What is Ultraviolet Light?

UV lights are the mainstay of the Fluorescent Mineral hobby. These lights are used in the field to collect these beautiful minerals and are an essential tool used on our yearly “Geo-Adventure” tours to Greenland. The results are the myriad of glowing colored specimens found on the MinerShop web site. UV lights are not only used by hobbyists to find these treasures but have been used by prospectors in the past to find minerals such as uranium and scheelite - primary ores used for many purposes. In the early days of mining at Franklin NJ UV lights were an essential tool in locating the ore veins.


Fluorescence is a “cold light” (luminescence) caused by electromagnetic radiation. There are many forms of electromagnetic radiation: visible light, radio, infrared, ultraviolet, X-rays, and gamma rays are all forms of electromagnetic radiation. All are like light in many ways; but most occur at wavelengths that the human eye cannot see.


Visible light can cause luminescence; shine a pure violet light at a ruby, and it glows red. It absorbs the violet light energy and uses it to create red light. But this light-induced luminescence is uncommon and hard to see. Radio and infrared supply too little energy to cause visible luminescence. X-rays can cause dramatic luminescence, but X-ray sources are expensive and dangerous - not practical for ordinary use.


The form of electromagnetic radiation that is most widely used to observe fluorescence is ultraviolet radiation, as generated by a "black light" or Ultra Violet lights. Ultraviolet light is that portion of the electromagnetic spectrum that lies beyond the purple edge of the visible spectrum and has wavelengths between 100 and 400 nm.


The UV spectrum is further divided into ranges as follows:




 Range Name  Wavelength Range (nm)
 UV-A             315nm to 400nm
 UV-B 280nm to 315nm
 UV-C 200nm to 280nm
 Vacuum UV (V-UV) 100nm to 200nm


Ultraviolet lights



For the fluorescent mineral collector there are three useful wavelengths of ultraviolet light; Long wave, Mid wave, and Short wave. A few minerals fluoresce the same color in all wavelengths, others fluoresce in only one wavelength (usually SW), and yet others fluoresce different colors in different wavelengths.


Longwave (LW) - compared to Shortwave, only a relatively few minerals fluoresce in longwave (perhaps only 15% of the total). Sometimes a significant difference in fluorescing color can be observed between the two. Longwave UV is closest to visible light and is the type of UV light that most people are familiar with (everyone has seen the “blacklight” bulbs used in discos, to light up posters, etc). The light easily passed through most types of glass and plastic. Thus Longwave lamps are fairly inexpensive compared to shortwave - and the filters are significantly cheaper.


Filter - Although ultraviolet light is “invisible”, the lamps used to generate UV light emit a significant amount of bluish/white visible light. This visible light must be blocked (filtered, typically by a piece of “black glass”) so that the light does not overwhelm the fluorescence. A longwave filter is relatively inexpensive - in fact, some fluorescent lamps have a filter built-in (such as the common “blacklight fluorescent bulb” used to illuminate posters and “glow” products.


The best (most efficient) Longwave UV source to use for mineral fluorescence is a special white phosphor coated blacklight bulb, combined with an external visible light blocking filter. While this approach is considerably more expensive than using ordinary blacklight bulbs, it provides much purer UV light, allowing less visible light to mask the fluorescence.


Recently advances in LED technology have resulted in a wide selection of longwave UV LEDs. Most are rather weak in power, and some are very expensive, but as in all technology these products will continue to improve and come down in price. Soon, a solid state longwave UV light will be a practical alternative to fluorescent bulbs


Roithner-Laser is a leading innovator in this technology and has recently announced a 1 watt LED (M3L1-HU-120, UV, 395 nm, 350 mA, viewing angle 120°) - expensive, but powerful and a step in the right direction. http://www.roithner-laser.com/LED_diverse.htm


(see this article on Blacklight US for a more in-depth explanation)


Midwave (MW) - The Midwave ultraviolet spectrum lies in between Longwave and Shortwave. Midwave UV is partially stopped by clear glass just as Shortwave UV. Midwave UV is passed by existing Shortwave ultraviolet filters, and, since Midwave lights (tubes) have become more readily available, the properties of Minerals under Midwave UV are starting to be noticed.


Certain Minerals that do not exhibit a strong Fluorescence under either LW or SW may FL strongly under MW. Occasionally a color change may occur, or fluorescence may be seen where none was observed under the other two lamps.


Shortwave (SW) - Shortwave UV is the most popular light source for displaying fluorescent minerals. The number of minerals that fluoresce under SW far exceeds those that fluoresce under LW or MW. But the lamps (bulbs and filters) required for SW illumination are considerably more costly.


Shortwave ultraviolet is almost completely stopped by most forms of glass or plastic. Quartz or silica glass must be used in shortwave tubes to let the shortwave UV escape the tube. SW ultraviolet can, over time, cloud the shortwave filter used in the lamp assemblies. This is called solarization. Additionally, the ultraviolet light source has a tendency to lose output power after several hundred hours of use; heating and repeated on/off cycling can further degrade some of the lights.


CAUTION! Ultraviolet light is dangerous to your eyes and skin. Even a short exposure can burn the eyes, causing severe irritation at the least and possibly cause loss of site. Never look into a UV light source with unprotected eyes.


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