Terahertz imaging of large objects with high resolution

Abstract

For over 28 years, INO has been developing microbolometer arrays for the infrared and Terahertz (THz) domains. INO’s microbolometer array is the key component of INO’s broadband THz cameras. Using this detector, INO has developed active Terahertz imaging systems ranging from 250 GHz to 750 GHz. See-through THz imaging is particularly well suited for security screening of persons and non-destructive inspection of objects. Materials such as cardboard, plastic, leather and denim are transparent to THz radiation and can provide insights on objects hidden from the naked eye or from infrared cameras. In addition, Terahertz provides high resolution images and is non ionizing. In particular, the frequency range of 150 – 550 GHz is of interest for its properties of see-through imaging that enable a wide variety of potential applications. In this paper, we present images obtained around 400 GHz and 200 GHz (corresponding to wavelengths of 0.76 mm and 1.52 mm). We have chosen these two wavelengths to allow for a wide range of objects and obscuring materials to be tested. The 400 GHz wavelength allows better image resolution, while the 200 GHz provides better penetration through the materials. The THz imaging system can obtain images of objects with dimensions up to 1 meter x 0.75 meter with subcentimeter resolution. To achieve this, we use diffraction-limited imaging optics with high numerical aperture and a microbolometer array detector. For each object, multiple images are acquired that are then stitched together. Each instantaneous image can be seen in real-time during the acquisition and has the same resolution as the global reconstructed image. In the context of an application, the operator does not need to wait until the scan has been completed to identify a hidden object if the size of its features is compatible with the instantaneous field-of-view. When a more global image is required, the reconstructed image shows the features of the whole object under investigation without resolution loss. Images are acquired in two different configurations: transmission and reflection. Each imaging configuration provides different information about the features inside the object as well as its composition. In summary, this paper demonstrates the potential for our THz imaging systems by providing see-through high-resolution THz images of large objects. An analysis of the impact of wavelength and imaging configuration on the image results is also provided.