Ultraviolet Light (UV) is form of electromagnetic radiation that is naturally occurring in sunlight. Vampires in the Underworld universe suffer from a highly aggressive and acute form of photosensitivity to it; they are allergic to the ultraviolet radiation inherent in natural sunlight. Because of this allergic reaction, the movements of Vampires are restricted to the night.
Brief exposure to even indirect sunlight can quickly cause localised second-degree burns (as seen in Underworld: Evolution, before Selene and Michael Corvin reach an abandoned mining facility). Prolonged exposure to direct sunlight will completely reduce a Vampire into a charred, blackened corpse, as shown in Underworld and Underworld: Rise of the Lycans, during Sonja’s execution. Even just looking into indirect sunlight hurts their eyes.
As shown in the first and third installments, cremation via sunlight was used as a form of execution in the Old World Coven, as the ultimate deterrent against breaking the Coven's laws, the "Covenant."
In the first installment, UV ammunition is used for the first time; speculated by Kahn to have been adapted from stolen military prototype high-tech tracer rounds, using fluid irradiated with ultraviolet radiation. A Vampire handling UV ammunition will feel the UV radiation stinging their fingers through the individual ammunition's insulated casing.
Upon becoming a Vampire-Corvinus Strain Hybrid in Evolution, Selene gained an immunity to ultraviolent light. David gained this ability in Awakening when Selene resurrected him as a Hybrid like herself. The fact that both possess this ability appears to not be common knowledge as both Semira and Marius were shown to be shocked by it in Blood Wars. In Blood Wars, David is able to use Semira's fascination with her own recently-gained immunity to UV light to distract and defeat her.
- While the Vampires of Underworld are highly vulnerable to ultraviolet radiation, they do not possess any of the mystical weaknesses associated to that of mythological Vampires (i.e., crosses, holy water, garlic, stakes, etc).
- "The sky above was growing brighter by the second. Selene squinted at the spreading radiance. The daylight obviously hurt her eyes, making it hard for her to see." ~Evolution novelization, chapter 12
- "[Selene] swiped up one of the glowing clips. Even through its insulated casing, the glow from the irradiated fluid stung her fingers." ~Evolution novelization, chapter 15