Doctor Who: 10 Things Everyone Always Gets Wrong About The Daleks
6. They Only Have Two Dome Lights
Raymond Cusick's Dalek design is key to their legendary status, and it was criminal that he never saw more benefits for it. The two dome lights, the gunstick, the manipulator arm, and those gorgeous Dalek bumps are so great that the design has barely changed in 60 years.
However, it's a falsehood to say that all Daleks have just two dome lights, as there are one or two notable exceptions to that rule.
The red Dalek Supreme in The Stolen Earth/Journey's End and The Magician's Apprentice/The Witch's Familiar has three dome lights. It's very hard to see due to how it's shot in both stories, but if you find the right moment you can glimpse the third light on the centre of its dome.
From a behind the scenes perspective, the dome lights have been seen as a translation device. That's why they flicker in time with Dalek dialogue throughout the majority of their appearances.
With that in mind, it's unclear what function this third light has for the Dalek Supreme beyond being a bit of bling, though it's possible that extra dome lights denote superiority.