Abstract: A parallel light beam is introduced from a collimator lens into through holes defined in a honeycomb-shaped object, and any light emitted from the honeycomb-shaped object is read by a CCD camera through a Fourier-transform lens to produce a Fourier-transform image of the honeycomb-shaped object. The honeycomb-shaped object is tilted such that the axis of the through holes is inclined at a certain angle to the optical axis of the parallel laser beam. If the partitions contain interstices spaced at intervals and light introduced into the through holes passes through such interstices, the Fourier-transform image includes a stripe pattern induced by diffracted light from the interstices. Since the stripe pattern can easily be distinguished from a dot-matrix pattern representing light emitted from the through holes, the interstices contained in the partitions can reliably be detected.