Brief description

With the exception of Mercury Vapour lamps, High Intensity Discharge lamps are amongst the most efficient light sources. Producing a large amount of light from a very small space, they have been designed to compact dimensions so that light can be easily and accurately directed.

These lamps operate on the basis of the arc discharge. There is a constant arc between two electrodes that causes the gas filling to give light. This principle can be used with different metals and filler materials, including metal halides lamps, sodium and mercury vapour, each filler type giving distinct characteristics to the type of lamp in terms of colour appearance, colour rendering and efficiency.

Disadvantages

The main limitation of discharge lamps is they usually take about 5 minutes to achieve full brightness. In most cases if the power is interrupted or they are switched off when hot they will take up to 15 minutes to give full light again. However there are options known as 'hot-restrike lamps'  that avoid that situation.