BEQUEREL EFFECT: Intensification of a latent image by exposure to light to which
the emulsion is otherwise insensitive. An image on print out paper can be
intensified if it is exposed to green light before fixing, ... , a silver image
eventually builds up on the areas that have allready received an exposure. Apart
from this, silver bromide emulsions do not exhibit the effect. (The Focal
Encyclopedia of Photography, Vol 1, 1985, p.69.)
If I remember correctly (can't remember where I read it), that this effect may
also apply to other light sensitive systems.
Klaus Pollmeier