Diffractive Optical Element Ghost Control in Telephoto Lens
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Solution Overview
Problem
Conventional optical systems with diffractive optical elements often suffer from ghost generation issues, which are not adequately addressed, leading to performance problems.
Innovation Solution
An optical system with a diffractive optical element disposed on specific lens surfaces, where the combined focal length and radius of curvature satisfy certain conditional expressions, ensuring controlled ghost generation, and including a first lens group with positive refractive power and a second lens group with negative refractive power, with the diffractive optical element positioned on the image-side surface of the first lens group.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If a diffractive optical element is arranged on a lens surface in a conventional optical system, then the optical system can achieve compact size and corrected chromatic aberration, but ghost images are generated due to uncontrolled diffraction and reflection
Solution Approach 1:
The patent applies parameter changes by optimizing the conditional expressions for focal length ratios (0.30 < f1/fa < 0.80 and 0.05 < fd/f < 0.15) and radius of curvature (0.50 < fa/Rd < 1.50). These parameter constraints control the diffraction angle and reflection paths, preventing ghost image formation while maintaining the compact optical system design with the diffractive optical element on the first lens group's image-side surface.
2Length of stationary object
If the diffractive optical element is positioned closer to the object side, then the optical system length is reduced, but diffraction efficiency decreases and aberration correction becomes difficult
Solution Approach 1:
The patent optimizes the position of the diffractive optical element on the image-side surface of the first lens group by constraining the focal length ratio 0.05 < fd/f < 0.15. This parameter range ensures that the element is positioned at an optimal distance from the object, maintaining high diffraction efficiency and effective aberration correction while achieving a compact overall optical system length.
3Reliability
If the radius of curvature of the lens surface is increased, then the optical system achieves better aberration correction, but the conditional expression for ghost control becomes harder to satisfy
Solution Approach 1:
The patent resolves this contradiction by establishing the conditional expression 0.50 < fa/Rd < 1.50, which optimally balances the radius of curvature Rd of the lens surface with the combined focal length fa. This parameter range ensures that the lens surface curvature is sufficient for effective aberration correction while maintaining the diffraction angle control needed to prevent ghost image formation.
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 effectively controls ghost generation, enhances diffraction efficiency, and maintains a short total lens length while correcting various aberrations, including chromatic aberration, in optical systems like telephoto lenses and digital single lens reflex cameras.
Implementation Method 1
a diffractive optical element which has a diffraction grating
Implementation Method 2
correcting various aberrations, including chromatic aberration
Implementation Method 3
a plurality of lenses arranged in order from an object
Implementation Method 4
the generation of a ghost is not considered, hence problems may be generated due to a ghost
Data Source
AI summary
A telephoto lens TL having, in order from an object, a plurality of lenses L1, L2, . . . and a diffractive optical element DOE which has a diffraction grating having a rotationally symmetric shape with respect to the optical axis, wherein the diffractive optical element DOE is disposed on any one of lens surfaces of the plurality of lenses L1, L2, . . . , and conditional expression 0.50<fa/Rd<0.90 or 1.10<fa/Rd<2.00 is satisfied, where fa denotes a combined focal length of each lens from the lens L1, which is closest to the object, of the plurality of lenses L1, L2, . . . to the lens L2, on which the diffractive optical element DOE is disposed, and Rd is a radius of curvature of the lens surface on which the diffractive optical element DOE is disposed.