Electroretinogram measurements have shown reindeer retinal responses to wavelengths around 320 nanometres, which the human lens filters out almost entirely. Snow reflects ultraviolet with little loss, so a UV-sensitive eye sees a uniformly bright background. Objects that absorb ultraviolet, including lichen, fur and urine, appear as strong dark contrast rather than blending into the white.
The apparent danger of retinal ultraviolet damage seems to be offset in this species, though the mechanism is not fully worked out. Reindeer also possess a separate and unusual adaptation: the tapetum lucidum behind the retina changes colour seasonally, from golden in summer to deep blue in the polar winter. The blue state scatters light within the eye, increasing sensitivity at the cost of sharpness.
Both traits fit an animal living through months without direct sunlight in a landscape where the ground is white and the food is buried. Note that ultraviolet sensitivity is not unique to reindeer; many birds, rodents and insects share it. What is distinctive is the combination with a seasonally colour-shifting reflective layer.