Sodium (Na)

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Low Pressure Sodium
Once the most efficient form of artificial light. High efficiency and long service life are the good points that have kept this form of light in existence. These lamps emit monochromatic yellow/amber light limiting its suitability to street/amenity lighting and in specialized industrial applications where a monochromatic light of ‘safe’ wavelength is all that can be tolerated. Over the years low pressure sodium lamps have undergone numerous changes and today the only version manufactured is the ‘SOX’ type lamp. This lamp is now only available in wattages starting at 18w going right through to 180w.These lamps need to be operated with a ballast, ignitor and power factor correction capacitor or with an ‘auto-leak’ transformer and P.F. capacitor.

Sodium (Na)

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High Pressure Sodium

Not quite as efficient as the low pressure lamp but it does have the advantage of improved colour rendering. The arc tube is fabricated from aluminium oxide as this is one of the few materials that can endure the heat and chemical attack from a high pressure sodium arc. The high pressure lamp was invented in the USA and quickly gained popularity across the world. Light emitted is of a warm whitish golden/salmon colour and is certainly not unpleasant. Their predominant service is in street, road and amenity lighting. The lamps have also found place in areas where good colour rendering is not essential and the warm hue of the high pressure sodium lamp is appealing, for example lanterns in the style of gas lamps, architectural highlighting where the golden light enhances the overall look of the building or monument for example. These lamps not only limit themselves to exterior lighting but lend themselves well to lighting sports halls, gymnasiums, bus/train stations, factories, churches and other public buildings. They don’t seem to be as popular in warmer countries possibly because the cool white light of the cheaper mercury vapour lamp is considered more refreshing and the additional cost cannot be justified.High pressure sodium or ‘SON’ lamps are available in ratings from 35w to 1Kw. There are various designs of ‘SON’ lamp. First there is their shape or outer envelope variations, the common forms are the SON-E signifying an ellipsoidal bulb and SON-T signifying a single ended tubular shape bulb. A double ended tubular shape lamp is also available and a rare SON-R with a built in reflector can be found. There are also two classifications of SON lamp which indicates what control gear is suitable for the lamp. A lamp that is designed for operation on electronic control gear or inductive ballast, power factor correction capacitor and ignitor will have an ‘E’ inside a triangle signifying ‘external ignition’. A lamp that is intended for use on a simple inductive ballast and PFCC, and must only be operated in this manner will have an ‘I’ inside a triangle signifying ‘internal ignition’. In an internal ignition lamp, ignition is achieved by one of three means. In low wattage lamps it is likely that a glow-bottle bi-metallic lamp similar to that used in a fluorescent lamp circuit is connected in parallel with the arc tube. In higher wattage lamps it is likely that ignition is performed by a coil acting as a heater and a bi-metallic switch. When the lamp is energised the coil heats up causing the bi-metallic strip to flex breaking the circuit, the resulting influx of current that has built up in the ballast is enough to strike an arc. The heat created from the arc tube is enough to keep the bimetallic strip flexed and thus preventing a short occurring. The third form of ignition is not very common; it takes the form of a small electronic circuit housed in the outer envelope of the lamp. This is a very reliable and efficient form of ignition and also has the benefit of striking the arc quicker than glow-bottle or heater.