Bluethroat. Museum specimen of a bluethroat (Luscinia svecica) as seen in ultraviolet (UV) light. In visible light, the male bluethroat has a blue and red coloured bib under the throat. The image shows how the blue mark reflects strongly in UV light. This image was taken using a digital camera adapted to only filter and detect UV light waves. Non UV-reflecting areas appear dark blue to mauve. Objects reflecting UV appear pale blue or light purple. Birds are able to detect four wavelengths of light (tetrachromatic vision). In addition to normal colour vision, they can detect wavelengths in the UV part of the electromagnetic spectrum which allows them to view plumage markings that are invisible to human eyes. (See image Z892/619 for bluethroat in normal light).