High intensity discharge lamps or HID lamps are made of a discharge tube (generally of quartz glass) inside which there is a rare gas.

Two electrodes are placed at the tube’s two extremities. If the right potential difference is applied between the two electrodes, many high-speed bumps between the moving particles and the gas’ atoms happen inside the discharge tube. Bumps excite the gas’ atoms, which then return in their stable configuration, emitting energy in turn as photons. If the gases inside the discharge tube are metallic vapors of mercury or sodium, some of radiation is emitted at particular visible wavelengths. The light emission of a discharge lamp depends on the type of metallic vapours contained or on the pressure inside the discharge tube.

From an electrical point of view, the biggest problems of discharge lamps are the ignition and the discharge stabilization. To make ignition easy, an easily ionisable filling rare gas (xenon, neon, argon, helium, krypton) comes inside the discharge tube, together with the metallic vapours.
When the lamp is cold, the gas is not ionized very much an the pressure inside the tube is low, and so the likelihood of bumps is not high. To start the discharge, it would be necessary to apply to the electrodes a potential difference higher than the mains voltage (220V).
Ignition voltage is the minimum potential difference to apply to the electrodes in order to set off the discharge inside the gas.

It is possible to match the ignition voltage to the mains:
•    by reducing the distance between the electrodes through an auxiliary electrode: during ignition, the discharge takes place between one of the two electrodes and the auxiliary one, while during operation the discharge takes place between the two main electrodes.
•    by using a starter that creates a peak between electrodes during ignition.

After the initial discharge in the rare gas, temperature and pressure inside the tube begins to grow, until the metallic vapors are affected too. When the lamp is hot, the discharge would grow indefinitely, because of multiple collisions, raising more and more the temperature and pressure values and compromising the lamp’s integrity. A reactor is then necessary, which limits and stabilizes the discharge. For all these reasons, not only the discharge lamp ignition is not instantaneous, but the presence of electronic devices such as starters, reactors, makes the discharge lamps consume more energy than comparable incandescent sources, because the auxiliary electronic circuits are responsible of their own energy absorption.

The main characteristics of the discharge lamps are:
•    impossibility of instantaneous ignition when cold and instantaneous relighting when warm;
•    high luminous efficiency (from 35 lm/W to 200 lm/W);
•    long life (from 3000 hours to 12000 hours);
•    color temperature similar to average sunlight at noon (6500 °K).