Wide Angle Variable Neutral Density Filter With Counter-Rotating Polarizers
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Solution Overview
Problem
Variable neutral density (VND) filters in cameras introduce spatial/angular artifacts due to transmission non-uniformity and color shifts, particularly at high densities and wide angles, limiting their effectiveness in photography and video capture.
Innovation Solution
A rotationally reconfigurable opto-mechanical structure with multiple linear polarizers and an angle-of-incidence sensitive near-infrared reflecting filter, where the polarizers are arranged to minimize transmission and color non-uniformity across a wide range of densities and angles, using a mechanical structure with actuators for precise orientation and a transparent optical grease to match refractive indices, reducing the need for substrates and coatings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a variable neutral density filter uses polarization to vary transmission, then the transmission can be controlled independently of scene luminance, but spatial/angular artifacts are introduced that limit usefulness in practice
Solution Approach 1:
The filter system is divided into multiple functional layers: a first polarizer, a variable neutral density element (liquid crystal or mechanical), and a second polarizer. This segmentation allows each component to be optimized independently, with the first and second polarizers working together to control polarization while the variable element provides neutral density adjustment, thereby reducing artifacts while maintaining exposure control independence.
Solution Approach 2:
A quarter-wave plate is introduced as an intermediary element between the first polarizer and the second polarizer. This quarter-wave plate converts linear polarization to circular polarization, which helps eliminate the angular artifacts that occur with simple polarizer pairs while maintaining the ability to control transmission independently of scene luminance.
2Adaptability or versatility
If the VND filter is placed in front of the camera lens for wide-angle shots, then exposure control is improved, but contrast or color non-uniformity precludes wide-angle shots and/or high density
Solution Approach 1:
The filter system applies different optical properties to different parts of the light path: the first polarizer handles polarization control for the main beam, the quarter-wave plate provides localized phase correction to eliminate angular artifacts, and the second polarizer completes the polarization filtering. This local quality assignment ensures uniform contrast and color across the entire field of view while maintaining wide exposure control range.
Solution Approach 2:
The system dynamically changes the transmission parameter of the variable neutral density element to achieve different exposure levels while maintaining constant polarization characteristics. By keeping the polarization state unchanged and only varying the neutral density, the system avoids introducing angular artifacts that would compromise contrast uniformity in wide-angle shots.
3Reliability
If poly-vinyl-alcohol polarizer films are clad with cellulose-triacetate protective films, then protection is provided, but negative c-plate retardation is introduced that increases contrast of cross-pattern artifacts
Solution Approach 1:
The harmful cellulose-triacetate protective films are removed from the polarizer structure. Instead, a quarter-wave plate is introduced that provides the necessary protection and optical performance without introducing negative c-plate retardation. This extraction of the problematic element eliminates the cross-pattern artifact enhancement while maintaining film protection functionality.
Solution Approach 2:
The quarter-wave plate serves as an intermediary that replaces the problematic protective films. It provides mechanical protection and optical functionality without introducing the harmful retardation effects of cellulose-triacetate layers, thereby eliminating the cross-pattern contrast artifacts while maintaining reliability.
4Strength
If multiple air spaced elements and functional substrates are used in the VND filter, then structural integrity is maintained, but veiling glare is increased that reduces image contrast
Solution Approach 1:
Multiple functional elements are merged into a single integrated structure where the first polarizer, quarter-wave plate, variable neutral density element, and second polarizer are combined in a compact arrangement. This merging reduces the number of air spaces between elements, thereby minimizing veiling glare while maintaining structural integrity through the unified design.
Solution Approach 2:
The system uses a liquid crystal variable neutral density element that can be filled with index-matching fluid or operates in a vacuum-sealed environment to eliminate air spaces. This eliminates refraction and reflection at air-glass interfaces, thereby eliminating veiling glare while maintaining structural integrity through the fluid-filled or vacuum-sealed encapsulation.
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 reduces transmission non-uniformity and color shifts, maintaining high contrast and color neutrality over a wide acceptance angle and attenuation range, improving image quality by minimizing artifacts such as veiling glare and ghost reflections.
Implementation Method 1
VND filters use polarization as a means of varying transmission. When polarizer absorption axes are parallel, the transmission is maximum. As one polarizer is rotated with respect to the other the transmission drops
Implementation Method 2
An angle-of-incidence sensitive near infrared reflecting filter substantially reflects light at normal-incidence from the long-wavelength edge of the don't-care band to approximately 1,000 nm
Implementation Method 3
a retardation plate with multiple waves of in-plane retardation
Data Source
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AI summary
A three (or more) polarizer arrangement is used to demonstrate a wide-angle variable-neutral-density (VND) filter that has both contrast uniformity and color uniformity. According to one embodiment, the outer polarizers effectively counter-rotate with respect to a fixed center polarizer as a means of compensating for transmission non-uniformity associated with geometrical polarization distortions experienced by off-normal rays. In particular, the achromatic compensation arrangement enables angle uniformity relative to normal-incidence transmission when the number of stops of attenuation grows large (e.g. 10-stops, or OD3). The filters are useful for cameras or instrumentation allowing mechanical or electromechanical tuning.