Flash question

[Natha Congdon]

This is _very_ off topic, so my apologies, but it is for a good cause. (see be low) I've tried other groups to no avail, and, since this is the most knowledgea ble group I know of on general photo topics, I thought I would give it a try:

I am carrying out a scientific project, in which an ordinary camera flash is use d to bleach a subject (the opposite of having your eyes used to the dark), prior to doing a simple test of dark adaptation. We've been doing this test for near ly a decade now, and used the Promaster 2100 Flash when we started. These are n o longer made, we've lost our last original unit, and we need to get something c omparable (if the light output differs by much, our results in the future will n ot be interpretable vis a vis the past results). We measured the output of the original flash in absolute terms (in log candela/m2), but I didn't know enough a bout photography back then to do something as simple as record the guide number of the flash! Promaster is no help on this. So, as a test for all of you truly knowledgeable folks out there, what is the guide # on a Promaster 2100, and wha t current offering might be comparable? Alternatively, can a guide number be ca lculated from knowing absolute light output (eg, not f-stop) at a given distance ? I'd prefer not to try to take a bunch of flashes back into the lab to make me asurements, as we no longer have the equipment around to do this.

A large research effort to detect preventable blindness from vitamin A deficien cy in Asia is riding on this, so I would truly appreciate any help any of you ca n offer.

Thanks in advance, and again, my apologies for an off-topic post

Nathan Congdon.

[Michael_527]

Nathan,

You may have some difficulty determining the exact light exposure you have been giving to the subject. Candela/sq. meter is a unit of luminance, as I'm sure you know, but there is no time length associated with it. Therefore, you can probably get the correct luminance through testing, but there may be no way to determine the flash unit's length of flash, unless you can find an old one in working condition, of get some technical specifications.

That said, the bleaching may not be dependant on time, and be mostly dependant on the luminance of the light source. If this is the case, then I'm sure you can do some testing and get a new flash that will work.

Many better flashes have variable output now, and you may be able to adjust the output of a flash to match the conditions you have measured. One problem is the possibility that the flash output is controlled by length of flash, not intensity. But, I suspect some ND jells could be used in this case to correct the luminance to a reasonably close number.

Lastly, you may want to check with some smaller camera shops. There is always an old box of flash units, etc. that have been sitting there for the last 10+ years, and you may get lucky and find a replacement.

Good luck, I check my local shops the next time I'm in them.

---Michael

[Pete Andrews]

The guide number will be of no help whatsoever in matching the peak intensity of the flash. Photographic exposure is the product of time and intensity, and this is what the guide number tells you. You really ought to have taken some integrated time-intensity readings oughtn't you?