###### Abstract  
The focal length in a typical optical system changes with the angle of view, according to the size of the sensor. This study proposed an optical terahertz (THz) system application where the focal length changed while the angle of view was fixed; thus, the image height was variable and responded to various sensor sizes. Therefore, it is possible to respond to various sensors with one optical system when the inspection distance is fixed. The fundamental optical system was designed by arranging the refractive power, which was determined according to the sensor size using the Gaussian bracketing method. A zoom optical system that changed the image height by fixing the angle of view and changed the focal length by moving the internal lens group was designed. THz waves exhibit minimal change in the refractive index depending on the wavelength. Moreover, their long-wavelength characteristics facilitate the development of millimeter-level pixel sizes. Therefore, the root mean square size of the maximum spot was 0.329 mm, which corrected the aberration to less than 1 mm (smaller than the pixel size). Further, a lighting analysis at 3 and 6 m locations confirmed the expansion of the lighting area by the magnification of the sensor size. After turning off certain light sources, we checked the contrast ratio via lighting analysis and confirmed that the size of one pixel was clearly distinguishable. Consequently, this newly designed optical system performed appropriately as an optical inspection system for THz system applications. z  
202414909  
parts made of such materials exhibit a lens shape similar to that of general visual light [12]. The shape and thickness of a single lens and the arrangement of multiple lenses must be determined to focus the THz wave at the desired location with the desired energy distribution [13]. These processes are assumed to be the same as those used in the design process of a general optical system. In typical optical systems, an object is never fixed at a certain position; thus, a focus control function is essential [14; 15; 16]. This optical system for THz applications is characterized by almost no wavelength-dependent change in the refractive index, and the wavelength is very long; therefore, the pixel size of the imaging device is also very large. Consequently, the size of the imaging device becomes very large with the increase in the number of pixels compared to that of a general visible-light imaging device. Therefore, only imaging devices with fewer pixels than the quarter video graphics array (QVGA) class should be developed. For a pixel size of 1 mm, even if it was in the QVGA class, the size of the imaging device is a considerably larger area than that of a general visible-light imaging device. Currently, there is no standardized sensor size for THz applications, and several companies are developing various sensor sizes. Therefore, optical system design tailored to a certain fixed sensor size may be difficult to apply to another sensor size. In an inspection optical system, the inspection distance is usually fixed. If the sensor size changes, the field of view must be fixed. However, our proposed optical system with a fixed field of view can respond to various sensor sizes. In addition, no problems are encountered if the optical aberration is corrected only at the millimeter level. Thus, the optical system must be designed using optical system design software (CODE V 2023.03), and its performance must be evaluated using design software [17; 18]. Accordingly, the proposed optical system must be designed to respond to imaging devices of varying sizes. However, in these optical systems, the inspection distance and range are generally fixed. Therefore, an optical system with a fixed angle of view and variable imaging element size must be developed. In the last 15 years, studies related to lens fabrication using metamaterials or optical system designs for THz applications have been scarce.