###### Abstract
An aspherical lens was fabricated using polyethylene to obtain a large numerical aperture for THz imaging applications [19]. Using photolithography, an ultrathin lens was fabricated to obtain a large axial focal depth for THz applications [20]. Further, an ultrathin meta-surface multi-focus lens device comprising C-shaped antenna units was designed to have 2 or 3 mm focal spacing, thereby covering a range of 0.3-1.1 THz [21]. Using a three-dimensional printer, a lens group was fabricated based on a ridge prism to produce monochromatic light and a diffraction-free length within the THz range [22]. Using polymer materials, an f-\(\theta\) lens covering a wide field of view was fabricated to obtain heterogeneous samples and provide a constant phase over a 34\({}^{\circ}\) field of view (FOV) for 0.3-1.0 THz [23]. Using silicon material, an achromatic meta-lens composed of silicon pillars was fabricated for 0.6-1.0 THz [24]. Using a two-dimensional material, a tunable optical magnetic lens was fabricated to focus on the photon beam range and tune the phase shift [25]. A diffractive lens with concentric rings was fabricated using a three-dimensional printer to generate specific diffraction patterns for THz applications [26]. This study designed a novel optical zoom system with a fixed angle of view to support various imaging device sizes for THz applications. To accommodate imaging devices of various sizes, a zoom optical system was constructed wherein the image height was variable, the angle of view was fixed, and the focal length was changed. Based on the optimized design, the lens shape was improved, and aberrations were corrected. The focal length was varied using an internal focus movement method in which the overall length was fixed, and the internal lens group was moved to reduce problems such as driving weight or noise. Because the position of the sensor must be fixed, the focus is adjusted by calculating the amount of movement of the lens group using the zoom equation. Because of the nature of THz waves, their transmittance is very high; therefore, there is a strong possibility that sensor pixels will develop to be larger than 1 mm. Thus, the performance of the optical system was confirmed using a spot diagram. Infinite object points at both 3 and 6 m were confirmed according to the object distance in the spot diagram. In addition, the size of the focus was confirmed within 1 mm for all zooms at the wide-angle, middle, and telephoto ends. The largest focus was 0.329 mm at the telephoto end when the object distance was 3 m. Distortion was also checked at infinite object points of 3 and 6 m, depending on the object's distance. The distortion value and grid chart were checked for all zooms at the wide angle, middle, and telephoto ends. Because aberrations can be confirmed at the millimeter level, the performance was analyzed using light tools and a lighting optical system software package. The receivers were installed at object distances of 3 and 6 m, and lighting analysis was performed with a light source array at the sensor location. Because the size of the sensor was very large, a light source array of approximately 1/10 the size of the sensor was placed in the center and peripheral portions of the entire sensor size, and the illumination was checked at the central and peripheral receivers. The target lighting area enlarged by the magnification of the sensor was satisfactory for all zooms. The lighting analysis conducted after turning off certain lights indicated that the size of each pixel was clearly distinguished, and the contrast ratio was high in all zooms. Thus, the performance of the inspection optical system was satisfactory. The new optical system proposed in this study can be used regardless of the sensor size, thereby broadening the inspection range. Based on our research and previous studies, this is the first study to design a zoom optical system with a fixed angle of view that can support various imaging device sizes for THz applications. The primary contributions of this new design are as follows. 1.