Re: PHOTO-3D digest 1706

[P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

Ed Romney writes:

>    As per speed of response of CDS cells ,they are very fast, fast enough
>to fire a strobe flash and a slave to overlap at 1/1000 sec. No
>problem.Commercial shutter testers use them. In fact you should  buffer
>them with a capacitor to keep them from undulating with the sine wave of
>fluorescent light  in a good EE camera design.

They will in fact fire a slave strobe as you describe but the the intensity
of the strobe is very bright and this increases the speed of response of
the photocell as I mentioned in my previous post. Also photocells have
a much faster turn-on time. The turn-off time and turn-on time of CdS cells
are not symmetrical. The turn-on time can be an order of magnitude faster 
than the turn-off time. 
Practically speeds of 1/10,000 would be stretching the limits of photocells
and these figures would not be absolute and depend on a number of factors
inluding light level, light history, and ambient temperature. Even if a CdS
cell is connected to a digital clock and turns it on and off, running at
1/10,000 sec. the accuracy would be poor because of the reasons cited above
and also because of the turn on and turn off times. The turn-off time would
be much slower than 1/10,000 thus contributing to measurement error. In the
flash example above this is not as critical since it is on the turn-on and
even this does not trigger the slave instanteouly but greater than 1/10,000
seconds. My best estimate of the accuracy of CdS photocells as a speed 
testing sensor would be 75% accuracy tops. Using this as a commercial
application in testing camera shutter speeds is probably not critical
and there is a big leeway in the variance of shutter speed error allowable.
Practically, testing shutter speeds faster than 1/1000 would not be
recommended with a CdS cell. If they have a 1/10,000 sec resolution digital
timer than can resolve these units doesn't mean that it can do so accurately.
Resolution and Accuracy are two different entities.
As for light flickering, yes CdS photocells can definitely pick that up but
this is a much lower order of magnitude than 1/10,000 or even 1/1,000 sec.
They can also pick up the scan rates of television sets and computer monitors.

>> 1.You can buy 1/100 sec. wristwatch digital stopwatches very cheaply, a few
>> dollars. If you gated one with a CDS cell it should read slower shutter
>> speeds very well. How would you do this, would you turn the power on and
>> off? I'm not a solid state designer by any means. That is how a digital
>> shutter tester actually  works; a CDS cell turns on and off a digital clock
>> running at 1/10,000 sec.

As for using it with a 1/100 sec. wristwatch this would be within limits of
a photocell and sounds like a very good idea.

Gabriel reverting back to kiddding mode.


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