Discovery of Ultraviolet Light
In 1666 Isaac Newton allowed a sunbeam to enter into a darkened room through a small hole, pass through a prism, and fall upon the opposite wall. What appeared was a spectrum of very vivid and intense colours. By means of further experiments he was able to prove that white light was made up of several different components.
In 1800, Sir William Hershel discovered that the solar spectrum extended out beyond the portion which was visible to the human eye. He formed a spectrum and passed a thermometer through the various colours and compared the reading with that of a similar thermometer shaded from the spectrum. He found a rise in temperature as he passed from the violet to the red end. Still more astounding was the fact that the thermometer showed a higher temperature in the dark space beyond the red than it did in any part of the visible spectrum. This meant that there was some intense radiation beyond the region which was visible to the eye. This invisible radiation beyond the red end of the spectrum is now known as infrared radiation.
In 1801, J.W. Ritter investigated the other end of the spectrum and showed that chemical action was caused by some form of energy in the dark portion beyond the violet. This region is now known as ultraviolet.
Sources of Ultraviolet
The sources of ultraviolet may be didived into two classes: natural and artificial. The most important source of ultraviolet is the sun. The artificial source comprises a variety of arcs and incandescent lamps.
Ultraviolet disinfection of water generally employs low pressure mercury vapour lamps. They generate short wave ultraviolet in the region of 253.7 nanometers, which is lethal to micro-organisms including bacteria, protozoa, viruses, moulds, yeasts, nematode eggs and algae.
Ultraviolet dosage is measured in microwatt seconds per square centimetre (uWs/cm2). The higher the microwatt, the higher the dosage, the longer the exposure time, the higher the dosage. Increase the area exposed to the ultraviolet radiation the higher the kill.
Approximate dosage rates to kill:
- Bacteria 2,500 – 26,400 uWs/cm2
- Yeast 6,600 – 17,600 uWs/cm2
- Algae 11,000 – 330,000 uWs/cm2
- Viruses 2,500 – 22,000 uWs/cm2
The U.S. Public Health Service published a policy stating the dosage requirement for drinking water should be approximately 16,000 uWs/c2. This statement has formed the basis for a World Wide Standard. Every UVTA(TM) UV unit emits in excess of 36,000 uWs/cm2, which is more than double the recommended dosage of the U.S. Public Health Service.