Objective Funduscopic examination is a routine examination that can effectively screen for funduscopic lesions caused by systemic diseases such as diabetes mellitus and hypertension. However, current ultra-wide-angle high-resolution retinal imaging devices for funduscopic examination are usually difficult to combine the functions of long working distance and pupil-free dilation, which causes discomfort to patients and increases the difficulty of examination. In response to the need for wide-area retinal imaging in ophthalmological examinations, this study designed an ultra-wide-angle long exit distance pupil-free fundus imaging optical system based on confocal laser scanning fundus imaging technology. The system adopts a transmissive structure, taking into account the requirements of long pupil exit distance and no astigmatism, to achieve 110° ultra-wide angle imaging. The system has a 12 mm exit pupil distance, with a beam size of less than 2 mm at the pupil, and a built-in visual acuity compensation lens set, which can effectively compensate for the human eye with a refractive error of −15-+15 D. The design results show that the system's fundus resolution is 7.5 μm, with excellent imaging quality, and the imaging range and working distance meet the practical use requirements. This study provides a useful reference for the design of the ultra-wide angle confocal fundus imaging optical system, which is of significance for improving the quality of fundus imaging and the accuracy of clinical diagnosis.
Methods A scheme of a transmission confocal laser scanning fundus imaging system is proposed in this paper. Firstly, the technical specifications of the system are determined according to the actual demand (Tab.1), and then the wide-angle human eye model with adjustable diopter (Tab.2-Tab.3) is established. The objective is a common part of the imaging system and illumination system, and the initial structure consists of a wide-angle eyepiece and a long exit pupil eyepiece (Fig.2). The design problem of the objective is transformed into an optimization problem, and the image quality evaluation function for the multi-eye model with constraints is constructed, and the structure of the objective that meets the design requirements is obtained by optimization in ZEMAX.
Results and Discussions The optimized objective has nine lenses, including two aspherical lenses, and incorporates a dioptric compensation lens to correct for aberrations in the human eye at different diopters. The system is constructed by combining the objective, scanning galvanometer, and imaging lenses (Fig.5). Notably, the system achieves an angular magnification of 1. On one side of the object, the Numerical Aperture (NA) is 0.04. The full-field modulation transfer function (MTF) exceeds 0.2 at 67 lp/mm (Fig.7). Furthermore, the optical resolution is 7.5 µm, meeting the design requirements. Importantly, the system provides clear imaging of the human eye across a range of refractive errors from −15 D to +15 D. Even when compensating solely for refractive errors within the −10 D to +10 D range, the full-field RMS radius remains below 7.5 µm (Fig.8), indicating good imaging quality. Finally, tolerance analysis confirms that the system is feasible for manufacturing.
Conclusions A transmission confocal laser scanning fundus imaging system is proposed in this paper, which takes into account the ultra-wide angle and long working distance, with a field of view of 110° and a pupil exit distance of 12 mm. The system includes an objective lens, a scanning galvanometer, an imaging mirror, a collimating mirror, and a visual acuity compensating group in the objective lens, which can adapt to the refractive eyes of −15 D to +15 D. The optical resolution of this system is 7.5 μm, and the system exhibits relaxed tolerance, ensuring ease of manufacturability. This system serves as a valuable reference for the design and development of fundus imaging equipment.
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