Wow! There's a part of flash photography that I knew almost nothing
about. Lots and lots of chemistry and physics. Thanks for the info.
Chuck Norcutt
On 2/19/2014 11:29 PM, Mark Marr-Lyon wrote:
> On Wed, Feb 19, 2014 at 7:14 PM, Chuck Norcutt <
> chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
>
>> ps: There are some great gallery shots there but the one I found most
>> intriguing was this one of the rocket sled.
>> <
>> http://www.meggaflash.com/gallery?AG_MK=0&AG_form_paginInitPages_1=1&AG_form_albumInitFolders_1=/gallery2&AG_form_scrollTop=0&AG_form_scrollLeft=0&AG_MK=0
>> ?
>>
>> The slowest burn time bulb is the PF200 with burn time in excess of 40
>> ms. The rocket sled is traveling at 4800 ft/sec. or 4.8 ft per ms.
>> During the 40 ms burn time of the flash bulb the sled travels 192 feet.
>> So, unlike more conventional electronic flash scenarios we are used
>> to, even the fastest meggflash bulb can't freeze the action. Yet the
>> rocket sled does seem to be frozen... except for the background which is
>> very blurred horizontally. Conclusion: The camera was panning with the
>> sled, or more correctly, moving along with the sled. Somehow the camera
>> and flash had to be moved parallel with the sled at nearly the same
>> speed as the sled in order to freeze the motion. I wonder how that was
>> done!
>>
>
> Not the camera, but the image. It's a special high-speed camera called a
> smear camera. There's a little about it on Wikipedia:
> http://en.wikipedia.org/wiki/High_speed_photography
>
> We've still got some rotating-mirror cameras here at work, but they've been
> almost completely replaced by digital high-speed cameras. Many of our film
> cameras use 70-mm film, which given our Byzantine purchasing rules is
> effectively impossible to get. The last time I used a film camera was
> nearly 10 years ago. Our rocket sled track hasn't been used in probably
> close to 20 years.
>
> As for flash, back in the day argon candles were commonly used:
> http://en.wikipedia.org/wiki/Argon_flash
> Experiments often had more explosives in the lighting system than in the
> object being studied. Now we have a couple of Megasun flash systems:
> http://prismscience.com/system_components.php
> We're listed in the sample uses page:
> http://prismscience.com/sample_uses_lanl.php
> I mostly use the 15 kJ 700 us system. While not as fun as a big argon
> candle, they're much more convenient. We've also been known to destroy a
> few Alien Bees. They're cheap enough to be considered disposable, and while
> not nearly as bright as the Megasun sometimes they're enough.
>
> Mark
>
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