Visible light is a form of electromagnetic radiation. Other forms of electromagnetic radiation are like light in many ways; they are just at different wavelengths that the human eye cannot see. Since wavelength is what makes the difference between colors in visible light, you might say other wavelengths of electromagnetic radiations are "invisible colors of light". Radio, infrared, visible light, ultraviolet, X-rays, and gamma rays are all forms of electromagnetic radiation.
Fluorescence is luminescence caused by electromagnetic radiation. Visible light can cause luminescence; shine a pure violet light at a ruby, and it will glow red, as it has absorbed the violet light energy and used it to create red light. But light-induced luminescence is uncommon and hard to see. Radio and infrared supply too little energy concentration to cause visible luminescence. X-rays cause many dramatic luminescences, but X-ray sources are too expensive, not portable enough, and require too much shielding and attention to safety for ordinary use.
The form of electromagnetic radiation that is most easily used to observe fluorescence is ultraviolet radiation, as generated by a "black light" or UV lights.
Ultraviolet is closest to and just shorter than visible light
in wavelength.
Ultraviolet can be subdivided according to wavelength into
(from lowest to highest): longwave ultraviolet (UVA or near ultraviolet),
middlewave ultraviolet (UVB), shortwave ultraviolet (UVC),
and extreme ultraviolet.
Longwave ultraviolet is part of sunlight. It is the lowest-frequency ultraviolet, and thus the nearest to visible light. Longwave ultraviolet passes easily through most transparent types of glass and plastic. Longwave ultraviolet lights are available, and they are the cheapest and longest-lasting ultraviolet lights. They cause some fluorescent minerals (perhaps 15%) to exhibit fluorescence.
Midwave ultraviolet is also part of sunlight. Longer wavelengths of midwave ultraviolet cause suntans, while shorter wavelengths of midwave cause sunburn. Midwave, especially shorter wavelengths, is partially stopped by clear glass. Since midwave ultraviolet is passed by shortwave ultraviolet filters, and since midwave tubes have recently become widely available, some collectors are starting to use midwave to study mineral fluorescence.
Shortwave ultraviolet is emitted by the sun, but it is stopped in the upper atmosphere of the earth by the ozone layer. Shortwave ultraviolet can also cause burns resembling sunburns (they are often called sunburns, even though the sun did not cause them). Shortwave ultraviolet is almost completely stopped by most forms of glass or plastic. Quartz or special glasses must be used in shortwave tubes to let the shortwave UV escape the tube. Shortwave ultraviolet over time cause failure in the shortwave filter used in shortwave ultraviolet lights; this process is called solarization. Shortwave ultraviolet is the most popular for seeing mineral fluorescence, causing fluorescence in perhaps 90% of fluorescent minerals.
Extreme ultraviolet is also emitted by the sun,
but is stopped in the upper atmosphere, and in so doing forms ozone
from the atmosphere's oxygen.
It is this high ozone layer that stops part of the sun's
middlewave ultraviolet rays and all of its shortwave ultraviolet rays,
and which may be in danger from some commercial chemicals.
Extreme ultraviolet is closest to X-rays in frequency, and as with
X-rays there is no practical equipment for its use.
Few substances are transparent to extreme ultraviolet, and even air
stops it within a fairly short distance.