Color vision that extends into the ultraviolet wavelengths well beyond the range of human eyes is common among birds, fish, reptiles, and invertebrates. I think it would be astonishing to experience the capabilities this extended vision provides. Bees, for instance, can use their UV photoreceptors to navigate by the sun even on gloomy, overcast days by seeing rings of polarized light.

At least one predatory bird, the kestrel, uses its UV sensitivity to find small mammals, such as voles, by seeing tracks of urine and feces, which absorb UV light enough to stand out sharply from surrounding vegetation. The helmet gecko lizard can distinguish colors in dim light in which human vision is reduced to black and white. Feathers that, to us, appear only white can display a range of UV color to birds, and behavioural experiments show that the UV color is an important influence in mate choices.

Human eyes have three types of photoreceptor, each absorbing light maximally at wavelengths corresponding to what we perceive as red, green, and blue. (That’s why our computer monitors only need to mix red, green and blue light to present a full range of hues to human eyes). Bird have four types of photoreceptor, as do butterflies, and at least some fish and turtles. Mantis shrimp have ten types of photoreceptor, which may allow them to unmask the silvery camouflage of prey and predator fish.

The addition of these extra photoreceptors produces a qualitative change in the nature of color perception that is probably too alien for human minds to fully imagine. But a 2009 experiment suggests it might be possible to expand the range of human vision by hacking the genetic code of retinal cells. I wrote a news story about the experiment a few years ago and am still amazed by the findings. Male squirrel monkeys are normally red-green colorblind because they lack the necessary photoreceptors. Investigators packaged the human gene for a light sensing pigment called L-opsin into a harmless virus and injected trillions of copies of the virus around the retina in the eyes of the monkeys. About 20 weeks later, the genes began to direct the production of the color-sensing pigment at high levels, and the monkeys gained the ability to pick out red and green objects.

Maybe one day, we’ll have the option to mod our photoreceptors to see in ultraviolet. To make it work, we’d also need to replace the biological lenses we were born with, which filter out UV. But that’s no big deal. Lens replacement surgery  is already done routinely for people with cataracts.

[I originally posted a version of this as an answer to a question on Quora. I got a little carried away.]

Sources:

1. Honeybee navigation: following routes using polarized-light cues
2. Attraction of kestrels to vole scent marks visible in ultraviolet light
3. Color at Night: Geckos can distinguish hues by dim moonlight
4. Is the ultraviolet waveband a special communication channel in avian mate choice?
5. Spectral tuning and the visual ecology of mantis shrimps http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692847/
6. Exploring the fourth dimension
7. Gene therapy fixes colorblindness in monkeys, Seattle scientists report