The human eye can see light waves with lengths between 380 and 780 nanometers. Ultraviolet light is the part of the spectrum that has wavelengths between 30 nm and 300 nm. It is called “ultraviolet” because its wavelengths are closest to the violet part of the spectrum of visible light. There are three different parts of the ultraviolet spectrum: near, far, and extreme. Near is the area closest to where you can see light. The area closest to x-ray is extreme. Far is the zone in between. Light in the near area is further broken down into three types: UVA, UVB, and UVC. UVA is called black light or long-wave light. UVB is a medium wave, and UVC is a short wave that kills germs.
Johann Wilhelm Ritter, a German physicist, found at the beginning of the 19th century that invisible rays of light just above the violet end of the spectrum darkened paper that had been soaked in silver chloride. He called them “de-oxidizing rays” to tell them apart from heat rays, but they were really just ultraviolet light waves.
Scientists later found that ultraviolet light caused some molecules, called monomers, to join together chemically, making compounds called polymers. This discovery would be especially helpful for curing inks, coatings, and adhesives because it offered a fast-drying method that didn’t pollute and didn’t require solvents or volatile organic compounds, which do pollute (VOCs).
Photoinitiators, resins, and diluents that don’t use solvents are used to make inks, coatings, and adhesives that harden when exposed to UV light. When the photoinitiators are exposed to the right amount of energy and spectral irradiance, they start to polymerize with the resins. This makes the coatings harden, or cure. Most of the time, the curing process is very fast, and you only need a few seconds of exposure to get a full cure.
Mercury vapour lamps, which are also called medium pressure mercury arc lamps, are lamps that are made to make ultraviolet light. Mercury has been added to these gas discharge lamps. In order for the machine to work, an electric field is made in the quartz tube, which makes the argon ionise. As the electrical field gets stronger, heat and pressure build up inside the tube, which causes the mercury to vaporise. The free electrons of argon start to hit gas and metal atoms, which changes the energy states of the electrons in orbit around the mercury atoms. When an orbital electron in mercury moves from a higher energy state to a lower energy state, it gives off a photon of UV light.