Conservation DistList Archives [Date] [Subject] [Author] [SEARCH]

Subject: Flash photography

Flash photography

From: Stefan Michalski <stefan_michalski>
Date: Friday, August 9, 1996
I try to stay out of the Super Tower of Babel but this flashbulb
thing has dogged me throughout years of lighting seminars, and I
want it to go away. Besides, right now isn't a good time for our
profession to go to the wall  with yet another piece of revered
wisdom that turns out to be built on quicksand.

There have been reliable articles on an identical problem: the
effects of flash tubes in copying machines, that have already gone
through this issue, and laid it to rest (such as Jim Hanlan's about
twenty years ago).

The underlying principle for any such calculation is reciprocity,
the notion that light effects are cumulative in a simple additive
manner. Reciprocity has been found to hold with reasonable accuracy
in several industrial studies of textile light-fading, in at least
one copying machine flash tube study, and most recently in a paper I
just accepted for the upcoming ICOM-CC conference, by Saunders and
Kirby of the National Gallery London. It is far and away the best
study to date, on several historically important colorants, and
finds reciprocity holds very nicely. While it is true, as someone
noted, that some chemically measured effects linger in the dark
(e.g. free radicals have been shown to take many hours to decay in
photodegradation studies of wool) there is no evidence that this is
a significant correction to estimates based on simple reciprocity.
(Ezrati has a paper in the upcoming ICOM-CC conference showing
intermittency has no significant effect on dye fading). So, the rest
is arithmetic, and units.

An electronic flash on a camera is typically sized to use f8 for a
film of 100ASA at a subject distance of 3m (10ft). From photo
handbook data, this is equivalent to a light dose at the artifact of
about 30 lux seconds (lx-s). An alternate route to this estimate is
via the Illumination Engineering Handbook data: Xenon flash tubes
for photography range from 10 to 200 Joules rating. Given efficacy
of about 50 lumens per joule, a wide-angle reflector throwing the
light forward into about 1/4 of the sphere, this gives a range of 20
lx-s for little builtin flash to 400 lx-s for big fat studio tubes.

For convenience, round up to 50 lx-s for each amateur. Assuming the
gallery lighting is the lowest most museums can tolerate, 50 lux (5
foot candles) then each flash adds the equivalent of one second of
normal gallery exposure. So, 300 amateur flashes a day is equivalent
to adding five minutes to the display day. In order to actually
increase damage by 10% on a ten hour day, one would need to
experience 3600 flashes per day. Two large professional flashes
would raise the ante a little, they would need 225 flashes a day to
add 10%. For museums at 150 lux (15 footcandles) these numbers
become 10,000 amateurs, or 700 pros, every day. To actually double
fading would need 100,000 amateurs a day. Most museums would kill
for those attendance figures! As for the UV wrinkle, xenon is used
because it has a spectrum very close to daylight (6,000K). Given
typical glass tubes and plastic diffusers, the UV ratio will be a
little higher than properly filtered light, but UV type damage is
far from the Achilles heel of artifacts at controlled light levels,
it is color fading, and UV is not the issue here.

In other words, flash may very well be banned for reasons of
copyright, or as a disturbance to the act of contemplation (my
personal vote) but there is no preservation reason. I think the ban
started originally because flash bulbs (and their precursors the
open magnesium flash) were a genuine fire hazard, and an explosion
hazard (hot fragments) and a garbage problem. Of course, tripods,
hot studio lamps, and bulky equipment are still  hazards, and a
photography policy still necessary, but please don't wave the red
flag of conservation over flashcameras.

Stefan Michalski, Canadian Conservation Institute

                  Conservation DistList Instance 10:18
                 Distributed: Tuesday, August 13, 1996
                       Message Id: cdl-10-18-001
Received on Friday, 9 August, 1996

[Search all CoOL documents]