Luminosity discharger

The luminosity discharger, head lamp, radiation valve, dome light, vocalizer light or sound disc, was a device, typically appearing in twos, atop the dome of a Dalek's casing. They would light up whenever a Dalek spoke, much like the mouth slits of certain subspecies of Cyberman. (PROSE: Prisoner of the Daleks, Engines of War, Birth of a Legend, PROSE: The Dalek Problem, PROSE: Shadowmind) They were identified by the Time Lords as energy compensators. (PROSE: Dalek Combat Training Manual) Another source identified them as translation units, capable of translating Dalek speech into any language. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) They could also flash involuntarily in cases of shock, such as when the Tenth Doctor revealed his identity to a Command Dalek. (PROSE: Prisoner of the Daleks)

The Dalek Survival Guide acknowledged a theory which suggested that the luminosity discharges functioned as safety valves through which excess energy was released, though this did not account for the sychronisation with the speech circuits. While some documentary evidence did exist which suggested that the lights could flash when the Dalek was not speaking, the guide deemed the most plausible theory was that the lights simply served to indicate to other life-forms which Dalek was speaking. (PROSE: Dalek Survival Guide)

Number
Although the vast majority of Dalek casings possessed two luminosity dischargers, disposed symmetrically on either side of their eyestalks, (TV: The Daleks, et al.) there were exceptions to this rule.

The platoon of colourful asymmetrical Daleks whom the Second Doctor encountered while traveling with John and Gillian were distinguished by having only one, oversized luminosity discharger on their left side. The Doctor did not remark upon this peculiarity. (COMIC: Attack of the Daleks)

An increased number of speaking lights sometimes marked a heightened rank within the Dalek hierarchy: the unique, bulbous-headed Golden Emperor casing of the Dalek Prime had "about a dozen lights about the expanded dome instead of the average Dalek’s two". (COMIC: Genesis of Evil, PROSE: War of the Daleks) The Dalek Prime's Emperor Type 2 casing had seven medium-sized luminosity dischargers arranged around the back half of its dome, which flashed red. (COMIC: Invasion of the Daleks) During the Last Great Time War, one type of Skaro Degradation also had a golden head dome with four radiation valves. (PROSE: Engines of War)

The red Supreme Dalek model that would lead the bronze Daleks through the Time War and beyond was likewise distinguished by having three luminosity dischargers. (AUDIO: Out of Time, TV: The Stolen Earth/Journey's End, The Magician's Apprentice / The Witch's Familiar)

While his casing was partly modelled on the Golden Emperor, the Emperor of the Restoration had only three luminosity dischargers atop his dome, positioned much like the red Supreme Daleks. (PROSE: The Guide to the Dark Times)

Size
Most early Dalek types such as the War Machine Daleks and the Silver Daleks had small, bulb-shaped luminosity dischargers, (TV: The Daleks, The Power of the Daleks et al.) although even then this rule had exceptions: one Dalek Supreme of the Supreme Council was distinguished by its larger luminosity dischargers, (TV: Planet of the Daleks) and the singular dischargers of the asymmetrical Daleks encountered by the Second Doctor were similarly oversized. (COMIC: Attack of the Daleks)

By 2094, a photograph of Silver Daleks with a large, blue luminosity discharger not exhibited by later models, and an enlarged Dalek fender matching that of the later Dalek Earthforce, was freely available on Earth, where it served to illustrate a news article about the recovering of a set of story-cubes about the Daleks' history. Five of the Daleks were standing in a deserted street, with other ruined casings of the same type strewn about. (PROSE: The Dalek Chronicles Found!)

In their scrutiny of Dalek history, the Time Lords identified a group of Daleks who were engaged in the conflict against the Space Security Service, consisting of silver Dalek drones, a Red Dalek leader and its red-domed subordinate. These Daleks all blue, cone-shaped luminosity dischargers. (PROSE: Dalek Combat Training Manual)

The Grey Daleks' luminosity dischargers were also of limited size, but were slightly more elevated than the flat bulbs of the Silver and War Machine Daleks; (TV: Day of the Daleks et.al.) Some of these could vary in size and colour too. (TV: Destiny of the Daleks et al.) The very first Daleks created by Davros also had dischargers of this type. (TV: Genesis of the Daleks)

The white Imperial Daleks had yet another version, which appeared as small rings around small, golden and flat pieces of metal on the dome. Such luminosity dischargers were also seen atop the spherical dome of Davros' Dalek Emperor casing. (TV: Remembrance of the Daleks)

Daleks of the Last Great Time War and beyond, including the New Dalek Paradigm, had large luminosity dischargers of a size which had previously only been sported by the Supreme Council. (TV: Dalek, Victory of the Daleks)

Uniquely, the Black Dalek overseer had exceptionally short luminosity dischargers in comparison to even pre-Time War Daleks. (AUDIO: Planet of the Ogrons)

The robotic Proto-Daleks, which were constructed by the British and Germans as part of the Dalek Project during the First World War, had distinct luminosity dischargers; the British Proto-Daleks' luminosity dischargers were shaped like smaller glass hemispheres, while the German Proto-Daleks' were narrow, orange, cylindrical and larger than any true Dalek. (COMIC: The Dalek Project)

Colour
As well as size, the Dalek Supreme of the Supreme Council was distinguished by the purple colour of its luminosity dischargers. It had the unusual feature of a discharger in the eyestalk lens. (TV: Planet of the Daleks)

The Supreme Dalek of the Renegade Daleks during the Imperial-Renegade Dalek Civil War was distinguished by its luminosity dischargers, which were coloured orange (TV: Remembrance of the Daleks) or red. (PROSE: Dalek Survival Guide)

Despite the Recon Scout's rebuilt casing having basic white dischargers, the light inside would glow yellow when it spoke. (TV: Resolution)

Exceptions
The Special Weapons Dalek notably had neither luminosity dischargers nor an eyepiece. (TV: Remembrance of the Daleks) Instead the glass just below the dome, which also functioned as its vision, would light up when it spoke. (TV: The Witch's Familiar)

As a target
Ace used a baseball bat enhanced by the Hand of Omega to destroy the right luminosity discharger of an Imperial Dalek. (TV: Remembrance of the Daleks)

A radiation valve of a Skaro Degradation was detonated when Cinder struck it with her Dalek neutraliser, however, the Dalek survived. (PROSE: Engines of War)

Individual cases
"Ironside" Daleks deployed by the British Army against the Luftwaffe had covers placed over their luminosity dischargers. These covers were removed by the time they deserted and were subsequently destroyed by the New Dalek Paradigm. (TV: Victory of the Daleks)

Behind the scenes
The Daleks seen in the theatrical film Dr. Who and the Daleks (held to be an invalid source by this Wiki) were the first to have larger luminosity dischargers. Regardless of rank, from the Black Dalek to the Red Dalek and the Blue Daleks, all luminosity dischargers were coloured red.

In Daleks' Invasion Earth 2150 A.D., the size of the luminosity dischargers remained the same, but with different colours; the Gold Dalek had matching gold luminosity dischargers, both the Black Dalek and the Red Dalek had luminosity dischargers coloured red, while the Silver Daleks had blue luminosity dischargers.

The red King Dalek seen on an action card the board game War of the Daleks is distinguished by a pair of large luminosity dischargers, one coloured blue and the other red. However, the box artwork depicts the King Dalek with small, uniform luminosity dischargers.

DWM 77's History of the Daleks uses the term "head and relief valves".