*check this out
off wiki..
Sources of UV
Natural sources of UV
The
sun emits ultraviolet radiation in the UVA, UVB, and UVC bands. The Earth's
ozone layer blocks 97-99% of this UV radiation from penetrating through the atmosphere.
[5] Of the ultraviolet radiation that reaches the Earth's surface, 98.7% is UVA.
[citation needed] (UVC and more energetic radiation is responsible for the generation of the ozone layer, and formation of the ozone there). Extremely hot stars emit proportionally more UV radiation than the sun; the star
R136a1 has a thermal energy of 4.57 eV, which falls in the near-UV range.
Ordinary glass is partially
transparent to UVA but is
opaque to shorter wavelengths, whereas
Silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm.
[6][7][8]
The onset of vacuum UV, 200 nm, is defined by the fact that ordinary air is opaque at shorter wavelengths. 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, whereas 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 30.4 nm. 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; it was pioneered by the
NIXT and
MSSTA sounding rockets in the 1990s; (current examples are
SOHO/EIT and
TRACE) and for
nanolithography (printing of traces and devices on
microchips).
"Black light"
Main article:
Black light
A black light, or
Wood's light, is a lamp that emits long wave UV radiation and very little visible light. They are sometimes referred to as a "UV light". Fluorescent black lights are typically made in the same fashion as normal fluorescent lights except that only one
phosphor is used, and the clear glass envelope of the bulb may be replaced by a deep-bluish-purple glass called
Wood's glass, a nickel-oxide–doped glass, which blocks almost all visible light above 400 nanometres. The color of such lamps is often referred to in the trade as "blacklight blue" or "BLB." This is to distinguish these lamps from "bug zapper" blacklight ("BL") lamps that do not have the blue Wood's glass. The phosphor typically used for a near 368 to 371 nanometre emission peak is either
europium-doped strontium fluoroborate (SrB
4O
7F:Eu
2+) or europium-doped strontium borate (SrB
4O
7:Eu
2+) while the phosphor used to produce a peak around 350 to 353 nanometres is lead-doped barium silicate (BaSi
2O
5
b
+). "Blacklight Blue" lamps peak at 365 nm.
While "black lights" do produce light in the UV range, their spectrum is confined to the longwave UVA region. Unlike UVB and UVC, which are responsible for the direct DNA damage that leads to skin cancer, black light is limited to lower-energy, longer waves and does not cause sunburn. However, UVA is capable of causing damage to collagen fibers and destroying vitamins A and D in skin.
[citation needed]
A black light may also be formed by simply using Wood's glass instead of clear glass as the envelope for a common incandescent bulb. This was the method used to create the very first black light sources. Though it remains a cheaper alternative to the fluorescent method, it is exceptionally inefficient at producing UV light (less than 0.1% of the input power), owing to the
black body nature of the incandescent light source. Incandescent UV bulbs, due to their inefficiency, may also become dangerously hot during use. More rarely still, high-power (hundreds of watts) mercury-vapor black lights that use a UV-emitting phosphor and an envelope of Wood's glass can be found. These lamps are used mainly for theatrical and concert displays, and also become very hot during normal use.
Some UV fluorescent bulbs specifically designed to attract insects use the same near-UV emitting phosphor as normal blacklights, but use plain glass instead of the more expensive Wood's glass. Plain glass blocks less of the visible mercury emission spectrum, making them appear light-blue to the naked eye. These lamps are referred to as "blacklight" or "BL" in most lighting catalogs.
Ultraviolet light can also be generated by some
light-emitting diodes.
Ultraviolet fluorescent lamps
Fluorescent lamps without a phosphorescent coating to convert UV to visible light, emit ultraviolet light with two peaks at 253.7 nm and 185 nm due to the peak emission of the mercury within the bulb. Eighty-five to ninety percent of the UV produced by these lamps is at 253.7 nm, while only five to ten percent is at 185 nm. Germicidal lamps use quartz (glass) doped with an additive to block the 185 nm wavelength. With the addition of a suitable phosphorescent coating, they can be modified to produce a UVA, UVB, or visible light spectrum (all fluorescent tubes used for domestic and commercial lighting are mercury (Hg) UV emission bulbs at heart).
Such low-pressure mercury lamps are used extensively for disinfection, and in standard form have an optimum
operating temperature of about 30 degrees Celsius. Use of a mercury amalgam allows operating temperature to rise to 100 degrees Celsius, and UVC emission to about double or triple per unit of light-arc length. These low-pressure lamps have a typical efficiency of approximately thirty to thirty-five percent, meaning that for every 100 watts of electricity consumed by the lamp, it will produce approximately 30-35 watts of total UV output.
This section requires
expansion.
Ultraviolet LEDs
Light-emitting diodes (LEDs) can be manufactured to emit light in the ultraviolet range, although practical LED arrays are very limited below 365 nm. LED efficiency at 365 nm is about 5-8%, whereas efficiency at 395 nm is closer to 20%, and power outputs at these longer UV wavelengths are also better. Such LED arrays are beginning to be used for UV curing applications, and are already successful in digital print applications and inert UV curing environments. Power densities approaching 3,000 mW/cm
2 (30 kW/m
2) are now possible, and this, coupled with recent developments by photoinitiator and resin formulators, makes the expansion of LED-cured UV materials likely.
This section requires
expansion.
Ultraviolet lasers
UV
laser diodes and UV
solid-state lasers can be manufactured to emit light in the ultraviolet range. Wavelengths available include 262, 266, 349, 351, 355, and 375 nm. Ultraviolet
lasers have applications in industry (
laser engraving), medicine (
dermatology and
keratectomy),
secure communications, and computing (
optical storage). They can be made by applying
frequency conversion to lower-frequency lasers, or from Ce:LiSAF crystals (
cerium doped with lithium strontium aluminum fluoride), a process developed in the 1990s at
Lawrence Livermore National Laboratory.
[9]
Gas-discharge lamps
Main article:
Gas-discharge lamp
Argon and
deuterium lamps are often used as stable sources, either windowless or with various windows such as
magnesium fluoride.
[10]
Detecting and measuring UV radiation
Ultraviolet detection and measurement technology can vary with the part of the spectrum under consideration. While some silicon detectors are used across the spectrum, and in fact the US NIST has characterized simple silicon diodes
[11] that work with visible light too, many specializations are possible for different applications. Many approaches seek to adapt visible light-sensing technologies, but these can suffer from unwanted response to visible light and various instabilities. A variety of solid-state and vacuum devices have been explored for use in different parts of the UV spectrum. Ultraviolet light can be detected by suitable
photodiodes and
photocathodes, which can be tailored to be sensitive to different parts of the UV spectrum. Sensitive ultraviolet
photomultipliers are available.
Near UV
Between 200-400 nm, a variety of detector options exist.
Vacuum UV
Technology for VUV instrumentation has been largely driven by solar physics for many decades and more recently some
photolithography applications for semiconductors. While optics can be used to remove unwanted visible light that contaminates the VUV, in general, detectors can be limited by their response to non-VUV radiation, and the development of "solar-blind" devices has been an important area of research. Wide-gap solid-state devices or vacuum devices with high-cutoff photocathodes can be attractive compared to silicon diodes. Recently, a diamond-based device flew on the
LYRA (see also
Marchywka Effect).
).. lol
