Black light bulbs are bulbs that are designed to emit ultraviolet radiation, which isn’t visible to the naked eye. Black lights emit UV in the long wave, or UVA range, and are also designed to emit little light in the visible spectrum. It is possible to get UV bulbs that emit visible light as well, but these would not be considered to be black lights. Bulbs that are referred to as being black lights possess a dark blue filtering material in the glass that makes up the bulb. This filtering material, such as Wood’s Glass, stops most visible light from passing through the glass, but allows the UV light to penetrate.
Types of Black Light Bulbs
Bulbs that emit black light are available in many different types, which are usually used in different situations. They can be made into fluorescent bulbs, including low energy light bulbs, LEDs (light-emitting diodes) which are often found in hand-held torches as they require little power, mercury vapour lamps and the normal incandescent electric light bulb. The type of bulb used should be chosen to fit the situation – whilst an LED may be fine for a low-powered black light torch, something a bit more powerful will be required to illuminate an entire room.
Black lights have a variety of uses, from novelty to serious.
Wood’s lamps are used in healthcare in dermatology. By illuminating the skin with a UV light, the technician can then help make a diagnosis by making note of the fluorescence. The light can help identify skin conditions as well as fungal infections.
A black light will make many body fluids glow when they are exposed to it. In addition, a crime scene may be dusted with a fluorescent dye and then exposed to a black light source. This can help fingerprints stand out more readily. Forensic light sources will normally also be capable of emitting infra-red and visible light as well as UV light, but they come with the option to make the light only emit one type at a time, so that the same light can be adjusted to just emit visible light, or UV light or infra-red. This is because all three have different, but important, uses in forensics.
Glow in the Dark Stickers
If you have attached glow in the dark stickers to the walls or ceiling of a room, you may have found that these stickers will often not glow for very long. By using a black light in the room, this will provide a constant source of ultraviolet light to the stickers. This will keep them glowing for as long as the black light is turned on. The stickers may not be the only things that glow though.
A lot of paper money has a fluorescent strip embedded in it. This will glow when the note is exposed to a UV light source, allowing the detection of counterfeit money by the fact that it doesn’t glow.
Black light will also cause clothes to glow, especially white clothes. This is actually due to what is in effect a trick by the companies that manufacture washing machine detergents. In order to get that ‘whiter than white’ look for white clothes, they add phosphors to the detergent. These phosphors glow under ultraviolet light, making the clothes appear ‘whiter’ and therefore apparently cleaner than they actually are. As black lights provide a source of UV light without the same proportion of visible light that the Sun would provide these phosphors are very noticeable. This is effect commonly used in nightclubs.
White paper and some surface cleaners may also use these phosphors to make either the paper or the cleaned surface seem whiter than it is, so these may also glow under black light.
There are three spectra of ultraviolet light; UVA, UVB and UVC. Black lights emit the majority of their ultraviolet light in the UVA range which is the closest to the spectra of visible light and the safest. The majority is of course not the same as all. Some light is therefore emitted in the UVB range. Although UVA light is safer, that does not mean it is safe. UVA light has been shown to destroy Vitamin A in the skin, causing premature aging of the skin, damage collagen fibres and has the potential to indirectly cause DNA damage (UVB and UVC cause this damage directly). Long term exposure to a black light is definitely not recommended and you should seek appropriate advice before using a black light bulb for sustained periods of time. Having said that, UVA is still much safer than UVB and certainly more so than the deadly UVC.