Atmospheric filtering
Sunlight in space at the top of Earth's atmosphere, at a
solar constant output of about
1366 W/m2, is composed (by total energy) of about
50% infrared light,
40% visible light, and
10% ultraviolet light, for a total ultraviolet power of about
140 W/m2 in vacuum.However
, at ground level total sunlight power decreases to about 1000–1100 W/m2, and by energy fractions, is composed of 44% visible light,
3% ultraviolet (with the Sun at its zenith), and the remainder infrared. Thus, sunlight's composition at the zenith at ground level, per square meter, is about 527 W infrared radiation, 445 W
visible light, and
32 W UV.*
Since with the Sun at zenith the Earth's air and
ozone layer allows passage of a total of 32 watts/m2 (ground UV power) out of a vacuum value of about 140 watts/m2 (i.e., 23%) of Sun's UV light, this is equivalent to a minimal atmospheric blockage of 77% of the Sun's UV. However, most of the Sun's UV that is blocked by Earth's atmosphere lies in the shorter UV wavelengths. The figure rises to 97–99% of the Sun's UV radiation at the average mixture of other Sun angles encountered through the day.
The Sun's emission in the lowest UV bands, the UVA, UVB, and UVC bands, are of interest, as these are the UV bands commonly encountered from artificial sources on Earth. The shorter bands of UVC, as well as even more energetic radiation as produced by the Sun, generate the ozone in the ozone layer when single oxygen atoms produced by UV
photolysis of dioxygen react with more dioxygen. The ozone layer is especially important in blocking UVB and part of UVC, since the shortest wavelengths of UVC (and those even shorter) are blocked by ordinary air.
* Of the ultraviolet radiation that reaches the Earth's surface, up to 95% is UVA (the very longest wavelength), depending on cloud cover and atmospheric conditions.
5% of 32 Watts = 1.6 Watt / m^2 of UVB
Artificial sources
"Black lights"
A black light is a lamp that emits long-wave UVA radiation and little visible light. Fluorescent black light lamps are constructed in the same fashion as normal fluorescent lights, except they use a phosphor on the inner tube surface, which emits UVA light instead of visible white light. BLB type lamps use filtering glass with a deep-bluish-purple optical filter that blocks almost all visible light above 400 nanometres.[15] The color of such lamps is often referred to in the lighting industry as "blacklight blue" or "BLB", to distinguish them from UV lamps used in "bug zapper" insect traps, that do not have the optical filter coating. These are designated "blacklight" ("BL") lamps. The phosphor typically used for a near 368 to 371 nanometre emission peak is either europium-doped strontium fluoroborate (SrB4O7F:Eu2+) or europium-doped strontium borate (SrB4O7:Eu2+), whereas the phosphor used to produce a peak around 350 to 353 nanometres is lead-doped barium silicate (BaSi2O5
b+). "Blacklight Blue" lamps peak at 365 nm.
A black light may also be formed, very inefficiently, by simply using Wood's glass, a deep bluish-purple nickel oxide doped glass that filters out all light besides UV, 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 cheaper than fluorescent UV lamps, only 0.1% of the input power is converted to usable radiation, as the incandescent light radiates as a black body with very little emission in the UV range. Incandescent bulbs used to generate significant UV, due to their inefficiency, may become dangerously hot. High-power mercury-vapor black lights that use a UV-emitting phosphor and an envelope of Wood's glass are also made, in ratings up to 1 kW, used mainly for theatrical and concert displays.
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.
Short wave ultraviolet lamps
Lamps that emit shortwave UV light are also made. Fluorescent lamps without an internal phosphor coating to convert UV to visible light emit ultraviolet light with two peaks in the UV-C band at 253.7 nm and 185 nm due to the peak emission of the mercury within the lamp. Eighty-five to 90% of the UV produced by these lamps is at 253.7 nm, whereas only five to ten percent is at 185 nm[citation needed]. The quartz tube passes the 253 nm radiation but has impurities that block the 185 nm wavelength. These "germicidal" lamps are used extensively for disinfection of surfaces in laboratories and food processing industries, and for disinfecting water supplies.
Standard bulbs have an optimum operating temperature of about 40 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 forty percent, meaning that for every 100 watts of electricity consumed by the lamp, they will produce approximately 30–40 watts of total UV output. UVA/UVB emitting bulbs also sold for other special purposes, such as reptile-keeping.
Gas-discharge lamps
Main article: Gas-discharge lamp
Specialized UV gas-discharge lamps are sold, containing a variety of different gases, to produce UV light at particular spectral lines for scientific purposes. Argon and deuterium lamps are often used as stable sources, either windowless or with various windows such as magnesium fluoride.[16] These are often the light sources in UV spectroscopy equipment for chemical analysis.
The excimer lamp, a new UV light source developed within the last two decades, is seeing increasing use in scientific fields. It has the advantages of high-intensity, broadband radiation (no spectral lines) and operation at a variety of wavelength bands into the vacuum ultraviolet.
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 W/cm2 (30 kW/m2) are now possible, and this, coupled with recent developments by photoinitiator and resin formulators, makes the expansion of LED-cured UV materials likely.
http://en.wikipedia.org/wiki/Ultraviolet
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