Chocolate inspection by means of phase-contrast imaging using multiple-plane terahertz phase retrieval

Terahertz (THz) radiation has shown enormous potential for non-destructive inspection in many contexts. Here, we present a method for imaging defects in chocolate bars that can be extended to many other materials. Our method requires only a continuous wave (CW) monochromatic source and detector at relatively low frequencies (280 GHz) corresponding to a relatively long wavelength of 1.1 mm. These components are used to construct a common-path configuration enabling the capturing of several images of THz radiation diffracted by the test object at different axial depths. The captured diffraction-rich images are used to constrain the associated phase retrieval problem enabling full access to the wave field, i.e., real amplitude and phase distributions. This allows full-field diffraction-limited phasecontrast imaging. Thus, we experimentally demonstrate the possibility of identifying contaminant particles with dimensions comparable to thewavelength. (C) 2022 Optica Publishing Group

Automated summary

Terahertz radiation shows great potential for non-destructive inspection. Researchers have developed a method for imaging defects in chocolate bars using continuous wave sources and low-frequency detectors. By capturing diffraction images and solving the phase retrieval problem, they achieved diffraction-limited phase-contrast imaging and successfully identified contaminant particles with dimensions comparable to the wavelength.