The light whose wavelength (about 380 nanometers) is just long enough not to be x-rays, but just enough shorter than violet light so that it is not visible to the human eye. Ultraviolet is also known by the short form of "UV." It is sometimes called black-light because ultraviolet lamps (usually a mercury-vapor lamp) appear quite dark even when lit, and because of the peculiar way it illuminates certain kinds of surfaces, such as day-glo colors.Various colors of paint and layers of varnish fluoresce in different ways under UV light. This fluorescence can be photographed, both in color and in black and white. Recently applied paint absorbs UV and appears dark, allowing for the detection of areas of inpainting. Aged retouching, however, often goes undetected with UV. Examination under UV can sometimes also help in identifying pigments.Also see angstrom, color, sight, and spectrum.