Optical Low Pass Filter for Digital Camera Pixel Pitch
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Solution Overview
Problem
Existing digital camera systems face challenges in minimizing color artifacts and spurious resolution as pixel pitch decreases below 5 μm/pixel, particularly in single lens reflex photography, where the effectiveness of optical low pass filters is not systematically evaluated for pixel pitches between 4 μm/pixel and 3 μm/pixel, impacting overall image quality.
Innovation Solution
A digital camera system is designed with an optical low pass filter that undergoes light beam separation along diagonal directions to achieve frequency modulation, setting the extinction frequency band positions based on calculated multipliers for Nyquist frequencies, optimizing the filter's strength and position to address color artifacts and spurious resolution across various pixel pitch ranges from 2.5 μm/pixel to 3 μm/pixel.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pixel pitch is reduced to increase pixel density, then image resolution is improved, but color artifacts and spurious resolution increase
Solution Approach 1:
The patent applies preliminary anti-action by positioning the optical low pass filter to create an extinction frequency band that preemptively counteracts the aliasing effects before they manifest as color artifacts. The filter is designed with its extinction band centered at a spatial frequency that is 1.5 to 3.5 times the Nyquist frequency, which is the frequency where aliasing begins to occur. This preliminary counteraction prevents the folding of high-frequency components into the visible range, thereby suppressing color artifacts and spurious resolution while maintaining the benefits of reduced pixel pitch.
2Object-generated harmful factors
If optical low pass filter strength is increased to suppress color artifacts, then color artifact reduction is improved, but image sharpness and contrast deteriorate
Solution Approach 1:
The patent applies local quality by creating a localized extinction frequency band at a specific position in the frequency spectrum rather than applying a broad low-pass filter. The extinction band is positioned at 1.5 to 3.5 times the Nyquist frequency, which is a localized region where aliasing occurs. This targeted approach suppresses color artifacts only in the problematic frequency range while leaving other frequency components, particularly those responsible for image sharpness and contrast, relatively unaffected. The filter thus exhibits different transmission characteristics at different frequency locations, achieving selective suppression.
3Reliability
If optical low pass filter is designed for traditional pixel pitches, then filter effectiveness is optimized for larger pixels, but performance deteriorates for pixel pitches below 5 μm/pixel
Solution Approach 1:
The patent applies parameter changes by redefining the positioning parameter of the optical low pass filter in terms of the ratio between the extinction frequency band center and the Nyquist frequency. Instead of fixing the extinction band at a absolute frequency value optimized for traditional pixel pitches, the patent specifies that the extinction band center should be positioned at 1.5 to 3.5 times the Nyquist frequency. This parameter change makes the filter design scalable and adaptable to different pixel pitches, including reduced pixel pitches below 5 μm/pixel, while maintaining effectiveness in suppressing color artifacts.
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 optimized optical low pass filter design enhances image quality by minimizing color artifacts and spurious resolution, allowing for a higher pixel density in image sensors while maintaining sharp focus and contrast, even at reduced pixel pitches, thereby expanding the utilization range of optical low pass filters and ensuring high image quality with reduced pixel pitch.
Implementation Method 1
an optical low pass filter unit at which light having passed through the lens unit and yet to enter the image sensor undergoes light beam separation along two diagonal directions ((1/2)a, (1/2)b)×(√2/α) and ((1/2)a, −(1/2)b)×(√2/α) relative to (x, y) coordinate axes, so as to achieve frequency modulation
Data Source
AI summary
A digital camera includes: a lens unit that forms a subject image on an imaging plane; an image sensor that includes color filters each disposed at one of pixels disposed in a lattice-like pattern over pixel intervals (a, b) along two directions, an x direction and a y direction, extending perpendicular to each other, with color filters corresponding to a first color component among first through nth (n≧2) color components, disposed in a checkered pattern at pixels amounting to at least half an entire color filter density and color filters corresponding to remaining color components disposed at other pixels, and outputs image signals expressing the subject image; and an optical low pass filter unit at which light having passed through the lens unit and yet to enter the image sensor undergoes light beam separation along two diagonal directions ((1/2)a, (1/2)b)×(√2/α) and ((1/2)a, −(1/2)b)×(√2/α) relative to (x, y) coordinate axes, so as to achieve frequency modulation for the subject image to become extinct at a band formed by connecting spatial frequencies (α/(2a), 0) and (0, α/(2b)) at positions calculated by multiplying, by a multiplier α, a Nyquist frequency 1/(2a) and a Nyquist frequency 1/(2b) at the image sensor assumed respectively along the x direction and the y direction. When the pixel intervals (a, b) assumed along the two directions at the image sensor are both within a 2.5˜5 μm/pixel range, a position of an extinction frequency band is set for the optical low pass filter by setting the multiplier α for the Nyquist frequencies at the image sensor within a range of 1.5≦α≦3.5.