Ultraviolet Lamps - A Mineral Collector's Primer:

First, what’s the difference between a fluorescent light and an ultraviolet light?

This is confusing, because a lot of folks use the terms interchangeably. Typically a fluorescent light produces white light, and they are very commonly used in offices, schools and many homes as the primary artificial light source. Ultraviolet lights produce ultraviolet rays, and are used in germicidal applications, tanning salons, and to cause other objects (like posters) to glow (fluoresce). The confusion comes from the fact that fluorescent lights use ultraviolet rays internally to cause their phosphor coatings to produce white light externally, while ultraviolet lights produce ultraviolet rays externally to produce effects which white light doesn’t (like suntanning, or killing germs, or causing fluorescent objects to fluoresce).

OK, so what is ultraviolet light (UV)?

Scientists use a model of light that characterizes it by its wavelength; for example, the wavelength of red light is about 650 nanometers. Violet light has a shorter wavelength, about 400 nanometers. However, visible light is only a small part of the electromagnetic spectrum. Wavelengths just longer than red are called infrared (and invisible to humans), while light below the wavelength of violet light is called ultraviolet (also invisible to humans). Longwave UV (UVA) peaks at about 360 nanometers, while shortwave UV (UVC) peaks at about 254 nanometers. Midwave UV (UVB) is generally considered to peak at about 312 nanometers.

What is the difference between longwave and shortwave UV lamps?

True longwave UV lamps produce mostly longwave UV light; they have a filter which screens out most (but not all) visible light. "Blacklights" are an inexpensive way of producing longwave UV; they too have a visible light filter, but it is much less effective than that on a true longwave UV lamp. For most hobby and display applications, "blacklights" work just fine. Longwave UV light is present in sunlight, and is not harmful to humans.

Shortwave UV lamps come in two types – filtered and unfiltered. Unfiltered shortwave lamps invariably produce a lot of visible light in addition to the shortwave UV light; they are fine for germicidal applications, or tanning applications, but are largely useless in examining or displaying fluorescent objects – the visible light produced simply drowns out any fluorescence produced. Filtered shortwave lamps use a very expensive filter which allows shortwave UV through, but filters out almost all visible light; they are the ones which are used by scientists or hobbyists observing fluorescent minerals. Midwave UV lamps use the same filter used by shortwave lamps, but use a special phosphor inside the tube to produce mostly midwave UV light. Although both midwave and shortwave UV are produced by the sun, our Earth’s atmosphere filters out almost all of the shortwave UV, and most of the midwave UV as well. This is a good thing, because long-term exposure to midwave and/or shortwave UV light is suspected to cause skin cancer and cataracts.  For protection, you should always wear safety glasses (which block UV) when using shortwave or midwave UV lamps. Limiting skin exposure is also essential....

What kind of lamp do I need for fluorescent mineral collecting?

The good news: for beginning hobbyists, an inexpensive "blacklight" is a good way to get started; they are widely available, fun, and will work fine for minerals which fluoresce brightly when exposed to longwave UV.  Small battery-powered "blacklights" are available for field collecting.

The bad news: Most minerals don’t fluoresce at all, and of those that do, only a few are really bright under longwave UV. In order to see all the bright colors which make fluorescent minerals so attractive, eventually you will want to buy a good shortwave UV lamp. These are generally much more expensive than longwave UV lamps, and also should be used with care – protective eyewear should always be worn.  A good battery-powered shortwave UV field lamp will cost $200+, and higher-powered display lamps start at $400 or so, and can run to $1500 or more (!).  Midwave lamps are usually purchased only by advanced hobbyists, and are at least as expensive as shortwave UV lamps.

The power (wattage) of the lamp determines both how far from the lamp you can see fluorescent effects, and how brightly the minerals will fluoresce.  The amount of UV radiation reaching the illumination target is inversely proportional to the distance from the lamp - in other words, you want the lamp as close as possible to the rocks you are trying to light up!  So if you are examining rocks in your (gloved) hand, a low-powered shortwave lamp might be fine.  If you are setting up a display, the power you will need is determined by the area you want to view - I've found that a 36-watt lamp will adequately light up an area of about 2 feet by 3 feet, as long as the display area is protected from ambient light.  And for field collecting, I use a WTC 9-watt lamp - on a dark night, bright-fluorescing minerals can be seen easily as you walk around with the lamp waist-high.  Hope that helps, but the subject is complex, and the lamps are expensive - so if you have questions, send me an email....

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