Ultraviolet (UV) is that part of electromagnetic light bounded by the lower wavelength extreme of the visible spectrum and the X-ray radiation band.The spectral range of UV light is, by definition between 100 and 400 nm (1 nm=10-9m) and is invisible to human eyes. Using the CIE classification the UV spectrum is subdivided into three bands:

UVA (long-wave) from 315 to 400 nm
UVB (medium-wave) from 280 to 315 nm
UVC (short-wave) from 100 to 280 nm

In reality many photobiologists often speak of skin effects from the weighted effect of wavelength above and below 320 nm, hence offering an alternative definition.

A strong germicidal effect is provided by the Light in the short-wave UVC band. In addition erythema (reddening of the skin) and conjunctivitis (inflammation of the mucous membranes of the eye) can, also be caused by this form of Light. Because of this, when germicidal UV-Light lamps are used, it is important to design systems to exclude UVC leakage and so avoid these effects.
Self evidently people should avoid exposure to UVC. Fortunately this is relatively simple, because it is absorbed by most products, and even standard flat glass absorbs all UVC. Exceptions are quartz and PTFE.
Again fortuitously, UVC is mostly absorbed by dead skin, so erythema can be limited. In addition UVC does not penetrate to the eye’s lens; nevertheless, conjunctivitis can occur and though temporary, it is extremely painful; the same is true of erythemal effects.

Where exposure to UVC Light occurs, care should be taken not to exceed the threshold level norm. Figure 9 shows these values for most of the CIE UV spectrum. In practical terms, table I gives the American Congress of Governmental and Industrial Hygienist’s (ACGIH) UV Threshold Limit Effective Irradiance Values for human exposure related to time. At this time it is worth noting that radiation wavelengths below 240 nm forms ozone, O3 from oxygen in air. Ozone is toxic and highly reactive; hence precautions have to be taken to avoid exposure to humans and certain materials

UV light bulb application

1.Air purification

Good results are obtained with this form of purification because air has a low absorption coefficient and hence allows UVC to attack micro-organisms present. In addition, two other beneficial conditions are generally present, viz. random movements allowing bacteria etc. to provide favorable molecular orientations for attack and high chances of “closed circuit” conditions, that is second, third and more recycle opportunities. From this, it is evident that air purification is an
important application for UV light.
Even in the simplest system (natural circulation) there is an appreciable reduction in the number of airborne organisms in a room. Thus the danger of airborne infection, a factor in many illnesses, is considerably reduced.
However, it should be remembered that purified air is not, in itself, a purifying agent.
Presently, there are five basic methods of air purification using UV lamps viz:
a. Ceiling or wall mounted Philips TUV lamps
b. Philips TUV lamps (in upwards-facing reflectors) for upper-air irradiation.
c. Philips TUV lamps (in downwards-facing reflectors) for irradiation of the floor zone (often in combination with b.).
d. Philips TUV lamps in air ducts sometimes in combination with special dust filters.
e. Philips TUV lamps, incorporated in stand-alone air cleaners with a simple filter.

2. Surface purification
Surface purification generally requires high-intensity short-wave UV light. Mostly this means TUV lamps are mounted close to the surface requiring to be kept free from infection or to be purified. The success of surface purification depends largely on the surface irregularity of the material to be purified, because UV light can only inactivate those micro-organisms that it hits with a sufficient dose.
Thus purification can only be successful if the entire surface is exposed to UV light. Micro-organisms sitting in “holes” in a surface are not likely be overcome by reflections from the hole walls, as can be deduced from the reflectances shown in table 3.
In practice, solid surfaces, granular material and packaging (whether plastic, glass, metal, cardboard, foil, etc.) are purified or maintained germ-free by means of intensive, direct irradiation. Additionally, purified material can be kept largely germ-free throughout its further processing by irradiating the air along its path.

3. Liquid purification
Germicidal energy radiation is capable of penetrating liquids with varying degrees of efficiency. From a treatment view, liquids can be regarded as similar to air so the further the UV light is able to penetrate the liquid, the more efficient is its action. The degree of efficiency thus greatly depends on the liquid and more particularly its absorption coefficient at 254 nm (table 4).As an example, natural water’s transparency to 254 nm may vary by as much as a factor of
10 or more from place to place. Polluted industrial water often needs purification followed by disinfection; here UVC is growing with many thousands of systems in use in North America and Europe, each with a multitude of lamps. Often UV light may supplement or replace conventional chlorination measures (see later). UVC has advantages over chlorinating techniques, because it produces far fewer noxious by-products and is it unaffected by the pH of the water or its
temperature. The reader should note that the latter comment refers to the radiation, not to the lamp, or its environment as described earlier. Micro-organisms are far more difficult to kill in humid air, or in a liquid environment, than in dry air.This is because they limit transmission of 254 nm radiation. In more quantitative terms liquids decrease the germicidal intensity exponentially according to the formula

4. Water purification
A wide variety of micro-organisms in the water can cause disease, especially for young and senior people, who may have weaker immune systems. UV light provides purification without the addition of chemicals that can produce harmful by-products and add unpleasant taste to water. Additional benefits include easy installation, low maintenance and minimal space requirements.
UV has the ability to inactivate bacteria, viruses and protozoa. Each type of organism requires a specific dose for inactivation.
Viruses require higher doses than bacteria and protozoa. Understanding the organisms to be neutralised will help to determine to size of the UV system that will be required. For example, to kill 99,9% of E.coli, a UV dose of 90 J/m2 or 9 mW.sec/cm2 is required.
UV installations are suitable for industrial, municipal and residential markets.
The quality of the water has an important effect on the performance of UV systems.The common factors that have to be considered are iron, hardness, the total concentration of suspended solids and the UV transmittance.Various organic and inorganic compounds can absorb UV. When there is uncertainty about what may be present in the water, the UV transmittance should be tested. Most drinking water supplies have U V transmittances between 85% and 95%.