In the film K-PAX, Kevin Spacey plays a psychiatric patient who claims to come from another planet. Although he appears human, he can see ultraviolet light. Does this prove his extraterrestrial origin or could it mean something else?

Professor Bill Stark, of the biology department of Saint Louis University, has carried out extensive research on ultraviolet vision in animals - and can see ultraviolet.

Light consists of electromagnetic waves. Visible light is measured in nanometres - billionths of a metre. Light with a wavelength of around 700nm is red, at 500nm it is green, 400nm is blue-violet, and anything below that is usually invisible. You can see this invisible light indirectly by fluorescence.

"Black light" used in discos is UV; some surfaces absorb it and re-emit it in the visible spectrum, giving off a vivid glow. Washing powders contain fluorescent phosphors for this reason; your clean shirt does not just reflect white light, it also has an added glow from the UV in sunlight. Thus it really does appear "whiter than white" in daylight. Just because you cannot see UV does not mean it has no effect on your eyes.

You can absorb large amounts of invisible UV without realising it. Exposure to high levels of ultraviolet - glare from snowfields or sunlamps - can cause snowblindness when the cornea (the clear part of the eye) is effectively sunburned. This inflammation can cause loss of vision and makes the eyes painfully sensitive to light. The effects usually only last a day or two, but with intense UV, there is a risk of permanent damage.

These harmful effects are reduced by the lens, which absorbs UV and prevents it entering the eye. When the lens becomes opaque due to cataracts, it may be surgically removed, and can be replaced with an artificial lens. Even with the lens removed (a condition known as aphakia) the patient can still see, as the lens is only responsible for about 30% of the eyes' focusing power.

However, aphakic patients report that the process has an unusual side effect: they can see ultraviolet light. It is not normally visible because the lens blocks it. Some artificial lenses are also transparent to UV with the same effect. The receptors in the eye for blue light can actually see ultraviolet better than blue. Military intelligence is said to have used this talent in the second world war, recruiting aphakic observers to watch the coastline for German U-boats signalling to agents on the shore with UV lamps.

However, the origin of the story has proven hard to track down. Ultraviolet vision was discovered in ants in 1882. It was thought to be confined to insects and some birds, but was later found in mice, lizards and other animals. Some flowers have distinctive patterns only visible in the ultraviolet, and some birds have colours in their plumage that are invisible to us but may be important in attracting a mate.

Other animals have more exotic reasons for seeing into the ultraviolet. Kestrels and other raptors can roam over a large area searching for food. From a great height, they need to identify likely hunting grounds. Rodents mark their runs with trails of urine that absorbs UV, and in 1995, Finnish researchers found that kestrels can see these trails. It seems the birds can spot areas criss-crossed by recent rodent trails and zero in on them. Smaller rodents such as voles urinate almost continuously, so a predator could simply follow a fresh trail to find prey.

It appears we are blind to wavelengths that are useful to animals, and we would expect an evolutionary reason. One suggestion is that without a UV-absorbing lens, there would be cumulative damage to the retina; but aphakic patients do not seem to suffer seriously even after many years.

Another possibility is that cutting out UV gives us sharper vision. This is because a lens can only focus a limited range of colours at the same time. Increasing the range of wavelengths leads to a distortion called chromatic aberration, which will be familiar to people with cheap camera lenses.

The eye represents a compromise between clear focus and breadth of spectrum. What does ultraviolet look like? Prof Stark possesses UV vision because he is aphakic in one eye and, with Professor Karel Tan, has published research on the nearest visible equivalent. His conclusion is that it looks whitish blue or, for some wavelengths, a whitish violet.

This appears to be because the three types of colour receptor (red, green and blue) have similar sensitivity to ultraviolet, so it comes out as a mixture of all three - basically white, but slightly blue because the blue sensors are somewhat better at picking up UV. Our sensory system does not appear to be geared to revealing additional colours beyond the violet, though other animals will see things differently.

An illustration of how ultraviolet appears is provided by the Impressionist painter Claude Monet. Following cataract surgery in 1923, his colour palette changed significantly; after the operation he painted water lilies with more blue than before. This may be because after lens removal he could see ultraviolet light, which would have given a blue cast to the world.

Birds, bees, biology professors and Impressionists may have the ability to see into the ultraviolet, but it is more likely to be a sign of cataract surgery than having come from another world.