## 3 Results
Therefore, the target distortion value was determined to be within 5% using the optimized design. As shown in Figures 9-11, the distortion aberration at each zoom according to the object distance using a distortion grid was verified. In a distortion grid chart, the \(x\)- and \(y\)-axes represent the long- and short-change angles, respectively. The red grid line, which represents the actual FOV, is the actual image created using the optical system, and the black grid line (Paraxial FOV) is the standard image. The larger the difference between the two grid lines, the greater the distortion. The shape wherein the center of the image  
Figure 7: Optical path diagram for height-variable zoom lens. appears to protrude outward is referred to as the barrel type, and that wherein the center of the image appears to be pressed inward and the edges appear to bulge outward is referred to as the pincushion type. Figure 9 shows a distortion grid chart for each zoom when the object distance obtained using the CODE V built-in macro is infinite. Figure (a)a shows the barrel-type distortion at the wide-angle end. Figure (b)b,c show the middle and telephoto ends, respectively, which exhibit a pincushion-type distortion. Figure (c)c shows that the distortion was the largest. As the distortion was 4% at the maximum image height, the value was confirmed within 5%, which was the target performance. Figure 8: Zoom locus of height-variable zoom lens. Figure 10: Distortion grid at an object distance of 3 m at (**a**) wide, (**b**) middle, and (**c**) tele ends. Figure 9: Distortion grid at object distance infinity at (**a**) wide, (**b**) middle, and (**c**) tele ends. Even in Figure 10, where the object distance was 3 m, a barrel-type distortion was observed at the wide-angle end (Figure 10a), and the mid-end and telephoto end (Figure 10b,c) had a pincushion-type distortion. Figure 10 shows that the overall distortion was small. Even at the largest wide-angle end (Figure 10a), the distortion was within 1%. In Figure 11, the wide-angle end picture (Figure 11a) exhibited a barrel-type distortion, whereas the mid-end and telephoto end (Figure 11b,c) had a pincushion-type distortion. The distortion was observed to be the largest at the telephoto end (Figure 11c). Further, the distortion was 2.3%, which was less than the target distortion value. After checking the distortion in all zooms, the distortion at the telephoto end had the largest value of 4% at the infinite object point in Figure 9. However, the target performance was satisfied because it was within the target distortion of 5%. Because the pixel size of the imaging device was greater than 1 mm, optical aberrations were corrected at the millimeter level through an optimization process. The resolution was confirmed using a spot diagram. Spot diagrams are a geometric optical jump-spread function. The higher the density, the better is the imaging performance. Because the pixel size of the imaging device in the THz range was 1 mm or more, the device was designed and checked whether the focus was within a boundary of 1 mm \(\times\) 1 mm. Parts a, b, and c in Figures 12-14 show the spot diagrams for the wide-angle, middle, and telephoto ends, respectively. The \(x\)-axis (the defocus range) was confirmed to be within the range of \(\pm\)0.05 mm, whereas the \(y\)-axis represents the fields of 0, 0.3, 0.5, 0.8, and 1 when the height was standardized to 1. The diffraction MTF (modulation transfer function) was also checked. The \(x\)- and \(y\)-axes represent spatial frequency (cycles/mm), and the MTF is normalized to one. The block dotted line in F1 represents the diffraction limit. Figure 12 shows a spot diagram for each zoom at an infinite object point. The minimum value of the wide-angle end (Table 2a) based on defocusing 0 was 0.055 mm in 1 field, and the maximum value was 0.201 mm in 0.8 field. The minimum value of the middle stage (Table 2b) was 0.081 mm in 1 field, and the maximum value was 0.2 mm in 1 field.