The first LEDs in the early 60s emitted infrared light. Over the years, it was possible to develop LEDs for shorter wavelength, higher energy radiation and today it stands at the beginning of the development of the UV spectral range.
The main body of UV-LEDs consists of the semiconducting compounds GaN, InN and AlN and their mixed crystals. When current passes through specially doped layer sequences of these semiconductors a part of the electric current can be converted directly into UV radiation. Such semiconductor devices are known as light-emitting diodes or "LEDs". LEDs emit radiation only in a very narrow energy range around a central wavelength and their position (eg, the number of atomic layers in the individual layers) can be used to selectively adjust the composition of the semiconductor material and the design. The composition of the various layers must be accurately taken and their thickness can be accurately adjusted to one atomic layer.
UV LEDs are only at the beginning of their development and consequently LED-based applications are still being created. The shorter the wavelength of the radiation, the higher are the scientific and technical requirements for the material development and LED device technology. Therefore, in general, power and efficiency of short-wave UVB / UVC LEDs are behind the LEDs that emit in the near-UV region.
Advantages of UV LEDs
- Emission wavelengths of UV-LEDs are adjustable on the composition of semiconductor materials
- narrow-band emission without spurious peaks (eg no unwanted ozone production)
- Radiation intensity electrically adjustable easy (ie power scales linearly with the current flow or is digitally controlled via TTL technology)
- Generating short pulses (ms to a few 10 ns) easy to implement, so that eg new algorithms possible in metrology
- Radiation of UV-LEDs can be tailored (eg point-like radiation sources for sensor applications)
- compact design of the UV LEDs and UV photodetectors allows considerable freedom in the design of the radiator modules or UV systems
- Operating at low DC voltages and currents (ie operation with batteries or solar cells easily possible)
- Durable and maintenance free (expected lifetimes of many 10,000 h)
- immediate full function without preheating (important for the use of point-of-use systems for water disinfection)
- do not contain toxic materials (such as heavy metals such as Hg)
- no heat radiation in the emission direction (eg by treatment of heat-sensitive biological substances possible)
- extremely robust, compact (eg no protection against glass breakage necessary, mobile use possible)
Alternative UV sources
Currently the generation of UV radiation by excitation of gas discharges dominates technology. The most common are low and medium pressure mercury (Hg) –vapor lamps. Its housing is made of fused silica (quartz), partly because the necessary UV transmission, partly because of the high operating temperature. Low-pressure Hg lamps primarily show discrete emission wavelengths at 254 nm and 185 nm. Reach high conversion efficiencies (up to 40%), however, are limited in the power density.
With medium pressure Hg lamps, higher output powers can be achieved, however, the conversion efficiencies are low (typically 15-30%). The Hg-line spectrum widens in medium and high pressure lamps in the deep UV to a continuum and in addition shows discrete emission lines in the visible and UVA / UVB spectrum.
Mercury lamps require high voltages to operate and radiate toward the UV emission a lot of heat, for example, the surface temperature of medium pressure lamps from 600 to 950 oC. Moreover, these UV-radiation sources have very often residual intensities in the UVC range, which greatly restricts the medical application, because human skin has no protection mechanism. Besides Hg lamps and excimer lamps and excimer lasers for the generation of UV light can be used which are able to emit according to the used gas mixture at 193 nm, 222 nm, 248 nm, 282 nm, 308 nm or 351 nm. However, the efficiency of the excimer lamps is even lower (typically <15%) and the lifetime is restricted to a few thousand hours. Excimer lasers are extremely expensive and complex to operate and are therefore usually used for special medical applications.
Even if gas discharge lamps currently represent the most common source of UV radiation, they have a number of disadvantages that set limits to their applications.
For more information about alternative UV sources and UV radiation click here.