  Photographic Nuclear Emulsion Detector (PNED) has been in use in nuclear and
particle physics experiments from the begining, often as the major detector
system. However, direct measurement of impact parameter in this detector does
not seem possible due to some limitations. This paper discribe a simple yet
strong method to estimate the impact parameter of events on event-by-event
basis. Though this method is develpoed specifically for the photographic
nuclear emulsion detector, we envision that it should also be applicable to
other multi-target detector systems.
