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.

(Please note: the two following paragraphs were written before any mid- or short-wave LED lamps were available)

True longwave UV lamps produce mostly longwave UV light; they have a special 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 longwave hobby and fluorescent display applications, "blacklights" will work; they just cast a blue cast on everything, which is not fluorescence but reflection.  Longwave UV light is present in sunlight, and is generally not considered harmful to humans.

Shortwave (and midwave} UV lamps come in two types – filtered and unfiltered. The unfiltered lamps invariably produce a lot of visible light in addition to the UV light; they are fine for germicidal 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 as shortwave lamps, but use a special phosphor inside (or outside) 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 virtually all of the shortwave UV, and most of the midwave UV as well. This is a good thing for us, 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 - I have personally received a bad "sunburn" on the back of my hand from < 5 min. of sporadic exposure to a powerful shortwave lamp while simply moving specimens around in a display.... 

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 inexpensive (~$10) battery-powered "blacklights" are available for field collecting.   Recently, LED battery-powered longwave and midwave flashlights (which project a very bright beam of filtered UV) are being widely marketed - while not particularly useful for a display (because of the narrowly focused beams), they are amazing tools for collecting, and for differentiating between fluorescent and non-fluorescent specimens.  One caveat - the UV flashlight beams are so powerful that it is dangerous to point them directly at people - any young rockhound's use of them should be supervised...

The bad news:  most minerals don’t fluoresce at all, and of those that do, only a relatively small percentage are really bright under longwave UV.  In order to see all the bright colors which make fluorescent minerals so attractive, eventually you may 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 $2-300+, and higher-powered display lamps start at $400 and can run to $1500 or more (!).  Midwave display lamps are usually purchased only by advanced hobbyists, and are at least as expensive as shortwave UV lamps.  But stay tuned:  the new LED's are beginning to appear in display lamps - and soon they may take over the market in all three UV bands!

The power 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 square of 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 conventional (tube) 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've used a 9-watt battery-powered lamp with some success - on a dark night, bright-fluorescing minerals can be seen easily as you walk around with the lamp held waist-high.  The new UV flashlights have a smaller field of view than my field lamp, but are MUCH brighter, which can be a significant advantage!  Hope that helps, but the subject is complex, and the lamps are expensive - so if you have specific questions, send me an email....