Can light emitting diodes be used to generate UV wavelengths? I don't know much about light-emitting diodes, but I imaging if you had a panel of RGB diodes you could produce any wavelength of color within the visible light spectrum.  However, if I
also wanted to generate specific wavelengths of UVA or UVB (anywhere from 290 to 400nm), could I also accomplish this using diodes?  Essentially, I am interested in making a small panel of diodes in which I could produce any specific wavelength of visible light, UVA, or UVB.
Thanks in advance.
 A: RGB diodes, the kind you find in an LED computer monitor or television, don't actually produce any wavelength of light you want. As you probably know, the diodes come in triplets, one each of red, green, and blue. Each of those individual diodes only emits one specific wavelength of light.
However, by varying the relative intensities of the three colors, it's possible to simulate the effect of any other wavelength of light on the human eye. Since there are only three types of cones on the retina, for any given wavelength which the eye detects, you can induce the same chemical reaction in the cones as that wavelength naturally would, using only three colors.
The point is, in RGB LEDs there are only three wavelengths ever being produced, and it's only by "fooling" our eye that they create the illusion of the full spectrum. Since human eyes aren't sensitive to UV radiation, the trick wouldn't work for ultraviolet wavelengths. You can't actually create a diode that will produce any wavelength you want dynamically.
Of course, there are a wide range of materials that you can put in a diode in order to affect the wavelength of radiation it does produce. From a physics perspective, UV radiation is no different from light except that it has a higher frequency (shorter wavelength), so by choosing the right material, you can in principle create a diode that will get you any pre-chosen UV wavelength you want. The Wikipedia article on LEDs gives a table of various materials grouped by the wavelengths they can produce, and there are several options for UV radiation. (There may be a lower limit on the wavelength obtainable from an LED due to energy considerations, but I'm not familiar with the details of how LEDs work so I couldn't tell you whether that's the case or what wavelength it might be.)
A: 
Light-emitting diodes (LEDs) can be
  manufactured to emit light in the
  ultraviolet range, although practical
  LED arrays are very limited below 365
  nm. LED efficiency at 365 nm is approx
  5-8%, whereas efficiency at 395 nm is
  closer to 20%, and power outputs at
  these longer UV wavelengths are also
  better.

http://en.wikipedia.org/wiki/Ultraviolet#Ultraviolet_LEDs
There are not much options in production currently. Mainly ~395nm is available and that's it. Spectrum is quite wide. You can change wavelength a little by altering current/temperature, but that's tricky. 
You might want to look at non-linear crystals too (KTP for example).
A: Leonidas is completely wrong! The color of light produced from a light emitting diode is manly dependent of the band gap of the semiconductor material that is used to produce the LED.
For example:
• Infrared: GaAs or AlGaAs
• Red: GaInAlP or GaAlP
• Yellow/Orange: GaInAlP or GaAlP.
• Green: GaAlP or InGaN
• Blue: GaN or InGaN.
• Ultraviolet: GaN.
You may note that some of the materials are the same for different colors of light. This is due to the change in band gap that can be produced by changing the doping concentration and doping materials.
Some LEDs have a layer of fluorescent material though. To shift the color slightly in an easy way.
And to answer the original question:
Yes, as BarsMonster pointed out, there are Ultraviolet LEDs, but the selection is not that big.
A: RGB diodes are actually LED with fluorescent dye which is stimulated and thus mixed to different colours. LED-light itself is nearly monochrome.
So unless you find LEDs with dye that fluoresces in UV (I doubt that there are some for the normal market) you'll have to buy some UV-LED for these wavelengths.
