Neutral Density Filter Anti-Reflection via Periodic Structures
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional optical filters with high spectral reflectance in the visible light wavelength range cause ghost and flare issues in image pickup systems, as reflected light can be re-directed towards the image sensor, and existing anti-reflection coatings are inadequate in reducing reflectance across all wavelengths and on substrate boundaries.
Innovation Solution
An optical filter with a non-reflective periodic layer, such as a fine uneven periodic structure, is applied to the substrate surface opposite to the thin film, with anti-reflection structure bodies arranged at pitches shorter than the light wavelength to minimize reflection, and this configuration is used in an image pickup apparatus to reduce ghost and flare.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an ND filter with high spectral reflectance is used to control light quantity, then the aperture can be maintained at a reasonable size, but ghost and flare occur due to light reflection
Solution Approach 1:
The invention converts the harmful reflective property of the ND filter into a beneficial anti-reflection effect by forming a fine uneven periodic structure on the filter surface. This structure has a period shorter than the wavelength of light, causing incident light to be diffracted into higher-order beams rather than being reflected, thereby eliminating ghost and flare while maintaining the ND filter's light quantity control function
Solution Approach 2:
The invention changes the surface morphology parameter of the ND filter by forming a fine uneven periodic structure with a specific period (shorter than light wavelength) on the filter surface. This parameter change transforms the optical interaction from reflection to diffraction, resolving the contradiction between maintaining aperture stability and preventing ghost/flare
2Object-affected harmful factors
If a single-layer anti-reflection film is formed on the ND filter, then reflectance decreases at a specific wavelength, but reflectance remains high at other wavelengths
Solution Approach 1:
The invention changes the surface morphology parameter by forming a fine uneven periodic structure with a period shorter than the wavelength of light. This structural parameter change creates a diffraction-based anti-reflection effect that operates across a broad wavelength range, providing universal anti-reflection performance without the wavelength-specific limitations of single-layer films
Solution Approach 2:
The invention replaces the conventional thin-film interference-based anti-reflection mechanism with a diffraction-based mechanism. By substituting the optical principle from interference to diffraction through the periodic structure, the system achieves broadband anti-reflection performance that is not limited to specific wavelengths
3Quantity of substance
If ND films are formed on both sides of the substrate, then light attenuation is improved, but reflected light cannot be prevented from returning to the image sensor
Solution Approach 1:
The invention converts the harmful reflection from the ND filter's rear surface into a beneficial diffraction effect. By forming a fine uneven periodic structure on the rear surface, incident reflected light is diffracted into higher-order beams that do not return to the image sensor, thereby eliminating the harmful effect while maintaining the dual-sided ND film configuration for optimal light attenuation
4Ease of manufacture
If conventional anti-reflection methods are used, then manufacturing process is simple, but ghost and flare cannot be effectively suppressed
Solution Approach 1:
The invention changes the surface morphology parameter by forming a fine uneven periodic structure on the ND filter surface. This can be achieved through conventional manufacturing techniques such as adding a specific term to the substrate shaping step or incorporating it into the color filter formation process, maintaining ease of manufacture while effectively suppressing ghost and flare through diffraction-based anti-reflection
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 significantly decreases spectral reflectance across the visible light wavelength range, effectively reducing ghost and flare occurrences by suppressing light reflection on both the filter surface and substrate boundaries, improving image quality.
Implementation Method 1
anti-reflection structure bodies that prevent light reflection are arranged, on at least part of a surface which is opposite to a surface where the thin film is formed, the anti-reflection structure bodies being arranged at pitches with a period shorter than a wavelength of light
Data Source
AI summary
A spectral reflectance in the entire visible wavelength range may be decreased to decrease inconveniences in an image such as flare or ghost. A non-reflective periodic layer including a large number of fine uneven periodic structure bodies is formed on a substrate, and a neutral density (“ND”) film is formed on the other surface of the substrate by vacuum deposition, to improve an anti-reflection function. Regarding a beam entering the ND filter, the MgF2 film of the formed ND film suppresses the spectral reflectance at the boundary of the atmosphere and the ND film and at the boundary of the ND film and the substrate. The non-reflective periodic layer decreases the reflectance at the boundary of the substrate and the atmosphere.