We present an experimental and theoretical analysis of vibration micro
scopy using an arbitrarily defined detection aperture. For the two lim
iting cases, incoherent operation (no aperture) offers intensity-gradi
ent information in the amplitude of higher harmonics of the vibration
frequency, whereas coherent operation (pinhole aperture) reflects the
phase of the object being scanned in the amplitude of the first harmon
ic. The possibilities of phase microscopy, when the system is used in
a coherent mode of operation, are discussed. (C) 1997 Optical Society
of America.