optical lightning detector

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I want to concoct a trigger to fire my camera when there's lightning in  
the area of where the camera is pointed. I do not want one of the RF burst  
detectors, as I don't care about lightning that's not in front of the  
camera itself. I'm thinking about some sort of photodiode mounted in a  
housing peeping through the viewfinder for actual flash detection.

Past that, I'm not too sure of what's next.

Has anbody build something like that before? I'd like to somehow be able  
to tune the detector's sensitivity to the flash or light as well as the  
risetime of whatever it's detecting.

I've not yet tested if photographic slave modules actually pick up on  
ligtning. Has anybody ever tested this?



Re: optical lightning detector
On Tue, 24 Jun 2014 17:51:17 +0000 (UTC), Cydrome Leader

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Roll your own... might have to use a photo-tube... that was what I
used in my first slave-flash when I was ~15 years old ;-)
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: optical lightning detector
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Did these tubes act like a giant light activiate SCR?

Re: optical lightning detector
On Tue, 24 Jun 2014 18:38:38 +0000 (UTC), Cydrome Leader

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I don't recall the details now... it has, after all, been 60 years,
but I used it to directly fire a flashbulb.  (I didn't have a strobe
until a few years later.)
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: optical lightning detector
Cydrome Leader wrote:

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Digital, camera?  Most of them have massive delay between pushing
the button and actually taking the picture.  There often is a sports
mode that reduces the delay.  But, I suspect this will be a problem
with most digital cameras, they will take a picture of black sky, a
large fraction of a second AFTER the lightning.

Jon

Re: optical lightning detector
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the camera is fast, so lag won't be a problem, but figuring out how to  
adjust everything will be, as I can't summon a lightning storm at any  
time.

Re: optical lightning detector
Cydrome Leader wrote:

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The DSLR shutter lag really is significant. We're talking on the order of  
100ms, and this is assuming you have a mirror lock capable camera and are  
not using autofocus.[Prefocus and leave the camera in manual.]  
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The lightning flash lasts about 100ms
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The way these shots are done at night is you just leave the shutter open (B)  
and stop it down a bit. That gives you a long exposure time. When the flash  
occurs, close the shutter.  

In the daytime, they use a neutral density filter. Same basic technique.

To do what you want to do will require an external camera shutter. Some can  
fire in under 20ms.

Uman has a few books on lightning. Well worth the money.

  


Re: optical lightning detector
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the camera has shutter lag of 40ms or less, so it won't be too slow.  
Lightning really isn't all that fast, I just want an electronic trigger  
system that only triggers the camera when a flash of light of the correct  
speed and intensity appears. It's fairly bright in Chicago so long  
exposures when you're inside a storm mostly pickup reflected light from  
the city itself, which is an ugly orange color. Yeah one could use blue  
filter and all that, but it's still very bright.

Cloud to cloud lightning causes lots of lower intensity flashes which I'd  
like to just tune out so to speak as there's nothing to see there but a  
flash of diffused light.

I will look for the book though.

Re: optical lightning detector
Cydrome Leader wrote:


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Are you sure that 40ms isnt that "trigger button halfway down" mode? Most  
cameras are in a sleep mode when just sitting there in manual. The link on  
this Mk III shows this (model specific of course).  

The stepped leader has a number of 1us pulses. That is what you would want  
to trigger on. There is a very high frame rate video of a lighting strike on  
the internet, but I can't find it at the moment. Too many false hits on  
google. The stepped leader is not as bright, but the very short duration  
steps would make for a good trigger, giving you some advance notice before  
the return strike hits. The video wasn't on youtube, but hosted on the  
authors website. I snagged a copy of it with a little hacking, but I need to  
locate the file.





Re: optical lightning detector
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It's a nikon d3s, so it's the league of fastest cameras out there.

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I'm under the impression the return strike or whatever it's called is the  
brightest one, so there is no loss having a small amount of lag.

Re: optical lightning detector
Cydrome Leader wrote:

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The return strike is the big deal, but what you want to do is detect the  
stepped leader. The stepped leader can be flicking for 10 to 30ms before the  
return strike hits. Ideally you would open the shutter within 10ms of  
detecting the stepped leader. If you researching this a bit further, this  
has been done using external shutters in front of "stock" cameras in bulb  
mode.

There is much research on the stepped leader since disturbing the stepped  
leader is a means of preventing lightning.

This paper covers the spectrum of the stepped leader if you want to enhance  
your set up.
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The paper you should really have is "Spectrum of the Lightning Stepped  
Leader" by Richard E. Orville. (Journal of Geophysical Research, Vol 73, No  
22, November 15, 1968.)  

If you read research papers on lightning, there are a small number of names  
that keep cropping up. Uman or course, but Orville is another.

There is a video by Tom A. Warner online that uses a high speed CCD camera.  
THe last Adobe flash update screwed up my linux desktop machine, and many of  
Tom Warner's videos don't play. When you find it, it will not be eye candy.  
It is scientific imaging. I have a still from it and it read "Fri Jul 10  
2009" on the lower third. You can watch the stepped leader build.

There is something odd about the shutter lag testing on the D3s. For  
instance, it is 47ms in the manual focus mode.
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The reason this looks odd is the time is nearly the same as the "button  
halfway down mode" of 43ms.  

I would think your first project would be to measure the lag time yourself.  
You want to measure the time with manual focus and the mirror locked. On the  
Canon remotes, you can make it look like the camera button is halfway down,  
so I assume Nikon can do the same thing. That gets around the wake up time.  
However Canon has a limit on how long you can sit with the mirror locked. It  
isn't much, like maybe a minute or two. There are hacks around that since  
the "Magic Lantern" crowd have reversed engineered EOS.
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For the shutter lag test, I'm guessing you would have two rows of LEDs that  
the camera would photograph. One row counts in a thermometer code. The other  
acts as an accumulator, also in thermometer mode.  


Re: optical lightning detector
Cydrome Leader wrote:


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Well, I think using a camera flash (especially cellphone camera) indoors
would be a good lightning simulator.  I'm pretty sure you could use a small
photocell as a sensor, reverse biased to some modest voltage, and
capacitively coupled to a current to voltage (transimpedance) amp.
Then, high-pass filter the output of the amp.  Final calibration
of the trigger threshold would need a storm, but you can probably get
it close with a flash.

Jon

Re: optical lightning detector
On Tuesday, June 24, 2014 10:51:17 AM UTC-7, Cydrome Leader wrote:

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With a vidicon-type camera, that could work: the image remains readable for a
few microseconds after the light is received.  With solid-state, though,
there's a cycle of erase/integrate/readout.  If you keep the camera in
erase mode, the flash is over before  you can get to integrate mode.

Best, might be to alternate two cameras, having one always in the integrate phase,
and capture the associated readout data only when a flash was detected during the
integrate phase.   So, you'd be discarding a lot of frames of a video feed.

It's easier with film: just open a shutter, and close it if/when enough incident light is
seen.   Probably a used point/shoot 35mm camera is the best way to proceed; with
a tripod, using an IR remote to start the exposure.   Extra points for electric
film advance...

Re: optical lightning detector
On Tuesday, June 24, 2014 1:51:17 PM UTC-4, Cydrome Leader wrote:
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One strategy is to just concoct a trigger like you envisioned, the camera is in  
"bulb" mode which opens the shutter for say 20 seconds each time. Since  
lightning often occurs in clusters of strikes, you have a chance at catching  
the follow-on strikes. 'Pro' phtographers do this manually.  

The trigger has to catch a sudden burst of light. How to do? A simple way is  
just trigger on any bright flash. Take a bwp34 photodiode and lens. connect to  
a transimpedance op-amp and then to a schmitt trigger circuit to trigger the  
camera open for 5-30 s. I'm assumimg you have dark condx.


Re: optical lightning detector
On Tuesday, June 24, 2014 5:34:25 PM UTC-4, snipped-for-privacy@gmail.com wrote:
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And, in case the lightning flash is weak, I wonder about the following  
comparator circuit. Assuming the photodiode is put in a transimpedance  
configuration, a standard way of wiring PD's to minimize capacitance effects.  

Now suppose you take a sensitive comparator and put the output signal of the PD  
amp on BOTH input pins of the comparator. ONE pin of the comparator has a cap to
 ground to provide a bit of time delay. When a fast changing signal comes  
through, one pin will lag and the comparator will fire. Slower signals will not  
fire it, and its schmitt one shot.

I wonder if anyone has more insight into this type of simple circuit - a delay  
comparator in time. Be interested in hearing about.

The two signals must obviously be isolated from each other -  



Re: optical lightning detector
On Tuesday, June 24, 2014 6:23:33 PM UTC-4, snipped-for-privacy@gmail.com wrote:
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OH, and bear in mind that if you get 1/500 hits with a digital camera, its OK,  
since you just erase the 499 and keep the one. This greatly relaxes the  
precision requirement of your detector in time and space. Of course the more  
you miss, the longer you have to wait for a good pic. A detector that shoots  
and misses often may be better.

I thought about this area a bit because I wanted to catch grasshoppers  
springing from grass into flight. A trick is to use a bright flash even in  
daylight to catch them. But they are very fast, so I had trouble with manual
shooting, as they can hear you coming and spring fast.

Re: optical lightning detector
Buy one of the consumer digital cameras that has a SDK. Set it to record video for 20-30 seconds at a time, and wait for a input from your flash detector via serial or USB. If there was a flash, save or download the video. If not, start recording again.

Steve  

Re: optical lightning detector
On Tuesday, June 24, 2014 6:46:25 PM UTC-4, the rain in spain. wrote:
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Yes, that's clever. An even simpler way is to not use a detector. Do time lapse  
photography every night, each 20 s. long, and if a thunderstorm happens, review  
your photos. If not, erase the photos. (total chip erase.)

simple. easy.

Re: optical lightning detector
snipped-for-privacy@gmail.com wrote:
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this might be ok out in the country, but 20s exposures are going to pick  
up too might stray light in a major city.

It may be possible to set a super low ISO in the camera and use a super  
dark ND filter to attentuate the garbage light, but for the hell of it, I  
want an electronic solution vs. brute force. I've not found a screen cap  
from an oscilliscope of the light intensity of a lightning flash as  
measured by a photodiots. This would be interesting to see.



Re: optical lightning detector
Cydrome Leader wrote:

  
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Note that for most cameras, video mode is not the same resolution as when it  
shoots stills. I mean, you could shoot lightning with a gopro, but the still  
frame wouldn't be epic.

Stray light may not be the problem you think it is. I've done star trail  
photos in the high desert with no moon. There isn't enough stray light to  
illuminate the object that you want to combine with the star trails.  

But you will need ND in the city. The other thing to consider is lens flare.  
If you stop the lens too much, you see the iris in the flare. That is why ND  
are important. You can learn a lot about the flare of the lens by shooting  
the full moon in various locations in the frame and at different f stops.  
Stating the obvious here, you want a prime lens.





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