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Pt.4: High Pressure Mercury Lamps Electric Lighting Developed in the early 1900s, high pressure mercury lamps are ideal for use where high light outputs are required. Unlike filament lamps, they create light by producing an arc discharge through a mercury vapour gas. By JULIAN EDGAR The high pressure mercury lamp emits an intense, white light. It is widely used in industrial, commercial and outdoor applications. In fact, I have two high pressure mercury vapour lamps in my workshop. Because it is the first lamp discussed in this series that is not instantly recognisable through widespread domestic 12  Silicon Chip use, it is helpful to show where the high pressure mercury lamp fits into the scheme of things. Fig.1 shows that electric lamps can gener­ally be divided into two categories – those that bring a filament to high temperature by passing an electric current through it (incandescent lamps) and those that produce light by the excita­tion of a gas contained between two electrodes (discharge lamps). A common example of an incandescent lamp is the general service light bulb, while an example of a discharge lamp is a fluorescent tube. However, rather than being a low pressure discharge lamp like a fluorescent tube, the high pressure mercury vapour lamp falls (as the name suggests) into the category of high pressure discharge lamps. Note that in some publications, the high pressure mercury vapour lamp is abbreviated to MBF, while Philips use an HPL pre­fix. History The first lamp using the principle of a mercury vapour arc was developed in 1901. Peter Cooper Hewitt’s tubular Fig.1: electric lamps can generally be divided into two categories: (1) those that bring a filament to high temperature by passing an electric current through it (incandescent lamps); and (2) those that produce light by the excitation of a gas contained between two electrodes (discharge lamps). (de Boer, J. & Fischer, D. Interior Lighting). lamp used a mercury pool cathode and an iron anode. When the lamp was tilted, a column of mercury bridged the gap between the cathode and the anode. As the lamp was righted, the mercury column broke and the electric arc discharge started. This type of lamp was called a Cooper-Hewitt lamp and was widely used in the early 1900s. A 385-watt, 4-foot long tube version had an efficacy of 12.2 lumens/watt (l/W) and was first used in the composing room of the New York Evening Post in 1903. Interest in mercury vapour lamps increased in the 1930s when 400W lamps were introduced in Europe. These were called “high pressure” lamps, even though the internal gas pressure was actually near atmospheric. The design consisted of an arc tube enclosed within an outer glass bulb and the lumen output matched that of a contemporary 750W incandescent lamp. By the time of the Second World War, 3kW mercury lamps had been introduced. There were, however, two major problems with mercury dis­ charge lamps. First, when the lamp was mounted horizontally, the arc bowed under the influence of gravity. And if the arc touched the glass wall of the discharge tube, it melted it! To prevent this, magnets were used to pull the arc away from the glass. Eventually, the development of small­ er quartz discharge tubes overcame this problem and did away with the need for magnets. The second problem was that a mercury discharge produces light at just four visible wavelengths – 404.7nm, 435.8nm, 546.1nm and 577-579nm. The result is a bluish-green-white light that gives poor colour rendering. In industrial settings, a simple solution to this problem was to mount a 750W tungsten lamp next to a 400W mercury vapour lamp. The excess of reds and oranges from the tungsten lamp counterbalanced their absence from the mercury lamp. Subsequently, in the 1950s, another Hard glass outer envelope Quartz discharge tube Main electrodes Starting electrode Fig.2: the structure of the high pressure mercury discharge lamp. (Murdoch, J. Illumination Engineering). February 1998  13 TOTAL IR RADIATION 260W UV 10W CONVECTION & CONDUCTION 80W VISIBLE RADIATION 50W Fig.3: this pie chart shows the energy balance of a clear glass high pressure mercury lamp. (Philips Lighting Manual). TOTAL IR RADIATION 226 W UV 15W VISIBLE RADIATION 67W CONVECTION & CONDUCTION 92W Fig.4: the energy balance of a phosphor-coated high pressure mercury lamp. Note how the phosphor coating increases the output of visible radiation. (Philips Lighting Manual). solution was found. If the inside of the outer glass bulb is coated with a phosphor, the ultraviolet mercury radiation is converted into visible radiation in the red portion of the spectrum. However the size of the re14  Silicon Chip sulting light source then made optical control difficult. Basic construction Fig.2 shows the basic construction of a high pressure mer­ cury vapour lamp. The lamp consists of an inner quartz discharge tube and an outer envelope made from borosilicate glass or, in lamps of less than 125 watts, soda-lime glass. Quartz is used for the discharge tube because it has low absorption of UV and vis­ible light and is able to withstand the high operating tempera­ture. In fact, the quartz tube must withstand an arc temperature of 1000°C, while the outer bulb operates at a maximum of 430°C. The space between the inner and outer bulbs is filled with nitrogen to thermally insulate the arc tube and to protect metal parts from oxidation. The discharge tube, on the other hand, is filled with distilled mercury and argon gas, the latter included to aid starting. Housed within the discharge tube are two main electrodes and a starting electrode. Each of the main electrodes consists of a tungsten rod upon which a double layer of tungsten wire is wound. During manufacture, the electrode is dipped into a mixture of thorium, calcium and barium carbonates and then heated to convert these compounds to oxides. The starting electrode is simply a piece of molybdenum or tungsten wire positioned close to one of the main electrodes. The electrodes are connected through the quartz tube by leads of molybdenum foil. Molybdenum is used because it forms a reliable, gastight seal with quartz at the high operating temperature involved. Mercury lamps are available with clear or coated outer envelopes. Un­ coat­ ed lamps have a compact light source and are commonly used where accurate directional control is needed; eg, floodlighting. As indicated earlier, an uncoated mercury lamp has an absence of light at red wavelengths. The visible wavelengths emitted are at four distinct wavelengths, corresponding to yel­low, green, blue and violet. A significant portion of its energy is also emitted as UV radiation. Fig.3 shows the energy balance of an uncoated high pressure mercury vapour lamp. A 400 watt lamp produces 50 watts of visible radiation, 10 watts of UV radiation and 260 watts of infrared (IR) radiation. Convection and conduction heat losses are respon­sible for the remaining 80 watts. Most mercury lamps have a white phosphor coating on the inner surface of the bulb. This improves colour The high-pressure mercury lamp goes through a distinct series of phases during start-up. Here the glow discharge is spreading through the discharge tube. rendering and also increases the light output because the phosphor converts much of the UV radiation into visible light. Special coatings are also available that give the lamp a lower colour temperature, improved colour rendering, a higher lumen output and higher luminous efficacy. However, the colour rendering properties of mercury lamps are generally poor, with a typical Ra of 45, while colour temperatures from 4200°K to 6000°K are available. Fig.4 shows the energy balance of a phosphor-coated high pressure mercury vapour lamp. The 400 watt lamp has a visible radiation output of 67 watts, a UV radiation of 15 watts, an infrared radiation of 226 watts and convection and conduction heat losses of 92 watts. The time between switch-on and a high-pressure mercury lamp producing 80% of its final light output is about five minutes. When the glow discharge reaches the furthest electrode, the current increases considerably. This heats the main electrodes until the emission is increased sufficiently to cause the glow discharge to change into an arc. The starting electrode then plays no further part in the process because the resistance of the main arc is far less than that of the starting arc circuit. At this stage, the lamp has a blue appearance. It is in fact operating as a low pressure discharge lamp, similar to a fluorescent lamp. (2). Run-up: as a result of the arc discharge, the temperature within the discharge tube rapidly increases. This causes the mercury to gradually vaporise, thereby increasing the vapour pressure and causing the discharge to be concentrated in a narrow band along the tube’s axis. As the pressure within the discharge tube increases, the radiated light is concentrated progressively towards spectral lines of longer wavelengths. Operating phases There are three distinct phases during the operation of a high pressure mercury lamp: ignition, run-up and stabilisation. (1). Ignition: when the lamp is switched on, a high voltage gradient appears between the main electrodes and also between the starting electrode and the nearest main electrode. This ionises the gas in the latter region in the form of a glow discharge, the current being limited by a high-value resistor (typically 25kΩ) wired in series with the starting electrode. The glow discharge then proceeds to spread throughout the discharge tube. Fig.5: one of the disadvantages of high pressure mercury lighting is the slow start-up. The initial current drawn (I) is high, while lamp power (P), lamp voltage (V) and luminous flux (Φ) take around four minutes to reach normal operating values. (Philips Lighting Manual). February 1998  15 Fig.6: the most common ballast system employed uses shunt compen­sation. (Philips Lighting Manual). At the same time, a small proportion of continuous radiation is introduced and so the light becomes whiter. When the internal pressure reaches 2-15 atmospheres (up to 220 psi), the arc stabilises. All the mercury is then vapor­ ised and the discharge takes place in unsaturated mercury vapour. The time between switch-on and the lamp producing 80% of its final light output is 4-5 minutes. The performance of the lamp during this period is shown in Fig.5. (3). Stabilisation: as with a fluorescent lamp, a high pressure mercury discharge lamp has a negative resistance characteristic; ie, the current flowing through it would continue to increase if left unchecked. A suitable ballast is therefore required to stabilise the current flow. Unlike low pressure mercury vapour lamps (fluorescent lamps), the output of a high pressure mercury vapour lamp is not significantly affected by changes in ambient temperature. The lamps are also not greatly affected by fluctuations in the mains voltage. A drawback is that once switched off, the lamp will not re-ignite for about five minutes. This is because the lamp must cool sufficiently to lower the vapour pressure to the point where the arc will re-strike. A typical coated high pressure mercury vapour lamp has an efficacy of 36-58 lumens/watt. 500W 300 400 500 600 700 nm Fig.8: the spectral output of a Philips HPL-N phosphor-coated mercury discharge lamp shows three clear lines. (Philips). 1000W 500W Control circuit Because a high pressure mercury discharge lamp includes its own starter, the circuit required is relatively simple. The most common approach is to use shunt compensation, as shown in Fig.6. The capacitor improves the lagging power factor from 0.5 to better than 0.85, with the circuit also reducing lamp current under starting and operating conditions by nearly 50%. Blended light lamps As the name suggests, a blended light lamp uses two sources of light. The technology combines aspects of both a high pressure mercury lamp and tungsten filament lamp. Instead of using an external ballast as a mercury lamp does, a blended Blended light lamp: basic construction Fig.7: a blended light lamp uses some elements of both mercury discharge and tungsten lamps: (1) hard glass outer envelope; (2) coiled tungsten filament; (3) quartz discharge tube; (4) support; (5) main electrode; (6) internal phosphor coating; (7) lead-in wire; (8) base. (de Boer, J. & Fischer, D. Interior Lighting) 16  Silicon Chip 1000W 300 400 500 600 700 nm Fig.9: the spectral output of the blended light lamp stills shows the three lines, but in addition there is the higher wavelength emphasis of the tungsten filament. (Philips) light lamp has a built-in ballast in the form of a tungsten filament connected in series with the discharge tube. Light is produced by both the discharge and the glowing filament. Fig.7 shows the make-up of a blended light lamp. This uses a higher gas pressure within the outer bulb than a mercury vapour lamp to minimise vaporisation of the tungsten filament. As with conventional incandescent lamps, the filling consists mainly of argon with some nitrogen added. Blended light lamps have the huge advantage of being able to be retrofitted to existing incandescent installations. The lamps have almost twice the efficacy and five times the operating life of incandescent lamps, although both of these characteris­tics are still much inferior to those of mercury vapour lamps. The colour rendering of a blended light lamp is much better than that of a mercury lamp, with the lamp having a much wider spectral distribution. Fig.8 shows the spectral output of a Philips HPL-N mercury lamp while Fig.9 shows the spectral distri­bution of Philips ML blended light lamp. The contribution of the tung­sten filament can be clearly seen. Next month, we’ll look at floodlightSC ing for buildings.