Black light


Black light

[
FWHM spectral bandwidth of the 370nm peak is about 20nm.] A Black light or UV Light is a lamp emitting electromagnetic radiation that is almost exclusively in the soft near ultraviolet range, and emits very little visible light. The black light was invented by William H. Byler.Fact|date=October 2008

In medical use, such a light source is referred to as a Wood's lamp.

Description

Fluorescent black lights are typically made in the same fashion as normal fluorescent lights except that only one phosphor is used and the normally 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 nanometers. 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 don't have the blue Wood's glass."

The phosphor typically used for a near 368 to 371 nanometer emission peak is either europium-doped strontium fluoroborate (SrB4O7F:Eu2+) or europium-doped strontium borate (SrB4O7:Eu2+) while the phosphor used to produce a peak around 350 to 353 nanometers is lead-doped barium silicate (BaSi2O5:Pb+). "Blacklight Blue" lamps peak at 365 nm.

European Black Light, UV-A, UV-B and Actinic Tubes possess different numbering systems to identify them, Philips use one system which seems to be falling into obsolescence, (German) Osram`s system seems to be dominating throughout the world outside North America. The table below for details.

[Various Philips, Osram and Sylvania Lighting Catalogues]

Peak Position pproximated to the nearest decade.

Width between 50% intensity points on shoulders of peak.

This table lists tubes generating blue, U.V.A and U.V.B, in order of decreasing wavelength of the most intense peak. Approximate phosphor compositions, major manufacturer's type numbers and some uses are given as an overview of types available.

Osram wood's glass tubes seem to use a fairly narrow band emitting phosphor, Europium activated Strontium Pyroborate, with a peak at about 370nm, whereas North American and Philips Wood's glass tubes seem to use the wider band emitting, Lead activated Calcium Metasilicate, with a shorter wavelength peak at about 350nm. These two seem to be the most commonly used, and different manufacturers offer either one or the other and often both.

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 percent 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. Often the filament is "overburned", and run at a higher temperature to increase the proportion of U.V.A in the black-body emission, this drastically reduces the life of the lamp from a typical 1000 hours to around 100 hours.

More rarely still, high power, 125 and 400 watt mercury vapor black lamps can be found, these do not utilize the use of phosphors, but rely on the intensified and slightly broadened 365nm spectral line of Mercury from high pressure discharge at between 5 and 10 atmospheres pressure depending upon the specific type. These lamps use envelope of Wood's glass to block all the visible lines of Mercury and the short wavelength U.V.C resonance lines, which are harmful. Some other spectral lines, falling within the pass band of the Wood's glass, at 312 and 404.5nm contribute to the output. These lamps are used mainly for theatrical and concert displays and although, like the filament lamps described above they become very hot during normal use, they far more effective U.V.A producers per unit of power consumption.

Some UV fluorescent bulbs specifically designed to attract insects for use in bug zappers use the same near-UV emitting phosphor as normal blacklights, but use plain glass instead of the more expensive Wood's glass, these are listed in the table above. Plain glass blocks less of the visible mercury emission spectrum, making them appear light blue-violet to the naked eye. These lamps are referred to as "blacklight" or "BL" in most North American lighting catalogs.European equivalents are the Philips TL-XXW/09, emitting a peak at 350nm, and the Osram LXXW/78, emitting a peak at 371nm, among others.

Ultraviolet light can be also generated by some light-emitting diodes, but wavelengths below 380nm are uncommon and the emission peaks are broad.

Safety

While "black lights" do produce light in the UV range, their spectrum is confined to the longwave UVA region. UVA is considered the safest of the three spectra of UV light. It is the higher energy (shortwave) light in the UVB and UVC range that is responsible for the DNA damage that leads to skin cancer. UVA light is much lower in energy and does not cause sunburn. UVA is capable of causing damage to collagen fibers, so it does have the potential to accelerate skin aging and cause wrinkles. UVA can also destroy vitamin A in the skin.

UVA light can cause DNA damage, but not directly like UVB and UVC. Due to its longer wavelength it is absorbed less and reaches deeper skin layers (the leather skin), where it produces reactive chemical intermediates, such as hydroxyl and oxygen radicals, which in turn can damage DNA and is a high melanoma (a dangerous skin cancer) risk. The strength of a black light in comparison to sunlight is minuscule, so it is doubtful that UVA light poses any significant health risks. The weak output of black lights should not cause DNA damage or cellular mutations the way sunlight can.

Uses

Ultraviolet radiation itself is invisible to the human eye, but illuminating certain materials with UV radiation prompts the visible effects of fluorescence and phosphorescence. Black light testing is commonly used to authenticate antiques and banknotes. It is extensively used in non-destructive testing; fluorescing fluids are applied to metal structures and illuminated with a black light. Cracks and other artifacts can easily be detected. It is also used to detect pet excreta for elimination, such as urine, vomit and other substances that are not always visible to the naked eye.

It is also used to illuminate pictures painted with fluorescent colors (preferably on black velvet to intensify the illusion of self-illumination). The fluorescence it prompts from certain textile fibers, especially those bearing optical brightener residue, is also used as a recreational effect (as seen for instance in the opening credits of the James Bond film "A View to a Kill").

In forensic investigations, black lights are used to reveal the presence of trace evidence, such as blood, urine, semen and saliva, by causing visible fluorescence in these substances. The use of this technique by exposé style television news magazines for reporting on the various unsanitary and mysterious stains found in hotel rooms has become such an oft-repeated stunt that it has been lampooned on comedy shows such as "Family Guy", "The Office (US)" and "The Upright Citizens Brigade".

In medicine, the Wood's lamp is used to check for the characteristic fluorescence of certain dermatophytic fungi such as "Microsporan" species which emits a yellow glow, or corynebacterium which have a red to orange color under the Wood's lamp. It is also used to detect the presence and extent of depigmenting disorders such as vitiligo. It can also be used to diagnose ringworm, fungal infections, corneal scratches, foreign bodies in the eye, blocked tear ducts, acne, erythrasma, tinea versicolor, microsporum canis, scabies, alopecia, porphyria, bacterial infections, and many other skin conditions [http://www.homephototherapy.com/woodslamp.htm] [http://www.ijdvl.com/article.asp?issn=0378-6323;year=2004;volume=70;issue=2;spage=131;epage=135;aulast=Gupta] .

In security, a pen with a fluorescent ink, generally a soft tip, is used to "invisibly" mark items. If the items are stolen then a black light can be used to search for the security markings.

Black lights are also used to differentiate real banknotes with counterfeit banknotes as, in many countries, real banknotes have fluorescent symbols on them that only show under a black light. Through this, security officers can check for counterfeit banknotes. The same is true for checking for fraudulent identification cards.

One of the innovations for night and all-weather flying used by the US, UK and Germany during 1939-1945 was the use of UV floodlights to illuminate the instrument panel, giving a variable intensity alternative to the radium painted instrument faces and pointers, without visible illumination that would give away an aircraft's position. This went so far as to include the printing of charts that were marked in UV-fluorescent inks, and the provision of UV-visible pencils and slide rules such as the E6B. However, the common failure of the light's power inverter on take-offs in bombers, and the night-blindness and crashes caused when the blacklight filter fell off on trainer aircraft led to the system being mostly abandoned.

Black light puppetry is also performed in a black light theater.

References

See also

* Wood's lamp
* List of light sources

External links

* [http://www.sylvania.com/BusinessProducts/MaterialsandComponents/LightingComponents/Phosphor/FluorescentLamps/ U.V.C to U.V.A conversion Phosphors for Black Light sources]
* [http://www.blacklight-theatre.com Black Light Theatre World Web]
* [http://www.beo.cc Black Light Photography]
* [http://chemistry.about.com/cs/howthingswork/f/blblacklight.htm About.com - "What Materials Glow Under a Black or Ultraviolet Light?"]
* [http://www.ijdvl.com/article.asp?issn=0378-6323;year=2004;volume=70;issue=2;spage=131;epage=135;aulast=Gupta - "Medical Uses for a Wood's Lamp"]


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