Design of optical system for disposable epidural endoscope

In order to obtain wide-field and clear image of the epidural space, the disposable epidural space endoscopic optical system was designed using Q-type aspherical surface. The operating band was visible light, the field of view was 90°, the F-number was 4, and the receiver was a 1/18 inch CMOS sensor. The system was composed of three plastic lenses and one cover glass, and the third lens adopted Q-type aspherical surface. Therefore, the structure was simple, compact and low cost. The image quality evaluation results show that the modulation transfer function of each field of view is greater than 0.27 at the Nyquist frequency of 200 lp/mm, which is close to the diffraction limit. The root-mean-square radius of each field is less than the pixel size of 2.5 μm, and all are within the Airy spot range. The maximum residual distortion is less than 20%, which meets the requirement of high image quality. Finally, the ghost image of the system was analyzed and evaded. It is shown that the energy of the spot formed by the pupil ghost image is diffused, which has little influence on the uniformity of image illuminance. The spot size of the focal ghost image is much larger than the spot diagram of the main image, which has little impact on the imaging quality and meets the imaging requirements.