Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of visible light, but longer than that of soft X-rays. The name means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the shortest wavelength of visible light. It is colloquially called black light, as it is invisible to the human eye.

UV itself can be subdivided into near UV (380-200 nm wavelength) and extreme or vacuum UV (200-10 nm). When considering the effects of UV radiation on human health and the environment, the range of UV wavelengths is often subdivided into UV-A (380-315 nm), UV-B (315-280 nm), and UV-C (280-10 nm). See 1 E-7 m for a list of objects of comparable sizes.

Ordinary glass is transparent to UV-A but is opaque to shorter wavelengths. Quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths.

The sun emits ultraviolet light in the UV-A, UV-B, and UV-C bands, but because of absorption in the atmosphere's ozone layer, 99% of the ultraviolet light that reaches the Earth's surface is UV-A. (Some of the UV-C light is responsible for the generation of the ozone.)

Partial electromagnetic spectrum; scale in nanometers

Ultraviolet Lamps

The principal ultraviolet light source in use today is based on light emission from ionized Hg vapor contained in a low-pressure glass or fused silica tube. As atoms of mercury within these lamps are excited they emit a very strong light at 253.7nm (shortwave ultraviolet). This may be passed through the walls of a fused silica tube (shortwave ultraviolet does not penetrate glass) and then through a special dark filter, providing a lamp rich in 254nm shortwave ultraviolet. Other lamps can be made with varying formulations of phosphors coating the inside of the tubes. These absorb the 253nm ultraviolet and fluoresce in other regions of the ultraviolet spectrum, depending on the phosphor. In fact, common household ‘fluorescent lights’ are based on similar technology. In that case, however, the tube is glass opaque to ultraviolet and the phosphors inside fluoresce in various parts of the visible spectrum - combined they appear as white light.

Mineral collectors use lamps or "blacklights" in the shortwave, midwave and longwave regions of the ultraviolet; far ultraviolet does not pass through air very well and is both dangerous and impractical. Most shortwave lamps in use today emit almost completely at 254nm (as below); longwave sources have more variable outputs dependant on phosphor mix and filtering.

The two longwave sources we use have tubes with peaks at 352nm and 368nm (upper right), but other wavelengths are present as well. Several midwave or "UVB" lamps have become available in recent years, and these also have outputs dependant on phosphor mix and filtering. The one used for midwave photographs on this website has the spectrum shown at right, which includes a fairly broad range of wavelengths.

Ilímaussaq Fluorescence

To best experience the fluorescent minerals of Ilímaussaq one should have access to lamps in all three ultraviolet regions. The local sodalite responds most brightly to longwave or midwave ultraviolet; while associated fluorescent minerals may glow best under shortwave. The tenebrescent effect in both sodalite and tugtupite is stimulated by shortwave ultraviolet. Tugtupite glows in different colors under differing ultraviolet sources. Some responds with a unique white color under midwave only. A large display of Ilímaussaq fluorescent minerals would best have a ‘full spectrum’ of ultraviolet light sources: short-, mid-, and longwave.

Activators

Some minerals, like scheelite, are intrinsically fluorescent - their ideal chemical formula and perfect crystal structure provide a fluorescent material. Most fluorescent minerals however owe their glow to the presence of minor/trace impurities or structural defects. Hence, the fluorescence of many species is variable; depending on a few ‘alien’ atoms or structural ‘holes’. At Ilímaussaq, the late-stage lujavrites are enriched in volatiles - including S_2-. The presence of minor amounts of sulfur has been identified as the cause of fluorescence in both sodalite and tugtupite.

© Herb Yeates, Luminous Minerals, http://luminousminerals.com/ 2006