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Re: [OM] OM4Ti meter

Subject: Re: [OM] OM4Ti meter
From: Winsor Crosby <wincros@xxxxxxx>
Date: Sun, 5 Jul 1998 17:08:48 -0700
>Kennedy wrote:
>>
>> 1/250,000th, or 4uS, is an age in electronics - your PC could probably
>> calculate pi to quite a few places in that time if programmed
>> appropriately, so I don't buy that one.
>>
>That depends. When you are only handling information, as in a PC, things
>can go very fast. When dealing with significant power levels, things are
>not so simple, just ask any designer of switch mode power supplys. An
>electronic flash has a high current running through the flash tube. This
>not something you just turn off, even with modern electronics.
>Incidently, one of the main reasons for the high speed of modern computers
>is the very low power level they work at.
>
>> Afterglow?  Its a long time since I did any plasma physics, but I think
>> the time for the spark to decay once the energy source has been removed
>> (via the thyristor in the flash circuit) is of the order of pico-
>> seconds.
>>
>Maybe, it's been some years since I last had any datasheets on flash
>tubes handy, so I don't have any hard information on this. But to get
>all the exited atoms back to their normal state in picoseconds? I'm
>sceptical.
>
>Anyway, we need hard facts for this to make sense. Physics is very
>interesting, but does not improve the pictures beyond a certain point.
>Computer flash works, that's the most important part.
>
>> My guess is that the times at which reciprocity starts to reappear are
>> closer to 1uS mark or less, rather than any failing of the circuit to
>> control the flash fast enough.  Of course reciprocity failure might just
>> have a slower fall off at the high speed end than it has at the long
>> time period - the mechanisms are probably completely different - so its
>> a bit less noticeable.
>> --
>> Kennedy
>> Yes, Socrates himself is particularly missed;
>> A lovely little thinker, but a bugger when he's pissed.
>> Python Philosophers         (replace 'nospam' with 'kennedym' when replying)
>>
>
>Regards
>Lars


I remember seeing a few articles on reciprocity failure with flash when the
first thyristor autoflash units came out a long time ago. Some even had
some horrible examples. I suspect that film manufacturers reformulated to
extend short exposure reciprocity safely beyond the shortest anticipated
flash duration so that it was just a temporary problem. They certainly have
worked on the other end as well when you look at newer films such as Provia
compared to a legacy film like Kodachrome.

Winsor

Winsor Crosby
Long Beach, California
mailto:wincros@xxxxxxxxxxx





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