Retro Focus Objective Optical System for Thin Video Scopes
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Solution Overview
Problem
In extremely thin video scopes, there is a challenge in securing a short overall length for the objective optical system while maintaining a long back focus and effectively correcting longitudinal chromatic aberration, which affects image quality during both white light and narrow-band light observations.
Innovation Solution
An objective optical system comprising a first negative lens and a second positive lens, with specific refractive power and Abbe number conditions, is designed to achieve a short focal length of about 0.3 mm or less and an overall length of 3 mm or less, incorporating a retro focus type to secure a long back focus and minimize lens count, and an image processor for lateral chromatic aberration correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If an objective optical system with a short focal length is adopted, then the overall length of the insertion portion can be reduced, but it becomes difficult to secure the predetermined space for disposing optical elements
Solution Approach 1:
The patent applies the retro focus type optical system which inverts the conventional arrangement by placing a negative lens before the positive lens, creating a long back focus distance despite the short focal length. This inverted configuration allows sufficient space to dispose optical elements like the optical filter while maintaining the compact overall length required for extremely thin video scopes
Solution Approach 2:
The patent utilizes the back focus distance as an additional dimensional space to accommodate optical elements. By extending the optical path in the axial direction through the retro focus design, the system creates space for disposing the optical filter and other components without increasing the radial dimensions, thus maintaining the thin profile
2Length of moving object
If the number of lenses is reduced to minimize the overall length, then the compactness is improved, but the correction of longitudinal chromatic aberration becomes more difficult
Solution Approach 1:
The patent carefully selects specific parameter ranges for the two lenses, including the focal length ratio (0.3 < fL2/fL1 < 1.5), Abbe numbers (30 < νd1 < 70, 20 < νd2 < 60), and refractive indices (1.5 < nd1 < 1.8, 1.4 < nd2 < 1.7). These parameter constraints enable effective correction of longitudinal chromatic aberration while maintaining the compact two-lens configuration
Solution Approach 2:
The patent employs lenses with different Abbe numbers and refractive indices to correct chromatic aberration. By combining a negative lens with specific dispersion characteristics (νd1) and a positive lens with different dispersion characteristics (νd2), the system achieves chromatic aberration correction using only two lenses, effectively using optical material properties as composite solutions
3Device complexity
If a retro focus type optical system is adopted to secure a long back focus, then the space for disposing optical elements is improved, but the overall length of the optical system increases
Solution Approach 1:
The patent optimizes the focal lengths of the individual lenses within specific ranges (0.15 mm < fL1 < 0.45 mm, 0.10 mm < fL2 < 0.35 mm) to achieve the desired back focus distance while controlling the overall length. By adjusting these parameters, the system secures sufficient space for optical elements without excessive length increase
Solution Approach 2:
The retro focus configuration inverts the conventional lens arrangement, placing the negative lens before the positive lens. This inversion creates a long back focus distance that provides space for disposing optical elements, while the compact focal lengths of the individual lenses ensure the overall system length remains acceptable for extremely thin video scopes
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
The solution enables a compact, high-performance objective optical system that maintains image quality across different light observations, improving contrast and reducing chromatic aberration, thus enhancing the imaging capabilities of extremely thin video scopes.
Implementation Method 1
An objective optical system consisting of, in order from an object side to an image side, a first lens having a negative refractive power, an aperture stop, and a second lens having a positive refractive power
Data Source
AI summary
An objective optical system includes in order from an object side to an image side, a first lens having a negative refractive power, an aperture stop, and a second lens having a positive refractive power. An image-side surface of the first lens is a surface which is concave toward the image side, and an image-side surface of the second lens is a surface which is convex toward the image side, and following conditional expressions (1), (2), (3), (4), (5), (6), and (7) are satisfied:0.6<LL1is/LsL2i<1.25 (1)−3<(LL1is/RL1i)×(LsL2i/RL2i)<−1.25 (2)−2<fL22/(f×fL1)<−1.35 (3)2<(nd1−1.63)×(νd1−31) (4)5<(n2−1.45)×(νd2−31) (5)1.63<nd1, and, 31<νd1 (6)1.45<nd2, and, 31<νd2 (7).