Objective Optical System for Endoscope with Long Back Focus
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Solution Overview
Problem
Conventional objective optical systems for endoscopes face challenges in maintaining a long back focus while correcting aberrations and supporting high-resolution imaging, particularly with increased pixel density leading to a narrow depth of field and asymmetric refractive power configurations that complicate lens design.
Innovation Solution
The objective optical system comprises a first group with negative refractive power, a movable second group with positive refractive power, and a third group with a specific configuration of positive refractive powers, including cemented lenses, to achieve a long back focus and correct aberrations such as chromatic aberration and astigmatism, with conditional expressions defining optimal ratios of focal lengths and refractive powers to balance performance and lens size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a polarizing prism is disposed on the optical path to expand depth of field, then depth of field is improved, but back focus becomes excessively long
Solution Approach 1:
The objective optical system is divided into multiple lens groups (first group with negative refractive power, second group with positive refractive power, third group with positive refractive power) that can move relative to each other. This segmentation allows the system to achieve both long back focus and expanded depth of field without requiring a polarizing prism, by coordinating the movement of different lens groups to adjust focal properties dynamically.
Solution Approach 2:
The patent implements a focusing mechanism where the second lens group (with positive refractive power) moves along the optical axis to change the in-focus position. This dynamic adjustment capability allows the system to maintain a favorable back focus while achieving expanded depth of field through focusing control, eliminating the need for a polarizing prism that would otherwise be required to expand depth of field.
2Measurement precision
If pixels of image sensor are increased to improve resolution, then measurement precision is improved, but depth of field becomes narrow
Solution Approach 1:
The movable second lens group provides dynamic focusing capability that works synergistically with high-resolution image sensors. By enabling precise control over the in-focus position through lens group movement, the system can maintain adequate depth of field even when using high pixel density sensors that would otherwise produce extremely narrow depth of field.
3Device complexity
If asymmetric refractive power configuration is used to simplify lens design, then device complexity is reduced, but aberration correction becomes difficult
Solution Approach 1:
The patent employs an asymmetric configuration where the first lens group has negative refractive power while the second and third groups have positive refractive power. This asymmetric arrangement is specifically designed to correct various aberrations including chromatic aberration and astigmatism. The asymmetric power distribution allows different lens groups to address different types of optical aberrations, achieving superior correction while maintaining manageable design complexity.
Solution Approach 2:
Each lens group is assigned a specific refractive power characteristic (negative for first group, positive for second and third groups) to address particular optical aberrations locally. This local quality assignment allows each group to optimize its function for correcting specific types of aberrations, achieving comprehensive aberration correction through coordinated action of groups with different local optical properties.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively secures a long back focus, allows for the disposition of a polarizing prism, and enhances the correction of aberrations, thereby supporting high-resolution imaging and maintaining optical performance across various observation states.
Implementation Method 1
an objective optical system includes, in order from an object side, a first group having a negative refractive power, a second group having a positive refractive power, and a third group having a positive refractive power
Data Source
AI summary
An objective optical system includes, in order from an object side: a first group having a negative refractive power; a second group having a positive refractive power; and a third group having a positive refractive power, in which the first group and the third group are fixed and the second group is movable, the first group includes at least two lenses having a negative refractive power, the third group includes, in order from the object side, a 3-1st group having a positive refractive power, a 3-2nd group having a negative refractive power, a 3-3rd group having a positive refractive power, and a 3-4th group having a positive refractive power, and the following conditional expression (1)′″ is satisfied:1.5≤Bk/f3≤6 (1)′″.