In various areas of modern life, it becomes necessary to obtain low-power UV radiation. It:
- medical equipment, incl. dental fillings hardening under the influence of ultraviolet radiation;
- cosmetology, where ultraviolet light is used as a means of hardening varnishes;
- installations for chemical drying of various compositions (an example is shown in Figure 1);
- scanners to determine the authenticity of bills and documents;
- greenhouse farms where ultraviolet light stimulates plant growth.
Classic lamp sources of ultraviolet radiation have unsatisfactory weight and size characteristics and consume high power. As their effective replacement in recent years, UV LEDs have become widespread, which are free of these disadvantages and additionally differ in ease of use.
Principle of operation and characteristics
The principles of operation of an ultraviolet light-emitting diode (LED) and LED of the visible wavelength range are identical. The devices contain a pn junction through which a direct current is passed. During its passage, charge carriers find themselves at high energy levels of the semiconductor structure. Their transition to lower levels, with appropriate execution, is accompanied by the generation of light quanta.
When the structure is formed by additions of aluminum, gallium, indium and other rare earth elements, the distance between the radiative transitions is selected so that the radiation is in the invisible near-UV wavelength range (usually the spectral range from 100 to 400 nm).
The described principle of operation immediately determines the list of parameters that are normalized for ultraviolet LED. The passport indicates:
- working wavelength, sometimes complemented by the width of the spectral line;
- maximum luminous flux, indicated in lumens for high-aperture sources, or given in candellas (flux-to-solid angle ratio) in the case of directional radiation;
- maximum forward current;
- forward voltage.
Due to the use of a special pn-junction structure, the forward voltage on UV diodes is usually noticeably higher compared to conventional ones and sometimes reaches 5 V.
The operating time of the diode is several tens of thousands of hours, the typical temperature range is from -20 to + 100 ° С.
Structural performance
For reasons of minimizing the range of components, ultraviolet LEDs are produced in standard cases, which at the same time helps:
- solve the problem of polarity due to the presence of a key protrusion;
- provide the necessary heat dissipation and elements with high forward currents, provided that a metal case is used.
For low power emitters, a cheaper plastic housing is used, such as shown in Figure 2.
The most common diodes have lead wires, while SMDs have ribbon leads. The last option is shown in Figure 3.