Help me understand sparks (for flash photography purposes)

[nsled]

So even at same voltage some sparks are wimpy dull blue ones, and others are white bright and bang when they go off. What is the difference in these discharges? For example comparing an ignition coil spark to a stun gun spark over same distance.

If I assume the voltage and resistance are the same over the spark gap, the current must also be the same. So is it just the duration of the spark that is changing to make it whiter and louder?

 

[alan sailer]

I can't say from measurements but I don't think that the resistance is constant. As more current goes through the air
gap more ions are created lowering the resistance.

The sparks that I have made range from pale blue sparks from a Van De Graff generator to a blinding white million candle power
25kV of a spark gap flash. The former has a capacitance of about 40pF for a 12 inch sphere. The spark gap flash has a capacitance of
.03uF. The blinding brightness of the later is due to the very high peak currents.

[nsled]

Thankyou Alan. It's funny I know your name from a long time ago, I have one of Maurice Ribbles air gap flash units here. I guess its somewhat obsolete these days with LED technology but that was the thought in my question, what exactly makes that flash bright and a ignition spark so dull.

[klugesmith]

>>I guess its somewhat obsolete these days with LED technology

OK, who can fill us in on progress using LED technology to make microsecond flashes for photography? I've given it some thought but never got my hands dirty.

In phones, and compact digital cameras, LED's have generally replaced tiny xenon flashlamps.   It's a backwards step, time-wise, replacing millisecond flash with maybe 1/20 second.  Because the cheap LED's aren't happy or efficient operating at more than 100 amperes, even briefly.

[nsled]

>>I guess its somewhat obsolete these days with LED technology

OK, who can fill us in on progress using LED technology to make microsecond flashes for photography? I've given it some thought but never got my hands dirty.

There was one sold about 8 years ago that went down to 0.5us flashes called the Vela One, it was originally $700US. It seems the site is still up but now $1600. I'm not so sure the site/store is still active though. The trigger that goes with it has been out of stock for many years, and I had tried to purchase one when it was but money just disappeared.

I have personally measured the flash though just by flashing it into a LED wired to an oscilloscope and it indeed did produce a signal from the LED of 0.5us. It's quite dim at this speed though and has output wire for stringing more units together.

[alan sailer]

nsled,

Yes I helped Maurice quite a bit with that flash. It's a hard thing to market because the air gap flash requires frequenct tear-down and readjustment. Humidity can also make it go down.

Kklugesmith,

In addition to the Vela there a quite e few people who have managed to get working microsecond LED flashes.

https://hackaday.io/project/165622-edgerton-a-high-speed-led-flash

The problems I had with the Vela were two fold. First, unlike the air gap flash, the light field was much more
dispersed due to the multiple emitters. As Ted Kinsman once said anything that you need to shoot at a
microsecond is going stupid fast. By that he means that a lot of energy is involved. So any realistic microsecond
subject is going to be small* and thus require a concentrated source of light.

Second was that for some darn reason the LEDs kept dying.

* Large fast moving objects are incredibly dangerous.

[davekni]

In addition to the Vela there a quite e few people who have managed to get working microsecond LED flashes.

Short LED flashes are easier for backlit photography since less total light is needed.  At work (Xerox) I built 0.5us LED flash units for imaging ink drops ejecting from print heads.  Before that we used short-arc xenon flash units.  The xenon units had a more exponential decay shape to light, around 0.5us time constant and about 1.5us until flash fully ended.

BTW, in 2015 I made three Xenon flash units at home for capturing images of our supersonic ping pong ball launcher.  I had two short-arc xenon flash lamps.  For the third I used a standard cheap disposable-camera flash tube at much higher voltage than rated, around 1.5kV if I recall correctly instead of nominal 300V.  (Didn't need long life.)  Time constants were around 1us for the short-arc flashes and around 2-3us for the camera tube.  Images below are triple flashes within a single camera exposure, 500us flash spacing: