Abstract
High definition and magnification rigid endoscope plays an important role in modern minimally invasive medical surgery and diagnosis. In this paper, we present the design and evaluation methods of a high definition rigid endoscope, specifically an arthroscope, with a large depth of field (DOF). The incident heights and exit angles of the sampled rays on the relay lens are controlled during the optimization process to ensure an effective field view (70°) and a normal ray path within the limited lens diameter of 2.7 mm. The lens is set up as a multi-configuration system with two extreme and one middle object distances to cover a large DOF. As a result, an entrance pupil of 0.3 mm is achieved for the first time, to bring the theoretical resolution to 23.1 lps/mm in the object space at a working distance of 20 mm, with the wavelength of 0.532 um. The modulation transfer function (MTF) curves approach diffraction limit, and the values are all higher than 0.3 at 160 line pairs/mm (lps/mm) in the image space. Meanwhile, stray light caused by total internal reflection on the inner wall of the rod lenses and the objective lens is eliminated. The measured resolution in the object space at a 20 mm working distance is 22.3 lps/mm, and test results show that other performance characteristics also fulfill design requirements. The relay lenses are designed with only one type of the spacer and two types of lenses to greatly reduce the fabrication and assembly cost. The design method has important research and application values for lens systems used in modern minimally invasive medical surgery and industrial non-destructive testing area.
Highlights
A medical endoscope is an elongated optical device used to observe otherwise inaccessible areas within the human body through a small orifice in either a non-invasive or minimally invasive manner [1]
The loss ray may be absorbed by the spacer or may be reflected back to the successive lens by total internal reflection (TIR) if the ray hits the inner wall of the lens, especially the rod lens
All images from the endoscope vary in intensity from the center to the edge, the image center is always the brightest portion of the image for most of the designs, but in this design, the peak value happens at the field of 26°, this helps to balance the relative illumination (RI) across the field, the RI value of the marginal field is over 94%
Summary
A medical endoscope is an elongated optical device used to observe otherwise inaccessible areas within the human body through a small orifice in either a non-invasive or minimally invasive manner [1]. Only in 1986 did Mouret developed a video computer chip that allowed the magnification and projection of images onto television screens This technique of endoscopic surgery became integrated into the discipline of general surgery [5]. Endoscopes have been connected with a video camera and displayed on large and high-definition (HD) screens rather than being directly viewed through the eyepiece. Using this new technology enables better observation and recording of images for further analysis and remote diagnosis. Details on the design methods of a cost-effective and high-performance rigid endoscope are seldom discussed.
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