Anyone seriously using phosphorescent or fluorescent paint will also need to understand black lights and the long wave ultraviolet light they emit. Black lights are by far the most efficient way to charge glow in the dark paint and it is mandatory for fluorescent paint.
Unfortunately, manufacturers use the words “Black Light” and “UV” carelessly and confusion follows. This problem has increased recently with the advent of falsely labeled UV LED’s. This article will define black light, discuss various bulbs and give you the tools to choose an efficient black light for your project.
The colors we see are determined by the wavelength of light energy. Unlike some insects, humans can only view the spectrum from red to violet. However, other invisible “colors” exist above and below this spectrum. The “color” above red is called infra-red and the color below violet is called ultraviolet. Ultraviolet light will cause fluorescent or phosphorescent pigments to fluoresce, emitting visible light.
The ultraviolet spectrum is broken down into subcategories depending on wavelength:
- 450 - 400 nm Violet, (visible light, shown for reference)
- 400 - 320 nm UVA, Long Wave, Black Light
- 320 - 280 nm UVB, Medium Wave
- 280 - 100 nm Short Wave, Germicidal
Long wave ultraviolet (UVA) used for true black lights is relatively safe on the eyes. Medium and short wave ultraviolet light can do damage to eyes. For legal reasons, I will refer you to other sources for the safety of using various UV products.
Therefore, an ideal black light has the shortest wavelength possible without going below the dangerous 320 nm threshold and has nominal visible light (above 400 nm). Traditional black lights emit an average wavelength of 365 nm and have a filter (often ordinary glass) to eliminate anything below 320 nm.
Ordinary glass filters medium and short wave ultraviolet light, while passing visible and long wave ultraviolet light. Our earth’s atmosphere filters short wave ultraviolet light. A special type of glass called 'Woods Glass' filters visible light along with medium and short wave ultraviolet light. Due to its high cost, it is only used in very high-end black lights. Most black lights use a coating with similar properties.
BLACK LIGHT VS. BLACK LIGHT BLUE
Terms for black light bulbs are a bit different than for the fixtures. Black light fixtures contain a bulb called a "Black Light Blue" or BLB bulb. This bulb uses cobalt blue glass and emits very little purple visible light in addition to the long wave ultraviolet light.
There are also "Black Light" or BL bulbs. These bulbs are made of a clear glass and emit a relatively large amount of blue visible light along with the long wave ultraviolet light. BL bulbs are often used in bug catchers.
BL bulbs emit more long wave UV than BLB bulbs. However, it is hard to see objects fluoresce due to the large volume of visible light. In addition, BL bulbs will cause your 'night vision' to readjust to the bright light. The result is that when you turn a BL light off, your glow in the dark items will appear to glow less.
When most people refer to black light, they really mean black light blue. For this article, outside of this section, we will use the common term black light to refer to black light blue as well.
For a bit of trivia, the famous Tron arcade game in the 1980’s included both a traditional BL and a BLB bulb. Since most of these machines have been repaired by technicians that do not understand BL bulbs, it is rare to see a Tron game that looks as it was intended. A prized possession of the Glow Inc. staff is a properly restored Tron game.
Black light does not affect or alter your 'night vision'. In addition to its efficiency, this makes it the ideal charging spectrum for glow in the dark items. High powered, long wave ultraviolet light can often penetrate clothing and illuminate white underwear causing embarrassing situations. This is due to phosphors added to white clothing dye to make them look whiter in sunlight.
Long wave ultraviolet light does reflect efficiently off of mirrored surfaces. Like visible light, it retains 80-90% of its brightness with a good mirror. Most polished metal surfaces and cheap plastic mirrors only reflect 60-70%. Medium and short wave ultraviolet will not reflect in a mirror due to passing through the glass.
ULTRAVIOLET LIGHT SOURCES
The sun and almost all types of white light bulbs emit some long wave ultraviolet light. That is why the sun and most light bulbs can charge phosphorescent paint. However, this article is going to focus on bulbs, known as black lights, that are designed to primarily emit long wave ultraviolet light.
The wavelength of light emitted from an LED is very specific. When you buy an LED, you can specify exactly what wavelength you wish it to emit. Ideally, for black lights, you would use 365 nm LED’s. Therefore, you would assume that manufacturers of black light products would stick to that wavelength. Why do they not do that?
A 365 nm LED is only about 20% efficient as a 405 nm LED. Individual 365 nm LED's are also relatively expensive. Therefore, manufacturers need a massive number of expensive LEDs to create a decent LED Blacklight. Furthermore, light from true 365 nm LED’s is not visible and therefore uneducated consumers are not happy with the 'light output'.
The popular stage lighting company, Chauvet, released a “black light” consisting of 192 “UV” LEDs. This product is a great VIOLET light using 405 nm, visible light, violet LED’s. However, it is almost completely useless as a black light. You can understand their reasoning when several DJ’s reviewed this light as much 'brighter' than more powerful fixtures emitting true long wave ultraviolet light. The reviewers were making this determination by how much light they could see. Properly, they should have used fluorescent paint to determine the best fixture.
Flashlight manufacturer Inova, makes a “UV” flashlight with 3 365 nm LED’s and 2 405 nm LED’s for the same reason. LED’s use relatively low electricity and emit almost no heat. They turn on instantly and can be very bright. They also have an extremely long bulb life. If you select an LED fixture, you just need to make sure that the product you are buying includes 365 nm LED’s.
Currently, LED fixtures of similar power are 10-20 times more expensive then standard fluorescent black lights.
Incandescent black lights are created by taking a regular white light bulb and adding a coating to filter the visible light, passing only long wave UV light. Since less than 2% of the light is emitted as long wave UV, they are very poor black lights. As such, you would think they would not ever be sold.
Incandescent bulbs are popular because they are cheap and can screw into any home fixture. Unfortunately, they are useless for charging glow in the dark items. In fact, they are almost completely useless unless you want a violet light bulb.
Black light and white light fluorescent tubes are very similar. The mercury inside of all fluorescent tubes primarily emits UV light (long, medium and short).
For white lights, the inside of the tube is coated with fluorescent phosphors to convert UV light to visible white light. The glass of the tube filters any remaining medium and short wave UV.
The phosphors in a black light fluorescent tube convert all of the UV light to long wave UV. For most black lights, a coating on the exterior filters any excess visible light (above 400 nm). The result is an extremely efficient, inexpensive black light.
Fluorescent tubes vary by both length and thickness. Popular black light thicknesses are T-8 (1”) and T-12 (1.5”). Contrary to instinct, T-8 bulbs, using newer technology, are brighter, more efficient, flicker less, and can start in colder temperatures. Therefore, there is no reason to buy a T-12 bulb or fixture. Of course spectrum, brightness and quality vary by brand. The only problem with full size fluorescent tubes as black lights is that they are large and delicate.
The new CFL (compact fluorescent) bulbs are just miniature fluorescent tubes. True black light versions are available and share the characteristics of the larger tubes. Light output and wavelength varies greatly between brands and styles.
Black light CFL bulbs screw into a standard light socket and typically use 25-40 watts of power. Larger wattage units are typically sold in their own specialty fixture.
Unfortunately, the majority of CFL 'black lights' are actually regular white CFL bulbs dipped into a coating that filters all non-violet light. Yes, this is absurd. The mercury inside these bulbs creates ideal UV light. The phosphors convert the UV light to white light. The coating filters all of the white light except violet. Only about 4% of light makes it out of the bulb rendering it completely useless. Yes, companies go out of their way to make this crappy product.
Several stage lighting manufacturers offer a 400 watt HID mercury black light. They function very similar to a black light fluorescent tube. However, they take 5-10 minutes to turn on, they get very hot, they are very heavy, and they use a lot of electricity. The bulbs have a short life and are relatively expensive to replace.
These large, powerful lights are sometimes preferred because they can project ultraviolet light a far distance. Otherwise, their drawbacks make them unappealing compared to fluorescent and CFL fixtures.
If you need an inexpensive black light to use with your glow in the dark or fluorescent creations, then stick to full size or compact fluorescent tubes. LED fixtures are a viable, yet expensive alternative. However, you must make sure they incorporate 365 nm LED’s. In general, stay clear of incandescent or HID.
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