Electronic flash duration is *about* 1/500-1/2000 of a second.  If the
energy from the flash was 1000 times as strong as sunlight (very high
estimate), then there would be an average of one second of exposure to
sunlight for each flash.  If an object receives 60 flash photos a day,
then it would receive the equivalent of 1 minute of sunlight a day.
There is substantial literature on this issue in the CoOL archives, see
my previous posting for address.

Flash photography for documentation is desirable because the light
exposure is significantly shorter that incandescent *hot lights.*  The
other problem is that normal incandescent bulbs give off massive amounts
of heat.  Incandescent lights do change the local relative humidity
during photography.  They can also cause direct damage to a sensitive
art work under the worst conditions, i.e., to many watts and too close,
humidity sensitive thick watercolor or low Tg (softening temperature)
resin coating on a painting.  Professionals routinely use photographic
umbrellas and soft-boxes on flash heads to soften the light and modify
its tone and color rendering abilities.  Flash photography does require
proofing for new setup, otherwise it is the desirable mode of museum
photography.  Hot lights are easier and cheaper to work with, but they
have significant disadvantages.  Most photographic lighting systems are
workable, given the proper precautions.

Timothy Vitale
Paper and Photograph Conservator
& Preservation Consultant