Roger Weller, geology instructor
Ultraviolet (UV) Lights (Lamps)
Specialized ultraviolet lights, producing a very narrow wavelength are used for observing fluorescent mineral specimens. These lights consist of a power supply, an ultraviolet bulb, mechanical enclosure, and a UV filter. Specific bulbs and filters are selected for best operation in a specific portion of the UV spectrum desired. Power supplies are available for operation from house current and/or from a battery pack for field use.
Ultraviolet bulbs are similar to common fluorescent bulbs used in office lighting systems, containing a low pressure mercury vapor. These bulbs function by striking an electric arc through the bulb, which produces a light which is mostly shortwave ultraviolet, but includes some middlewave ultraviolet, longwave ultraviolet and some visible light. A fluorescent bulb used in the office has its tube coated inside with a material that fluoresces white. Thus the ultraviolet produced by this bulb is mostly converted to visible light. The glass tube of the lamp itself blocks all the shortwave ultraviolet light.
A longwave ultraviolet bulb is much the same, except the material coating the inside of the tube is made to fluoresce in the longwave ultraviolet portion of the spectrum. But since the coating also emits a fair amount of visible light, an external filter is needed to block the visible light. Only a dim purplish glow usually remains.
With a shortwave ultraviolet bulb, no fluorescent coating is used, and the glass tube must be made of quartz or a special glass which allows shortwave ultraviolet wavelengths to pass. As with the longwave bulb, a filter is added to suppress any visible light. The modest amounts of middlewave and longwave ultraviolet produced by the mercury vapor action also pass through the tube and filter, accounting for about 7% of the total radiation produced. Wavelength filters age with exposure to ultraviolet lighting and will eventually 'solarize' and begin to block shortwave ultraviolet light. Shortwave lights without a filter are commonly used by doctors to kill germs and by the electronics industry to erase erasable-programmable-read-only-memory (EPROM) memory chips.
The same mineral will fluoresce different colors under different wavelengths. Some minerals will only fluoresce under one wavelength, and most minerals will not fluoresce at all.
Calcite, normal light, Courtesy of:
short-wave light, Courtesy of: Georgia Tech
Calcite, long-wave light, Courtesy of:
Calcite, medium-wave light, Courtesy of: Georgia Tech
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