Black light

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Spectrum of a fluorescent black light source. FWHM spectral bandwidth of the 370nm peak is about 20nm.

Black light (also Wood's light) is the common name for a lamp emitting electromagnetic radiation that is almost exclusively in the soft near ultraviolet range, and very little visible light.

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–, cobalt-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 blacklite blue.

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⁺). Blacklite blue lamps peak at 365 nm.

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 (a mere few lumens per watt) owing to the black body nature of the incandescent light source. Incandescent UV bulbs, due to their inefficency, may also become dangerously hot during use.

Note: Lamps referred to as simply blacklite have a different spectrum with much stronger visible emissions, and are used in such devices as bug zappers. They appear light blue to the human eye, and they are not useful for UV applications.

Uses

A black light bulb. To the human eye, the light looks much more violet.

"Midnight bowling" utilizing blacklights and the fluorescence of pins to enhance ambience.

The 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 bank notes. 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 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.

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.

See also

• Wood's lamp
• List of light sources

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