CHAPTER # 1
INTRODUCTION TO ULTRA VIOLET RADIATIONS
Ultraviolet (UV) light is electromagnetic
radiation with a wavelength shorter than
that of visible light, but longer than soft X-rays.
It can be subdivided into near UV (380–200 nm wavelength), far or vacuum UV
(200–10 nm; abbrev. FUV or VUV), and extreme UV (1–31 nm; abbrev. EUV or
XUV).
When
considering the effect of UV radiation on human health and the environment, the
range of UV wavelengths is often subdivided into UVA (380–315 nm), also
called Long Wave or "blacklight"; UVB (315–280 nm), also
called Medium Wave; and UVC (< 280 nm), also called Short
Wave or "germicidal". In photolithography, in laser
technology, etc., the term deep ultraviolet or DUV refers to
wavelengths below 300nm.
The
name means "beyond violet" (from Latin
ultra, "beyond"), violet being the color
of the shortest wavelengths of visible light. Some of the UV wavelengths are
colloquially called black light, as it is invisible to the human eye.
Some animals, including birds, reptiles, and insects such as bees, can see into the near
ultraviolet. Many fruits, flowers, and seeds stand out more strongly from the
background in ultraviolet wavelengths as compared to human color vision. Many
birds have patterns in their plumage that are invisible at usual wavelengths
but seen in ultraviolet, and the urine of some animals is much easier to spot
with ultraviolet.
The Sun
emits ultraviolet radiation in the UVA, UVB, and UVC bands, but because of
absorption in the atmosphere's ozone layer, 99% of the ultraviolet radiation
that reaches the Earth's surface is UVA. (Some of the UVC light is responsible
for the generation of the ozone.)
Ordinary
glass is partially transparent to UVA but is opaque to shorter
wavelengths while Silica or quartz glass,
depending on quality, can be transparent even to vacuum UV wavelengths.
The
onset of vacuum UV, 200 nm, is defined by the fact that ordinary air is
opaque below this wavelength. This opacity is due to the strong absorption of
light of these wavelengths by oxygen in the air. Pure nitrogen (less than about
10 ppm oxygen) is transparent to wavelengths in the range of about 150–200 nm.
This has wide practical significance now that semiconductor manufacturing
processes are using wavelengths shorter than 200 nm. By working in oxygen-free
gas, the equipment does not have to be built to withstand the pressure
differences required to work in a vacuum. Some other scientific instruments,
such as circular dichroism
spectrometers, are also commonly nitrogen purged and operate in this spectral
region.
Extreme
UV is characterized by a transition in the
physics of interaction with matter: wavelengths longer than about 30 nm interact mainly with
the chemical valence electrons
of matter, while wavelengths shorter than that interact mainly with inner shell
electrons and nuclei. The long end of the EUV/XUV spectrum is set by a
prominent He+ spectral line at 30.4nm. XUV is strongly
absorbed by most known materials, but it is possible to synthesize multilayer optics
that reflect up to about 50% of XUV radiation at normal incidence. This
technology has been used to make telescopes for solar imaging
(pioneered by the NIXT and MSSTA
sounding rockets in the 1990s; current examples are SOHO/EIT
and TRACE) and microphotolithography
(printing of traces and devices on microchips).
The solar
corona as seen in deep ultraviolet light at 17.1
nm by the Extreme
ultraviolet Imaging Telescope instrument aboard the SOHO
spacecraft
DISCOVERY
Soon
after infrared radiation had been discovered, the
German physicist Johann Wilhelm Ritter
began to look for radiation at the opposite end of the spectrum, at the short
wavelengths beyond violet. In 1801 he used silver chloride, a light-sensitive chemical, to
show that there was a type of invisible light beyond violet, which he called
chemical rays. At that time, many scientists, including Ritter, concluded that
light was composed of three separate components: an oxidising or calorific
component (infrared), an illuminating component (visible light), and a reducing
or hydrogenating component (ultraviolet). The unity of the different parts of
the spectrum was not understood until about 1842, with the work of Macedonio Melloni, Alexandre-Edmond
Becquerel and others. During that time, UV radiation was also called
"actinic radiation".
HEALTH CONCERNS AND PROTECTION
In
humans, prolonged exposure to solar UV radiation may result in acute and
chronic health effects on
the skin, eye, and immune system.
Tungsten-halogen lamps have bulbs made of
quartz, not of ordinary glass. Tungsten-halogen lamps that are not filtered by
an additional layer of ordinary glass are a common, useful, and possibly
dangerous, source of UVB light.
UVC
rays are the highest energy, most dangerous type of ultraviolet light. Little
attention has been given to UVC rays in the past since they are filtered out by
the atmosphere.
However, their use in equipment such as pond sterilization units may pose an exposure risk,
if the lamp is switched on outside of its enclosed pond sterilization unit.
Ultraviolet photons harm the DNA
molecules of living organisms in different ways. In one common damage event,
adjacent bases bond with each other, instead of across the "ladder".
This makes a bulge, and the distorted DNA molecule does not function properly.
SKIN
UVA,
UVB and UVC can all damage collagen fibers and thereby
accelerate aging of the skin. In general, UVA is the least harmful, but can
contribute to the aging of skin, DNA damage and possibly skin cancer. It
penetrates deeply and does not cause sunburn. Because it does not cause reddening of
the skin (erythema) it cannot be measured in the SPF testing. There is no
good clinical measurement of the blocking of UVA radiation, but it is important
that sunscreen block both UVA and UVB.
UVA
light is also known as "dark-light" and, because of its longer
wavelength, can penetrate most windows. It also penetrates deeper into the skin
than UVB light and is thought to be a prime cause of wrinkles.
UVB
light can cause skin cancer. The
radiation excites DNA
molecules in skin cells, causing covalent bonds to form between adjacent thymine bases, producing thymidine dimers.
Thymidine dimers do not base pair normally, which can cause distortion of the
DNA helix, stalled replication, gaps, and misincorporation. These can lead to mutations, which can result in cancerous growths. The mutagenicity of UV radiation can be easily
observed in bacteria cultures.
This cancer connection
is one reason for concern about ozone depletion and the ozone hole.
As a
defense against UV radiation, the body tans when exposed to moderate (depending
on skin type) levels
of radiation by releasing the brown pigment melanin. This helps to block UV penetration and
prevent damage to the vulnerable skin tissues deeper down. Suntan lotion that
partly blocks UV is widely available (often referred to as "sun
block" or "sunscreen"). Most of
these products contain an "SPF rating" that describes the amount of
protection given. This protection applies only to UVB light.
EYE
High
intensities of UVB light are hazardous to the eyes, and exposure can cause welder's
flash (photokeratitis or arc eye) and may lead to cataracts, pterygium, and pinguecula formation.
Protective eyewear
is beneficial to those who are working with or those who might be exposed to
ultraviolet radiation, particularly short wave UV. Given that light may reach
the eye from the sides, full coverage eye protection is usually warranted if
there is an increased risk of exposure as in high altitude mountaineering.
Mountaineers are exposed to higher than ordinary levels of UV radiation, both
because there is less atmospheric filtering and because of reflection from snow
and ice.
Ordinary
eyeglasses give some protection, and most
plastic lenses give more protection than glass lenses. Some plastic lens
materials, such as polycarbonate, block
most UV. There are protective treatments available for eyeglass lenses that
need it to give better protection. Most intraocular lenses help to protect the retina by absorbing UV radiation.
BENEFICIAL EFFECTS
A
positive effect of UV light is that it induces the production of vitamin D in the skin. Grant (2002) claims tens
of thousands of premature deaths occur in the US annually from cancer due to
insufficient UVB exposures (apparently via vitamin D deficiency). Another
effect of vitamin D deficiency is osteomalacia, which can result in bone pain,
difficulty in weight bearing and sometimes fractures.
Ultraviolet radiation
has other medical applications, in the treatment of skin conditions such as psoriasis and vitiligo. UVB and UVA radiation can be used, in
conjunction with psoralens (PUVA treatment).
Most effective in case
of psoriasis and vitiligo is UV light with wavelength of 311 nm.
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