Objective Given the increased prevalence of digestive diseases in recent years, the endoscope has been widely used for abdominal diagnoses, including those related to the stomach and intestines. Researchers are working to develop more effective and less invasive techniques for patients to benefit from endoscopy. A large field-of-view (FOV) and high resolution will reduce examination time and improve evaluation accuracy. Moreover, a compact endoscope structure is critical for minimising patient discomfort. In conventional wide-field camera lenses, a large panoramic scene needs to be focused onto an image sensor plane, to reduce the field curvature caused by the strong mismatch between the focal planes. The concentric lens consists of four refractive surfaces, and the centres of curvature of each refractive surface coincide at one point. Therefore, off-axis aberration does not exist. Only spherical and axial chromatic aberrations need to be corrected. Therefore, this structure can be applied to optical systems with miniaturisation, high image quality, and a large FOV; however, the image surface formed by the concentric system is curved. In this study, we correct the curvature of the field in the concentric sphere system by designing an annularly stitched aspheric surface to achieve flat-field imaging with a large FOV.
Methods In this study, an optical system with full FOV is regarded as a combination of multiple single- or small-FOV sub-system units, then the sub-field units are solved separately, and the formation of a complete complex surface is optimised to realize the construction of a complete optical system. First, the initial concentric structure is solved with well-corrected spherical and chromatic aberration. Then, based on the FOV, an annularly stitched surface is constructed by dividing the surface into rings and calculating the initial structure parameters of each zone based on the flat-field conditions. The Q-type aspheric surface characterises different annuli to ensure imaging quality while obtaining good splicing results. Simultaneously, the continuity constraint condition of the annularly stitched aspheric surface is derived. Finally, a complete surface is optimized to realize the construction of a complete electronic endoscope.
Results and Discussions The deviation of normal and sag between adjacent rings has been reduced to less than one-tenth of the test wavelength (typically test wavelength 632.8 nm) through optimisation. These rings are then fused after the optimisation. The system diagram is shown in Fig.10. Compared with the modulation transfer function (MTF) curve of the initial structure in Fig. 3, the MTF of the system after optimisation is more than 0.3 at the spatial frequency of 72 lp/mm (Fig.11). Thus, the curvature of the full FOV is reduced from 0.5 mm in the initial structure to within 0.1 mm [Fig.12(a)], the imaging requirements of electronic endoscope objectives are met. To validate the design results’ manufacturability, a Monte Carlo simulation analysis was performed 200 times within the tolerance range (Fig.14). Consequently, in the full FOV, considering mass production and assembly, a probability that an optical system with an average diffraction MTF greater than 0.3 at 72 lp/mm frequency can be obtained is more than 90%.
Conclusions Based on the concentric structure, multiple rings are superimposed on the last surface to obtain different optical powers to generate the initial surface shape of the splicing surface of the rings. The surface shapes of the multiple rings are fused to generate a complete continuous surface after the continuous conditions are optimized. In the design, the Q-type aspheric surface is used to characterise different ring zones to ensure imaging quality. An electronic endoscope objective lens operating in the visible band is designed using this method. The objective comprises only four refractive surfaces, with a total system length of 2.81 mm and FOV of 90°. The field curvature of the system is less than 0.1 mm, the distortion is within 20%, the MTF reaches 0.3 at 72 lp/mm, and the relative illuminance of the full FOV is greater than 0.5, which meets the imaging requirements of electronic endoscope objectives. The system uses the imaging advantages of the concentric objective lens with a large FOV and small volume. The annularly stitched aspheric surface is used to correct the curvature of the field caused by the spherical lens. Compared with the traditional structure, our electronic endoscope objective lens is more compact and readily manufacturable.
