A cw terahertz image beam is upconverted by a nonlinear optical process (e.g., sum- or difference-frequency generation with a near IR cw upconverting beam). The upconverted image is acquired by a near IR image detector. The bandwidths and center wavelengths of the terahertz image beam and the upconverting beam are such that wavelength filtering can be employed to permit an upconverted image beam to reach the detector while blocking or substantially attenuating the upconverting beam.