## 4 Conclusions  
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.  
1. The size of the focus was confirmed to be 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, demonstrating that sufficient results were obtained within 1 mm. The performance was confirmed up to the diffraction limit even when diffraction is considered within the diffraction MTF.  
2. 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. For an infinite object point, there was 4% distortion at the telephoto end. When the object distance was 3 m, the maximum distortion was approximately 1% at the wide-angle end. When the object distance was 6 m, the maximum distortion was 2.3% at the telephoto end. The maximum value of the total distortion was 4% for an infinite number of object points, satisfying the target performance within 5%. 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.  
3. 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.  
Conceptualization, J.P., J.R. and H.C.; methodology, J.P. and J.R.; writing--original draft preparation, J.P., J.R. and H.C. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Gachon University research fund of 2024 (GCU-202404100001). This work was supported by the Korea Industrial Complex Corporation (KICOX) grant funded by the Korea government (PKB22002).  
Not applicable for studies not involving humans or animals.  
Not applicable.  
The data presented in this study are included in the article.  
The authors declare no conflicts of interest. The funders had no role in the study design; collection, analyses, or interpretation of data; writing of the manuscript; or decision to publish the results.