High Voltage Forum
Science photography => DSLR => Topic started by: davekni on July 19, 2021, 06:32:35 AM
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Just a thought at this point. Does anyone have a global-shutter camera? It would be interesting to sync the camera exposure with DRSSTC sparks. With a properly timed and short exposure it should be possible to get good arc images in brighter background situations. Synchronized global-shutter video could allow daytime DRSSTC testing while still monitoring for errant sparks.
Two possibilities for synchronizing. The likely-easier option would be to use a camera output intended for strobe-light triggering to trigger single DRSSTC enable pulses. The down-side of this option is longer exposure times (unless the camera has an option to generate a trigger slightly prior to shutter opening.)
The other possibility is to trigger camera exposure from the DRSSTC enable pulse, perhaps with some controlled added delay. Then the exposure could be short, covering just the active arc portion of the DRSSTC pulse.
I've built similar systems at work for strobed microscope viewing of ink drops. Unfortunately, I don't think there's any equipment available to borrow. I see cheap (~$70) global-shutter webcams from China. Can't find any information about options for trigger input or strobe output. The cheapest global-shutter camera I've found that lists trigger capability is $200+shipping. Perhaps someday when my project list gets short ;) I'll buy a camera and experiment. That is if no one else has tried it first.
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Nice idea there.
I bet a typical visible arc is the sum of many DRSSTC pulses following the same ionized air channel, as with natural lightning. Maybe the first pulse is not the brightest.
Do global shutter digital cameras allow independent control of the shutter duration?
Using camera shutter as the master, to trigger the DRSSTC as it might trigger an electronic flash,
would work even with a film camera. Many film cameras have global shutter times down to 1/500 second, with flash trigger at beginning of the wide-open time.
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I guess the easiest way would be to trigger them the way flashes are triggered. Not quite sure why that should result in longer exposure times though. You can still adjust all parameters in full manual mode. And the few 100 microseconds probably don't make too much difference either I'd guess.
The camera flash approach could actually be really easy, just a monostable 555, a photodiode and an IR filter from a remote control or something like that. I remember some of the old photo-flashes detecting the main flash like that, without requiring a special emitter on the camera. I'm assuming they just look for the IR pulse from the flash tube of the camera.
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I bet a typical visible arc is the sum of many DRSSTC pulses following the same ionized air channel, as with natural lightning. Maybe the first pulse is not the brightest.
Yes, I'm confident you are right. With high detuning, the first few pulses make very short weak arcs. That shows up in scope measurements and sequential video frames. Also, each pulse's arc generally displaces slightly from the previous path due to air motion - showing up as beautiful parallel paths in images. Having the DRSSTC electronics trigger camera exposure would allow triggering on the Nth pulse where the arc is strong (presuming smart electronics).
Do global shutter digital cameras allow independent control of the shutter duration?
Not certain I know what "independent" means here. The cameras we use at work have options for either programmed exposure time started by an edge of an external signal or for exposure to match the width of the external pulse.
The ideal (in my mind) camera would allow multiple short exposures with independent control of exposure (shutter open and close) and when the exposure-generated charge is transferred to readout circuitry (CCD cells or CMOS capacitors). That would allow multi-arc images. Would be useful at work too. However, the global-shutter cameras I've seen always transfer charge at the end of the exposure window, with no separate control.
Not quite sure why that should result in longer exposure times though. You can still adjust all parameters in full manual mode. And the few 100 microseconds probably don't make too much difference either I'd guess.
Exposure times are longer because exposure starts (slightly) before DRSSTC pulse starts. The arc doesn't start until primary current has built up and a significant fraction of primary energy transferred to secondary. Starting exposure as or after the arc forms allows shorter exposure. Daytime is very bright. It will require short exposures to not swamp arc light with daylight. Longer (but still short) exposures would be useful closer to dusk when daylight is lower.
The camera flash approach could actually be really easy, just a monostable 555, a photodiode and an IR filter from a remote control or something like that. I remember some of the old photo-flashes detecting the main flash like that, without requiring a special emitter on the camera. I'm assuming they just look for the IR pulse from the flash tube of the camera.
Yes, I agree, the camera flash approach is easier. Just more limited.
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With high detuning, the first few pulses make very short weak arcs.
IIRC, the first 2 or 3 bursts of a longer sequence in Hydrons measurement https://highvoltageforum.net/index.php?topic=117.msg735#msg735 showed a considerably higher arc voltage and a bit lower arc current compared to the later ones. Higher voltages and lower currents likely imply a lower capacitance, so probably shorter arcs and possibly less bright ones. A dependency on the tuning conditions seems quite plausible.
The ideal (in my mind) camera would allow multiple short exposures with independent control of exposure (shutter open and close) and when the exposure-generated charge is transferred to readout circuitry (CCD cells or CMOS capacitors).
The CCDs I used to work with (long time ago) only had a single charge path from the photo sites. This went to the analog shift register. For a multiply shuttered exposure, you'd need to dump photo electrons somewhere else for the unwanted exposure time. The CCDs I remember did not have a path for this.