ND Filter Substrate Thermal Stability
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Solution Overview
Problem
High-density ND filters manufactured using conventional evaporation methods often suffer from deformation issues like wrinkles due to thermal expansion and contraction, especially when the substrate temperature exceeds the glass transition temperature of the norbornene-based plastic materials used, making it difficult to achieve smooth optical surfaces.
Innovation Solution
Employing a plastic substrate with a glass transition temperature of 200°C or more, such as a heat-resistant polyimide-based material, and using techniques like vacuum evaporation with a pattern-producing mask to form multi-layered thin films that reduce light transmission, ensuring the substrate remains stable during the evaporation process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the evaporation process is extended to achieve high-density ND filter, then the light transmittance is reduced, but the substrate temperature rises above the glass transition temperature causing wrinkles
Solution Approach 1:
The patent changes the material parameter (glass transition temperature) of the substrate from conventional norbornene-based plastic (120-170°C) to polyimide-based plastic (200-400°C). This parameter change allows the substrate to withstand the high temperatures required for extended evaporation processes without deforming, thereby enabling high-density ND filter production while maintaining manufacturing precision.
2Quantity of substance
If the thickness of multi-layered film is increased to reduce light transmittance, then the evaporation time is extended, but the substrate temperature exceeds glass transition temperature
Solution Approach 1:
By changing the thermal parameter (glass transition temperature) of the substrate material to polyimide-based plastic with Tg≥200°C, the patent enables the substrate to maintain dimensional stability at higher temperatures during extended evaporation. This allows increased film thickness for higher density while keeping substrate temperature below the new, higher glass transition threshold.
3Quantity of substance
If the number of layers in multi-layered film is increased to achieve high density, then the evaporation process duration is extended, but wrinkles occur due to thermal expansion
Solution Approach 1:
The patent modifies the thermal stability parameter of the substrate by using polyimide-based plastic with glass transition temperature of 200°C or higher. This material remains in a rigid glassy state during the extended evaporation process, preventing thermal expansion and contraction that would otherwise cause wrinkles, thereby maintaining substrate composition stability throughout the prolonged manufacturing cycle.
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 use of a polyimide-based substrate with a high glass transition temperature effectively prevents wrinkles, resulting in a smooth and high-quality ND filter with consistent light transmittance and reflectance characteristics, suitable for high-density applications without degrading image quality.
Implementation Method 1
a multi-layered thin film, obtained by alternately stacking thin film made of a light-absorbing material and thin film for the reduction of reflectance, is formed on the surface of the plastic substrate by the evaporation method
Implementation Method 2
evaporation process of depositing evaporation particles, melted and evaporated from an evaporation source, on the surface of the plastic substrate
Implementation Method 3
Thermal expansion coefficient of the plastic substrate and that of the thin film stacked on the plastic substrate are not always equal. The multi-layered thin film may thus interfere with thermal expansion and contraction of the plastic substrate
Implementation Method 4
a glass transition temperature of the plastic material is not less than 200° C.
Data Source
AI summary
This invention decreases deformation of a plastic substrate caused by thermal expansion and contraction in an evaporation process of forming a multi-layered film of an ND filter, thus helping prevent wrinkles. The plastic substrate serving as the base of the ND filter is made of a polyimide-based plastic material having a glass transition temperature of 200° C. or more, has a total transmittance of light of 90% or more, and has a haze factor of 0.5% or less. As the multi-layered film, Al2O3 films for reduction of the reflectance and TiOx films for reduction of the transmittance are alternately formed on the substrate.