Ultraviolet light (UV light) is a type of electromagnetic radiation that is located between visible light and X-rays in the electromagnetic spectrum. It is invisible to the human eye, and has a frequency greater than that of visible light but lesser than X-rays. It has a wavelength ranging from about 10 nm to 400 nm (ISO, 2007). UV light is a part of the radiation that comes from the sun. It can also be produced artificially by electric arcs, black lights, tanning lamps and mercury vapor lamps. The name Ultraviolet literally translates to “beyond violet”, as it has a frequency beyond the violet color in the visible spectrum.
A German physicist named Johann Wilhelm Ritter was the first to discover UV light in 1801. Inspired by the discovery of heat rays or infrared radiation by William Herschel a year earlier, Ritter searched for an opposite (cooling radiation) at the other end of the visible spectrum. Although he was not successful in finding the cool rays that he was looking for, after several attempts of experimentation, he observed that silver chloride turned from white to black faster when it was placed at the dark region past the visible spectrum beyond the color violet (Frercks, Weber, & Wiesenfeldt, 2009). This radiation was termed as “chemical rays” and was called so for some time after its discovery. Another scientist named John William Draper opined that this radiation was completely different from visible light (Draper, 1842). He named them as “tithonic rays” (Draper, 1844). The names “chemical rays” and “heat rays” were discarded and the terms ultraviolet and infrared were adopted respectively (Beeson & Mayer, 2008) (Hockberger, 2007). A German physicist named Victor Schumann made the discovery of UV radiation below 200 nm in 1893 (Lyman, 1914).
The effectiveness of short-wavelength UV radiation in sterilizing bacteria was discovered in 1878. By the turn of the 20th century, it was discovered that the wavelength of 250 nm was ideal for this purpose. A few decades later, in 1960, the effects of UV radiation on DNA were discovered (Bolton & Cotton, 2008).
The ultraviolet (UV) range of electromagnetic radiation can be subdivided further into a number of smaller ranges as suggested by the ISO standard ISO-21348 (ISO, 2007). In the range of wavelengths from 10-121 nm, it is known as EUV or extreme UV, and is completely ionizing radiation It is however, entirely absorbed by the atmosphere. The radiation in the range of 100-200 nm is also termed as Vacuum UV or VUV. It is mostly absorbed by oxygen in the atmosphere, but the radiation with wavelengths 150-200 nm have the ability to propagate through nitrogen. The UV radiation in the ranges 122-200 nm, 200-300 nm and 300-400 nm are known as Far UV, Middle UV and Near UV respectively. The light in the Near UV range is visible to insects, fish and birds. Alternatively, there is a different type of subdivision where wavelengths from 100-280 nm, 280-315 nm and 315-400 nm are termed as UV C, UV B and UV A respectively. UV A light is long wave and is the one used in black lights. It is not absorbed by the ozone layer. UV B radiation is medium wave and is almost completely absorbed by the ozone layer and the atmosphere. UV C radiation is short wave and is used as a disinfectant because of its germicidal effect. It is totally absorbed by the ozone layer and the atmosphere.
In nature, UV light is emitted by very hot objects. The Sun radiates UV light of all different wavelengths including the extreme UV range in the 10 nm, where it crosses over to the X-ray spectrum. About 10% of the radiation emitted from the sun is in the UV spectrum (Holton, Curry, & Pyle, 2003). There are many artificial sources of UV light. This includes black lights, short wave ultraviolet lamps, gas discharge lamps, Ultraviolet LEDs, Ultraviolet lasers, Tunable VUV and plasma and synchrotron sources of EUV.
UV light has a number of applications because of its properties that enable it to have chemical reactions and also cause fluorescence in materials. UV light is used for UV photography wherein images are recorded by using only UV light that is emitted from objects. The main reason for using UV photography is to acquire information about objects or materials that cannot be done with conventional photography using visible light. Only Near UV (200-380 nm) is used for UV photography as air is opaque to wavelengths below 200 nm and the glass lens used in cameras is opaque below 180 nm. The two types of UV photography are Reflected UV photography and UV induced fluorescence photography. Reflected UV photography involves illumination of the object that is to be photographed using UV light. It uses only long wave UV (320-400 nm) by using a filter that allows only this range to pass through the lens (Robinson, 2007). These types of filters usually block other wavelengths including visible light and infrared radiation. Reflected UV photography is used in the field of dermatology, where it can be used to expose differences in skin conditions (Eastman Kodak, 1972). It is also used in UV micrography as use of UV light instead of visible light provides increased sharpness and resolution in microscopes (Eastman Kodak, 1972). On the other hand, the principle behind UV induced fluorescence photography is that the visible light from fluorescence, induced by UV radiation, is used for capturing images of objects. Consequently, the filter used for this kind of photography needs to only allow visible light and block all types of UV and infrared radiation.
UV light is also used to detect ordinarily colorless fluorescent dyes that are seen to be of a certain color under UV illumination. The applications of this phenomenon include the use of optical brighteners in fabrics and paper, black light paints that are used in art and other aesthetic applications, watermarks that help prevent the counterfeiting of currency and other vital documents such as passports and driver’s licenses, forensics, certain brands of pepper spray that are not easily washable and help the police identify the felon later and in non-destructive testing (NDT) methods such as liquid penetrant inspection.
UV light also has many analytic uses. It is used by forensic experts to detect bodily fluids such as saliva, semen and blood at crime scenes, that are otherwise hard to locate or completely invisible (Springer et al., 1994). UV light from high power sources is especially useful in detecting ejaculated fluids irrespective of the type, structure or color of the surface on which it is deposited (Fiedler et al., 2008). It is also helpful in the detection of organic deposits that remain as a result of lack of sanitizing and periodic cleaning. Stains on carpets because of urination by pets are also detectable by UV illumination. These capabilities make UV light a very useful tool in revealing unsanitary conditions in public places, hotel rooms and toilets etc. UV light has biological research applications where it is used to quantify proteins or nucleic acids. Analysis of minerals and precious gems can be done with the aid of UV lamps. The ordinarily invisible emissions of nitrogen oxides, mercury, sulfur compounds and ammonia in the exhaust gases of industries can be detected by UV analyzers (Battikha, 2007). Owing to the high reflectivity of oil films and the presence of fluorescent materials, UV light is also used to detect thin sheets of oil on water (Fingas, 2011). UV detectors are used to detect certain types of fires that produce higher radiation in the UV spectrum than in the infrared (IR) region. UV light is also used in high resolution photolithography, which is used in the manufacturing of printed circuit boards, semiconductors and integrated circuit components (IBM Almaden Research Center, 2016).
UV radiation is used for biological purposes such as air purification, sterilization and disinfection. It is also used in the reduction of gaseous pollutants such as carbon monoxide and volatile organic compounds (VOCs) (Scott, Wills, & Patterson, 1971). UV light therapy is used in treating skin conditions such as vitiligo and psoriasis. Fluorescent UV lamps are used in reptile enclosures to help the reptiles synthesize vitamin D, which they require to metabolize calcium for the production of eggs and bones.
There are both positive and negative effects of UV light on human health. UV light is a primary source of vitamin D3 as well as mutagen (Osborne & Hutchinson, 2002). Excessive exposure to UV light has a number of harmful effects on humans. Skin, eyes and the immune system are adversely affected by overexposure to UV radiation (WHO, 2016). UV irradiation on normal human skin can cause local or systemic immune-suppression, tanning and sunburn inflammation erythema (Matsumura & Ananthaswamy, 2004). UVA radiation is the primary cause of almost 92% of malignant melanoma, which is caused by DNA damage (Davies et al., 2002). Sunscreen is generally helpful and recommended for protection against sunburn and risks associated with exposure to UV light. However, some studies have indicated that sunscreens may increase cell mobility and the combination of octyl p-dimethylaminobenzoate (o-PABA) with solar UV may selectively damage melanocytes in the skin (Xu, Green, Parisi, & Parsons, 2001). Other studies have even revealed that sunscreens could, while preventing sunburn, contribute to sunlight-related cancers (Knowland, McKenzie, McHugh, & Cridland, 1993). The human eye is very sensitive to UVC light, which is present in welder’s arc lights and other artificial sources of UV light. Exposure to this band of UV light can cause photokeratitis or welder’s flash, that can lead to cataracts. UVB radiation is also known to cause pterygium (Nolan et al., 2003). The harmful effects to the eyes by UV light can be avoided by wearing protective eyewear designed to block UV radiation. Thus, UV light has numerous uses in the modern world, but has to be used with appropriate precaution and care to avoid its harmful effects.
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