There are many kinds of diodes used for different purposes : as a one way switch, as a variable capacitor, to mix radio frequencies, as a voltage regulator, etc... Light emitting diodes (LEDs) of course convert some of the energy they receive as light and convert the rest to heat. They operate in a relatively narrow range of DC voltages, and especially at the higher power ratings are subject to thermal overload (basically when driven incorrectly heat causes them to use more power, which generates more heat, which builds to the point where they fry themselves). Specialized diode drivers have been created to handle these characteristics at higher powers. Some things to note:
1) LEDs are not always more efficient that fluorescents or mercury vapor lamps. It took a very long time to develop efficient LEDs and then even longer to develop bright ones. Also the material and technology changes as the frequency of the LED does. So where as it is easy to find a white light LED that's more efficient than a fluorescent bulb today, the same might not be true at all for a 365nm UV LED. In general the shorter the wavelength, the less efficient the LED will be any given year (they do get better over the years). LEDs tend to produce more directional light though which can be used to more efficiently illuminate the negative. So in total the efficiency is (efficiency of bulb * efficiency of transport to get that light onto the photo chemistry * efficiency of the chemistry at the wavelength put out by the bulb). This equation can (and I suspect often does) come out in favor of the mercury bulb. The price per watt of bulb is certainly higher for LEDs as well. I think LEDs have a place though.
2) LEDs do not always have long life. The ones used for lighting do, but the ones used for curing UV resin and the like only have moderate lives.
3) There are several places that sell electronic parts in small lots and these pages have the spec-sheets for the stuff they sell. It can be a great place to back of the envelop figure out what you're going to get with any given LED (Life time, efficiency, illumination angle, power requirements, heat sink requirements, etc).