Blacklight strobes?

last updated 1/20/99

Do you want to make a blacklight strobe?

First, there are a few little problems having to do with the human eye.

1. Ultraviolet is not the best stuff to expose your eyes to. Furthermore, at least one damage mode may be affected by peak exposure intensity, even for a given amount of average exposure intensity. Many UV-A wavelengths are believed to contribute to "nuclear cataracts", which is a permanent brown discoloration of the central portions of the lens of the eye. The damage requires affected molecules to receive at least two photons, and to receive the second one before it recovers from the first. Damage may therefore be some function of peak and average exposure intensity. You need your eyes to get no more UV than a fraction of what you would get from natural daylight.

1a. In dark/dim areas, the pupils of peoples' eyes get big, and are big when the strobe goes off.

2. It is the central regions of the lens that most attenuate lower-visibility deep violet and borderline UV wavelengths. With the pupils being more dilated from darkness between flashes, the thinner edges of the lens let you see more of some of the wavelengths used for the "blacklight" effect. With this light being more visible than usual, the "blacklight" effect is not as apparent.

3. Human vision often sees contrast less with strobe lights than with steady lights. This can reduce the apparent brightness of fluorescent objects illuminated by a strobe blacklight.

But if you want to try to make a blacklight strobe anyway, here is a way to make a small one using the glass bulb of a "blacklight" bulb. Please note that these bulbs let out some visible violet and blue light and may not look very dark when used.

NOTE: The bulb-butchery mentioned below may require practice, and you may want to practice this on 80-cent regular bulbs before doing it with $3-$4 blacklight bulbs.

CAUTION: The below method requires butchering a glass bulb, which has obvious hazards. Do so only if and when and where it is tolerable to spew out a few bits of glass. Use of safety goggles and gloves is recommended.

Get a 75 watt "blacklight" bulb, needlenose plyers, a flat-blade screwdriver, and a pair of diagonal cutters that you don't mind treating roughly.

1. Remove the metal contact from the bottom of the base of the bulb.

2. Break out the piece of glass that separates the above contact from the main part of the base of the bulb. This may require alternating between different tools.

3. Use the diagonal cutters to peel off the lower portion of the siding of the base, almost up to the cement inside the base.

4. This gets a bit tricky. Use the pliers or the cutters to break the exhaust tip in the bottom of the bulb, if this stem is still intact. Break off/out the portion of glass around this, making a large enough hole. You may have to break off/out a portion of the cement.

5. Get the stem assembly out of the bulb. There may be a heat shield which you may have to bend somehow before it will come out through the hole in the bottom of the bulb. Be sure the hole is big enough to get your flashtube in.

Now that you have an empty blacklight bulb with a hole in the bottom, put a flashtube into the bulb. Seal with silicone rubber if you want to make it permanent, but poke or drill a hole in the silicone (or in any other way be sure this is NOT airtight). You don't want a possibly weakened glass bulb to be pressurized if/when it heats up.

Another idea: Cut a length of dark violet glass tubing from a fluorescent style blacklight tube. This works better. For some more details, go to my Blacklight Tube Hacking File. You will probably want to set up a hot wire type glass tube cutter. Otherwise, just put the flashtube in a whole blacklight tube. You are probably better off leaving the phosphor coating in place, unless you have reasons for NOT having the light diffused such as for focusing it into a beam.

As for how to make this make as much "blacklight" as easily possible: Use a quantity of flash energy near half the maximum the flashtube will take, with the maximum voltage that the flashtube is rated for. You can probably get away with a bit of extra voltage. You may want to select a capacitor for extra voltage; many won't take over 330 volts and you probably want to try getting away with a little more for a small camera flash or strobe flashtube.
If the energy quantity is lower, the ideal voltage is less certain. Higher voltage favors a slight spectral shift towards UV, but smaller capacitance favors a line spectrum that largely lacks "blacklight" UV lines. If you have a spectroscope or a diffraction grating, you want the smallest capacitance and highest voltage that gives you mostly a continuous spectrum instead of spectral lines. If you use low capacitance and high voltage and must deal with a line spectrum, then it is probably best to go for really high voltage, which favors a violet and violet-blue cluster of spectral lines, which has some "blacklight" effect. Most non-blue fluorescent paints, inks, and dyes fluoresce from violet and blue visible light.
Small straight flashtubes made for cameras seem better for producing longwave UV than the popular U-shaped strobe tube. The U-shaped strobe tube more easily produces a line spectrum unless you have really high capacitance of at least several hundred microfarads. Also, camera flash tubes can easily fit into a 4 watt blacklight fluorescent tube which you may get for about $3 each from Hosfelt Electronics and Electronic Goldmine.

Hosfelt Electronics ( (800-524-6464, 740-264-6464) sells this 4 watt fluorescent blacklight bulb (F4T5BLB) which costs $2.49 and has the catalog number 10-114.

Electronic Goldmine is selling these also. (800-445-0697, 602-451-7454, Their catalog number for this bulb is G9598 with a price of $3.49.

For more info, go to my xenon strobe design guidelines.

I recommend seeing my xenon strobe safety info.

Written by Don Klipstein. Please see my Copyright Info.

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