A scanning microlens-array printer comprises an optical focus/alignment subsystem in which the optical sensor elements are integrated within a microlens printhead unit. The unit also incorporates an integrated spatial light modulator; thus the printhead incorporates all the critical optomechanical components necessary for high-resolution, maskless, lithographic printing. Alignment is detected by an interferometric process in which a reference diffraction grating on a printing surface coherently combines two optical beams to generate an interference signal that is sensitive to the grating's lateral position. Focus sensing is effected by using the reference grating to divide a normally-incident convergent beam into two obliquely-directed reflected beams, and detecting the focus-induced translational shift in the reflected beams' focal points.