Beyond violet. That part of the electromagnetic spectrum which lies beyond the violet end of the visible spectrum, i.e. of shorter wavelength than violet, and invisible to humans.
The problem with UV is that it will pass through camera lenses, film and CCDs are sensitive to it, but lenses are not designed to focus it. The result is a haze or fuzziness in pictures taken outdoors.
The solution is to fit a UV filter (sometimes called a UV-390.e.g. a filter designed to cut off at a wavelength of 390nm (nanometres), the end of the visible spectrum. Some photographers fit a UV filter also to protect the main camera lens from scratches. A UV filter is not needed underwater