressure HID light sources where relatively high- power density is needed to achieve a near –equilibrium plasma.
CHOICE OF FREQUENCY AND DISCHARGE TYPE
A few frequencies allocated for industrial applications such as 13.56,27.12, and 40.68 MHz in the RF frequency band, and to 2.45GHz in the microwave band, the frequency rage between 2.2 – 3.0 MHz (2.65MHz is standard) has reduced restrictions on EMI and has been specifically allocated for RF lighting devices.
An RF generator (RF ballast) is the essential yet most expensive part of a modern RF lighting system. Electronic ballasts for driving electroded fluorescent lamps operate at a few tens of kilohertz. For such frequencies, ballasts efficiencies is rather high (90-95%), its cost is quite reasonable, and EEMI levels comply with regulations that are more tolerant of lower frequencies. With increasing frequency, EMI radiations grows, regulations are more stringent, ballast efficiency decreases, and ballast cost increases. This is why there is no hope for microwave RF lamps for general lighting .To the contrary, decades of development of RF light sources shows a preference for inductive RF discharge and reduction of RF frequency.
With a limited choice of available frequencies for industrial and residential applications, the inductively coupled plasma source seems to be the most practical way to make a commercially viable lamp with alight output of over 1000 lm.
COMMERCIAL RF LIGHT SOURCES
Considering the difficulties created by fluorescent lighting, it should come as no surprise that researchers have been trying to develop a practical electrode-less light source for years .The main obstacle to the development of a commercial RF lamp was the lack of efficient and economical electronic components to drive the lamp frequencies as high as 60 Hz, the level necessary to produce visible light more efficiently.
Within the past 10 years, the following RF lamp designs have achieved varying degrees of commercial use:
4.1 MICROWAVE- POWERED SULFUR LAMPS
This light source is remotely energised by microwave power source of about 1.5kW at 2.45GHz generated by a magnetron. 1.5kW magnetron, similar to those used in microwave ovens, generates power at 2.45GHz and delivers energy to a quartz glass bulb through a short wave guide (Fig.4). Filled with argon at a small dose of sulphur, the bulb (about 3cm in diameter) is rotated for discharge stability within the resonant cavity. Providing about 135,000 lumens, or 95 lm/W, and a life rating of 15,000 hr, the compact sulphur lamp also features low –infrared and ultraviolet emission and good colour stability.
4.2 SPHERICAL EXTERNAL- COIL INDUCTION LAMP
This type of lamps employs a 4.5 –cm diameter fluorescent bulb driven inductively at 13.56MHz from an RF power supply housed in a base unit (Fig.5). An induction coil wrapped around the lamp energises the neon gas and a small quantity of mercury contained with in the lamp .In turn, a screen cage surrounds the lamp to reduce EMI emissions to an acceptable level. The system operate at 27 W, and the efficiency is 37 lm/W. Typical applications would be difficult –to-reach locations, such as a bridge or a room with a high ceiling.
4.3 RE-ENTRANT CAVITY INDUCTION LAMP
These lamps are operating at 2.65MHz and are available in three wattages –55W, 85W,and 165W.all three are shaped standard incandescent lamps (Fig.6). The 85W model is 11cm in diameter and 18cm long.
An induction coil is wound on a ferrite core with an internal copper heat conductor connected to the lamp base and located in the centre of the lamp. A heat conductor removes heat from the re-entrant cavity and the induction coil. A 40 cm coaxial cable delivers power from the electronic ballast to the base of the lamp. By separating the two components, the ballast operates cooler, which extents its life. With its vibration resistance, an efficiency of over 75 lm/W and a 100,000-hr average rated life, this induction lamp is particularly useful for applications where regular maintenance of lighting equipment is difficult .A typical application of 165W induction lamps would be pole-mounted luminaries for dusk –to-dawn illumination on a campus.
4.4 SELF- BALLASTED RE-ENTRANT CAVITY LAMP
This compact fluorescent RF lamp has a reentrant topology and is integrated with an electronic ballast operating at 2.65 MHz. The lamp power is 23 W at 48 lm/W, and lamp life is rated up to 15,000 hours. Significant efforts have been made in this lamp for suppressing magnetic and electric components of EMI to comply with existing regulations.
4.5 LOW- FREQUENCY EXTENTED-COIL INDUCTION LAMP
This lamps consist of a 5.4 cm diameter Pyrex glass tube constructed with a rectangular or stretched –donut shape (Fig.8). Two ferrite coils located on the shorter sides of the rectangular lamp provide the energy coupling. The power transfer efficiency of this unit is as high as 98%. For example, the 150W lamp offers 80 lm /W efficiency. The lamps operating frequency of 250kHz minimizes the problems associated with EMI, and the ballast design is much simpler than an RF system working at 2.65MHz. This induction lamp is particularly useful for applications on bridges, tunnels, high-mounted street luminaries, and similar difficult-to-reach locations.
§ Absence of electrodes.
§ Some RF elctrodeless lamps on the market today reach 100,000 hours.
§ Maintenance is low.
§ Gas pressure is optimized for maximal efficiency in RF lamps.
§ It have instant and harmless starting and are more convenient for dimming
§ Efficiency is high.
§ Cost is higher than fluorescent and incandescent lamps.
§ With increasing frequency, efficiency will decrease, so microwave RF lamps not used for general lighting.
§ It is woks efficiently for some particular frequencies, if frequency changes there are problems with electromagnetic interference (EMI).
Induction lamps are suitable for a range of installations, including general lighting within a plant, as well as out door areas. Energy conservation and environmental concerns will inevitably bring about a new generation of compact residential RF lamps. Induction lighting is an economical choice for many plants. While it costs two or three times more than a HID lamp and ballast system, induction lighting last six times longer. Payback, in maintenance savings alone, can be as fast as one year for hard-to-reach applications and two to three years for general ambient lighting.
It has taken nearly a century from the first ideas and the first RF lamp proposals to make commercially viable RF lamps. The elimination of electrodes opens up great opportunity for increased durability, light output and efficiency, and it removes many of the lamp-shape restrictions of conventional electrode discharge lamps .The initial cost of RF lighting products is the major barrier to the widespread RF lamps, but with further development of the many components of RF lighting technology, the range of applications should increase.
(1) Bright idea: Radio frequency Light sources By V.A.GODYAK from IEEE Industry applications magazine, May/June 2002.
(2) RF lighting tunes in improved Illumination By Joe Knisley from www.ecmweb.com.
(3) Website www.plantservices a good fit.com.
(4) Website www.ibl:gov.com
(5) McGraw Hill Encyclopedia for Science and Technology Volume-14.