Lamps

• Lamp Types
• Discharge Lamps
• Solid State and Induction Lamps
• Lamp Characteristics
• Comparison of Lamp Characteristics
• Mercury Vapour: Some Problems

A basic understanding of how lamps work can assist in choosing suitable lamp technologies. This section introduces and compares various lamp technologies, and provides some recommendations about which ones to use for different purposes.

There are three main types of lamp used in public lighting in Australia:

• Discharge lamps, which include: mercury vapour (MV), the most common lamp used on most minor roads; high pressure sodium (HPS), used on major roads and characterised by its orange glow; low pressure sodium (LPS), the most efficient light, characterised by a harsh orange “hard to see by” light which means it is now rarely used; metal halide (MH), characterised by a very “white” light and often used in areas with large amounts of mixed pedestrian/vehicle traffic, e.g. King Street in Melbourne; compact fluorescent (CF) and T5 fluorescent (T5), often used with solar lighting; and tubular fluorescent (FL), used widely in NSW.
• Solid state lamps, which include light emitting diodes (LEDs), most commonly used in traffic lighting applications.
• Induction lamps. Induction lamps are a relatively new type of lamp, used in ANZAC parade in Canberra and in Southbank in Melbourne.

More information about these lamp technologies, including relevant trials and case studies, can be accessed by clicking on the links above. Note that incandescent lights are not used in public lighting because of their low efficiency and short life.

In general, lamps tend not to be interchangeable in a given light fitting, because the ballast used in the fitting is matched to the lamp. So a 400W high pressure sodium lamp should not be used in a fitting designed for a 250W high pressure sodium lamps, and a 400W mercury vapour lamp cannot be used in a 400W high pressure sodium fitting.

Some kinds of metal halide lamps can be used on a mercury vapour fitting of the same wattage, and a new development in metal halide technology is making some of these lamps compatible with some high pressure sodium fittings of the same wattage. For fluorescent lighting some electronic ballasts are manufactured to be compatible with a range of lamp wattages.

As the ballast is usually an integral part of the luminaire, it is important to appreciate that changing the lamp type is usually not just a matter of screwing in a different bulb; the ballast usually will have to be changed as well, and an ignitor or capacitor may also need to be added to the circuit. This adds considerably to the expense of changing the lamp type, as a crew of two or three using a lifting platform work at a high cost hourly rate.

Moreover, there may not be space in the luminaire for larger or additional control gear. So if changing lamp type or size usually it is most economic to change the whole luminaire, rather than to change the control gear on site.

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Discharge lamps are the most common type of lamp in Australia. Discharge lamps work by passing a current between electrodes in the lamp, which ionises a gas containing small amounts of metals, usually including mercury. The ionised gas then emits radiation.

In some lamps, such as fluorescent, the inside of the lamp is coated with phosphor, which then glows when hit by the radiation emitted by the ionised gas.

The ballast in a discharge lamp circuit plays an important role in limiting the amount of current that flows through the lamp and preventing the lamp from quickly burning out. In addition to a ballast, high pressure sodium lamps may also have an ignitor. An ignitor supplies a high voltage pulse that establishes the discharge arc.

High intensity discharge (HID) lamps include mercury vapour, high pressure sodium and metal halide. They are a form of discharge lamp that contains an arc tube within the outer glass bulb. HID lamps emit radiation in the visible spectrum and do not necessarily require a phosphor coating on the outer glass bulb.

Sodium lamps use sodium rather than mercury, which gives the distinct orange glow. Increasing the pressure of the gas in a sodium lamp improves the colour rendering of the light, which is why high pressure sodium (HPS) lamps have better colour rendering properties than low pressure sodium (LPS). LPS lamps having the highest efficacy (efficiency) of all lamp types, coupled with a long life. They are generally located along major arterial roads and have a distinctive yellow light, but are being phased out in favour of HPS due to HPS lights' better colour rendition.

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Solid state lamps are comprised of semi-conductor materials that glow when a current is passed through them. Light emitting diodes or organic light emitting diodes are current technologies.

Induction lamps are similar to discharge lamps in that they rely on the ionisation of a mercury gas emitting ultraviolet radiation and a phosphor coated lamp. The difference is that the gas is ionised by an induced electromagnetic field, and not by an electrical discharge. This extends lamp life as there are no electrodes to wear out. Induction lamps are driven by an electronic high frequency generator.

Efficacy
The unit of efficacy is Lumens per watt. Efficacy measures how efficient the lamp is in converting electrical energy into light. Note that the energy consumed by the control gear is not included in this calculation.

On lower wattage HID lamps with iron-core ballasts, the ballast power consumption can be a considerable percentage of the total power consumption. For example a luminaire with a 80W MV lamp would consume a total of 95.8 watts when used with an iron core ballast and a electro-mechanical photocell at 250Volts.

To enable a true comparison between lamps, the power used by the ballast should be taken account of in calculations.

Light colour
Light colour is measured by both the lamp’s “temperature” and the colour rendition index (CRI) of the resulting light.

The CRI measures how close a colour is rendered by the lamp to the “real” colour, on a scale from 1 to 100. A CRI of 100 means that all colours are correctly rendered.

The lamp temperature measures the colour temperature of the light from the lamp, and is measured in degrees Kelvin. Metal halide lamps, with very good quality light, have lamp temperature of 3000 to 4000 K.

When specifying the colour of light from their lamps, some manufacturers use the CRI and others use the lamp temperature.

Lamp temperature performance
Lamp temperature performance measures the ability of the lamp to perform and to produce its rated output at a given temperature. Some lamps may produce less than their rated output if temperatures are too cold or too hot.

Lamp life
Lamp life measures the life of the lamp, usually expressed as the number of hours of operation by which a percentage of a group of lamps has failed. The specified percentage may vary from manufacturer to manufacturer, making comparisons difficult.

In general, however, the twin arc high pressure sodium lamp can last over 8 years. The 80W MV needs replacing every 3-4 years because of reduction in light output over time. T5 fluorescents currently need replacing after 4-5 years. (Some T5 fluorescents claim double this life span.) Induction lamps are being developed that may last up to 15 years. 

Lamp reliability and toughness
Lamp reliability and toughness is a subjective assessment of the lamp’s ability to deal with adverse conditions, such as over voltage, under voltage, spikes, transients, vibration and rough handling.

Its electrical reliability and toughness depends to a large extent on the ballast used and the degree of protection the ballast offers the lamp.

Other considerations
The ballast and starting requirements of a lamp, as well as its cost, are also important factors in public lighting applications.

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Mercury vapour lamps are very cheap, can operate on a simple low cost ballast, are tough, reliable, and have a light colour acceptable for pedestrian areas. These characteristics have made them the work horse of Category P roads.

Unfortunately, they also have several drawbacks, including low efficacy, rapid lumen depreciation and use of high quantities of mercury, which poses a disposal problem. For these reasons, they have been phased out in many countries in Europe, with a recent report recommending they be completed removed form use in Europe by 2010. They are also believed to have been phased out in most of the United States.

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