The total EXPOSURE, E, which a piece of film has received is defined as: -
E = I x t [Intensity x time]
Where I = Intensity of exposing light (usually in meter candles)
and t = Exposure time of the shutter (in seconds).
Thus Exposure units are meter candle seconds.
The density produced on a film by a given exposure is almost always the same
whatever the differing values of intensity or time and this relationship is known
as the Reciprocity Law and is substantially obeyed by photographic materials
over the normal exposure range for which the material is designed.
However at very high intensities and very short times or at the other extreme
at very low intensities and very long exposure times there are deviations from
the Reciprocity Law with most materials This phenomenon is called Reciprocity
Law Failure but is rarely relevant to our technology.