Abstract
Terahertz (THz) radiation meaning electromagnetic radiation in the range from 0.1 (3) to 10 (30) has the unique advantage of easily penetrating many obstructions while being non-hazardous to organic tissue since it is non-ionizing. A shortcoming of this domain is the limited availability of high-sensitivity detector arrays respective THz cameras with >1k pixels. To overcome the imaging limitations of the THz domain, compressive imaging in combination with an optically controllable THz spatial light modulator is a promising approach especially when used in a single-pixel imaging modality. The imaging fidelity, performance and speed of this approach depend crucially on the imaging patterns also called masks and their properties used in the imaging process. Therefore, in this paper, it is investigated how the image quality after reconstruction is specifically influenced by the different mask types and their properties in a compressive imaging modality. The evaluation uses an liquid-crystal display based projector as spatial light modulator to derive specific guidelines for the use of binary and true greyscale masks in THz single-pixel imaging setups respective THz single-pixel cameras when used in far-field applications e.g. stand-off security imaging.
Highlights
Imaging at low terahertz (THz) frequencies provides unique information in particular for applications in security imaging because at these frequencies a good compromise between penetration properties on one hand and spatial resolution on the other hand is possible
The Hadamard mask measurements provide a large modulation for specific masks but in a compressive imaging modality, where a random part of the measurements is not considered for reconstruction, their application was found to be not optimal
The measurement errors can arise due to output power drifts of the Tx itself, the coherent nature of the radiation causing interference effects or even standing waves
Summary
Imaging at low terahertz (THz) frequencies provides unique information in particular for applications in security imaging because at these frequencies a good compromise between penetration properties on one hand and spatial resolution on the other hand is possible. The general design of a SPC enables image acquisition with increased resolution[4], improved signal-to-noise ratio (SNR)[5] and larger depth of focus[6] All these advantages can be achieved with a single-pixel detector even without mechanical scanning with the help of a SLM, which, in this case, acts as a dynamic aperture[7]. The general design of a SPC is similar for the VIS and the THz domain It consists of a SLM, a radiation source and a single-pixel detector. In the illuminated regions electrons are excited into the conduction band and the semiconductor (partially) changes from a semiconducting to a metallic state becoming less transmissive for THz radiation This mechanism implies that the performance of TM an optically controllable THz-SLM is influenced by the quality and quantity of the OS’ s illumination and the OS material (the type and quality of the semiconductor) itself[14,15].
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