High-intensity discharge lamps offer an energy-efficient way to brightly illuminate large spaces - for instance street lighting, sports stadiums, indoor arenas and cinemas. They can also be used for indoor horticulture and aquariums, and are even seen lighting residential and retail spaces. HID lamps work by sending an electrical discharge between two electrodes and through a plasma (ionised gas). The current is regulated by a ballast. As well as producing excellent brightness for very little energy, they are very long-lasting, especially the high-pressure sodium varieties.