UV-A is UV radiation in the range of 330-400 nm. The eye cannot see the radiation and the skin will react to the exposure after a while with redness and sunburn. Tanning is a desirable after-effect under controlled exposure which may be harmless. Solar UV-A reaching the Earth’s surface plays a very strong biological role in plants, when they produce leaves as more UV-A reaches plants in summer time, i.e. UV-A could be measured with a meter to quantify UV-A for better orchard management or a plant growing operation.
UV-A is also emitted from specialist lamps and are sometimes casually referred to as ‘dark lights’. UV-A causes fluorescence in certain substances e.g. rocks and clothing that cause funky, bright colours in clothing at night clubs. UV-A light sources are successfully used by NDT industries (non-destructive testing) to find fine cracks in a metal using fluorescent dye, for example.
UV-A LEDs are already available on the market in the form of UV-A torches. UV-A light sources may also find use in causing fluorescence on bank notes to check if they are legit or may be applied for curing of glues and resins in certain manufacturing industries. UV-A light sources are also used in mineralogy, which can cause fluorescence in certain rock types that enable identification.
Fluorescence in rocks is rather spectacular in the remarkable bright colours produced in a dark room or outdoors at night – this is the same principle that fluorescent bulbs work on. Certain insects may also undergo a brief colour change when exposed to UV-A radiation.