  We investigate quantum interrogation techniques which allow imaging
information about semi-transparent objects to be obtained with lower absorption
rates than standard classical methods. We show that a gain proportional to log
N can be obtained when searching for defects in an array of N pixels, if it is
known that at most M of the pixels can have transparencies different from a
predetermined theoretical value. A logarithmic gain can also be obtained when
searching for infrequently occurring large structures in arrays.
