Blacklight / UV Strobes

last updated 12/16/2018

Do you want to make a blacklight strobe?

My older stuff as of 1/20/1999
My 2018 guidelines for UV from many quartz flashlamps
Related stuff, flashlamp guidelines, safety stuff

My old stuff from 1/20/1999

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 very high capacitance of at least several hundred microfarads. Also, camera flash tubes can easily fit into a 4 watt blacklight fluorescent tube.

My 2018 guidelines for many quartz flashlamps

Quartz xenon flashlamps generally favor higher xenon plasma temperature that favors more UV output. Also, quartz flashtubes (other than ones of varieties doped or coated for blocking UV) are more transparent to UV in or near the UVB and UVC ranges than glass flashtubes are. Then again, glass flashtubes and protective glass used with many quartz flashtubes pass most of the UVA range of ultraviolet, which is more useful for achieving "blacklight effects" and less dangerous than UVB and UVC.

One thing to consider is a "capacitance rule" that I have become aware of, for getting a xenon flashlamp to be a good blackbody/graybody emitter of a continuous spectrum including UV. Please note that there are also requirements for quantity of energy in the energy storage capacitor(s) and voltage that the energy storage capacitor(s) must be charged to in order to achieve good efficiency of a xenon flashlamp's production of UV.

The capacitance rule that I have noticed: It is necessary for the energy storage capacitor(s) to have capacitance of about or more than .15 farad, divided by the xenon pressure in torr, multiplied by the square of the flashtube's inside diameter (bore) in mm, divided by the flashtube's arc length in mm.

When the xenon pressure is close to or above the 450 torr typical of most quartz linear flashlamps of kinds used for pumping lasers, I have found that 10% less capacitance than recommended above works well. This means with 450 torr: Use at least about 300 microfarads, times the square of the flashtube's inside / bore diameter in mm, divided by the arc length in mm.

Another thing I have found: Achieving this amount of capacitance combined with a specific percentage of the flashlamp's "explosion energy" (often recommended to not exceed 30%), favors flashtubes with smaller inside / bore diameter.

The narrowest/smallest interior/bore diameter quartz xenon flashlamps that are reasonably common have this bore diameter being 4 millimeters. This means that my "capacitance rule" predicts that with most quartz xenon flashlamps with inside diameter 4 mm, outside diameter 6 mm, the capacitance required to achieve good broadband continuous spectrum including UV is 4800 microfarads divided by the flashlamp's arc length in millimeters. Flash durations around or a little over 100 microseconds are favorable for quartz flashlamps with bore diameter 4 mm (and xenon pressure of 450 torr) to be used with 20-30 % of their "explosion energy" with the minimum recommended capacitance and an appropriate series inductor. A conservative estimate of the "single pulse explosion energy" of a quartz flashlamp is 3.5 joules times the flashlamp's bore diameter in millimeters times its arc length in millimeters times the square root of the flash duration in milliseconds. There are higher figures as high as 7.78 joules times the arc length in millimeters, times the bore diameter in millimeters, times the square root of the flash duration in milliseconds in a critically damped circuit with an inductor, with xenon pressure of 450 torr, for high quality quartz linear flashlamps with bore diameter 8 mm or less.

Lamp life expectancy is typically around 27,000 flashes at 30% of the explosion energy, around 130,000 flashes at 25% of the explosion energy, and around 800,000 flashes at 20% of the explosion energy, according to a "one size fits all" formula. Actual life may be less due to peak current density exceeding 4,000 amps per square centimeter of arc cross section. Also, external triggering may cause or contribute to life being shorter than predicted by leading to an uncentered arc. On the other hand, when these percentages are percentages of the conservative 3.5 joules times bore diameter in mm times arc length in mm times the square root of flash duration in milliseconds, these life expectancies may be realistic.

Table for flashlamps with 4 mm bore diameter
Capacitance is 4800 uF divided by the arc length in millimeters

  % of        Flash       Energy         Volts      Inductance   Estimated
explosion    duration    joules per      per mm     uH per mm    Arc temp.
 energy    microseconds  mm arc length  arc length  arc length   Kelvin
------------------------------------------------------------------------------
   20         115           .95           19.9        .31         11100

   25         106          1.14           21.8        .258        10500

   30          98          1.315          23.4        .224         9800
------------------------------------------------------------------------------
Note: Usage of switching means to control flash duration is a separate issue with different equations, and generally incompatible with the scheme of series inductors historically used to maximize performance of quartz xenon flashlamps.)

Related stuff, flashlamp guidelines, safety stuff

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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