The very high luminous efficiency of discharge lamps has led to their almost universal application for industrial and commercial premises; the introduction of low rated types as direct replacements for filament lamps is beginning to see their wider use in domestic situations.
Discharge lamps are those which produce light as a result of a discharge in a gas. Included are:
Really low pressure mercury vapour lamps, very widely used for general lighting in homes, shops, offices, etc.
High pressure mercury
Provide a very intense lighting level for outside use in situations where the (sometimes) poor colour rendering is not important.
Low pressure sodium
The most efficient lamp of all, but its poor colour (orange) light output limits its use to street and road lighting
High pressure sodium
The acceptable golden light colour enables the lamp to he used for road and outside lighting in areas where better colour rendering is needed, as well as for large indoor industrial applications.
Discharge lamps, unlike their incandescent counterparts, require control gear in the form of chokes, ballasts, autotransformers and transformers. These devices result usually in a lagging power factor, which is corrected, at least partially, by connecting capacitance across the supply.