A black light is a source of electromagnetic radiation that emits near-ultraviolet light, usually in the 350-375 nanometer wavelength range. This range is just slightly below the 400nm that human eyes can detect. When this light is reflected off certain materials, it looses energy and its wavelength expands to over 400nm. The reflected light is then visible to human eyes, and the illuminated objects appear to glow. The glow effect is the result of the object reflecting more visible light than the ambient environment. Black lights have been used in entertainment since the 1930's, and have been finding new uses in industry since the mid 1970's.
Standard fluorescent black lights are constructed in an almost identical fashion to their white-light counterparts. Electrical stimulation of mercury releases ultraviolet light, which must be converted to visible light in order for us to see it. In a regular white-light fluorescent bulb, a phosphorous coating is added to the outer glass layer of the bulb. A phosphor is any material that reflects light in the 400-700 nanometer (visible light) range when stimulated by radiation. Black lights do not have this phosphorous coating, so they release little visible light. However, any material containing phosphors in the surrounding environment will react to the UV light, and reflect back visible light.
In addition to their entertainment value, black lights have been put to use in mechanical engineering and repair, criminal investigation, forgery detection, and many other fields. Phosphorous dyes added to liquids will leak from the same places that the liquid itself leaks from, thus creating a phosphorous stain at any leak point. This is very easily detected with a black light. Airplane fuel lines and automotive air conditioner lines are two applications where this method is used extensively. Also, many bodily fluids (such as blood, semen, urine, and saliva) are phosphor-rich. Criminal investigators have been using black lights for years to detect these fluids on crime victims and at crime scenes. Paper manufacturers had begun adding phosphors to almost all white paper in the 1950's. Thus, exposure to a black light will reveal forged documents that are purposed to predate the mid-twentieth century. Money counterfeiters face a similar problem- many governments mark large bills in distinguishable, yet hard to reproduce, patterns of phosphorous materials to thwart forgery.
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