Nondestructive material testing techniques have been applied extensively to cultural heritage objects with various purposes, from research in history of arts, to practical conservation aims. In this work, we present data obtained with a monochromatic confocal microscope working at 0.3 THz, but easily upgradable up to 0.6 THz (the monochromatic source is a RF-multiplier chain), where the sample-to-objective mirror distance z can be varied micrometrically. The sample and the emitter horn antenna act as the two mirrors of an optical cavity and the Airy function for cavity interference is indeed measured by finely scanning z. Useful information can be retrieved both by analyzing z-scans and by imaging objects in x-y planes at constant z. The device is lightweight and compact, therefore it can be made portable and/or installed on a robotic arm.