Optical Low-Pass Filter Frequency Control for Single-Chip Sensor False Color Reduction
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Solution Overview
Problem
Single-chip image sensors struggle with accurate color reproduction due to the tendency to produce false colors, as they cannot set the cutoff frequency of the optical low-pass filter lower than half the sampling frequency without sacrificing resolution or increasing the size and complexity of the device.
Innovation Solution
An imaging device with an optical low-pass filter and color filters that separate imaging light into color components, where the frequency characteristics of the filter allow a controlled false color passing ratio, enabling multiple pixel signals to correspond to one output image signal, thereby improving color reproduction and resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the cutoff frequency of the optical low-pass filter is set lower than half the sampling frequency to reduce false colors, then color reproduction accuracy is improved, but the resolution of the output image is reduced and the image becomes blurred
Solution Approach 1:
The patent applies parameter changes by setting the cutoff frequency of the optical low-pass filter to a specific value (0.25 times the sampling frequency) and adjusting the decibel level (-6dB or -12dB) to optimize the balance between false color reduction and resolution preservation. This quantitative parameter adjustment resolves the contradiction by finding the optimal operating point.
2Measurement precision
If a three-chip image sensor is used to eliminate false colors, then color reproduction accuracy is improved, but the device size and complexity increase
Solution Approach 1:
The patent extracts the false color suppression function from the sensor structure itself and implements it through an optical low-pass filter with specific frequency characteristics. This separates the color filtering function from the pixel array structure, allowing a single-chip sensor to achieve three-chip sensor color accuracy without the complexity of multiple chips.
Solution Approach 2:
The optical low-pass filter acts as an intermediary component between the imaging lens and the single-chip image sensor. It mediates the optical signal before it reaches the sensor, preprocessing the light to prevent false colors while maintaining the simplicity of the single-chip sensor structure.
3Manufacturing precision
If the cutoff frequency of the optical low-pass filter is set around half the sampling frequency to preserve resolution, then image resolution is maintained, but false colors occur because frequencies higher than the Nyquist frequency pass through
Solution Approach 1:
The patent changes the cutoff frequency parameter from the conventional half-sampling-frequency setting to a lower value (0.25 times the sampling frequency). This parameter change effectively suppresses false colors by ensuring that frequencies higher than the Nyquist frequency are adequately filtered, while the associated resolution loss is managed through the specific -6dB or -12dB decibel setting.
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
This solution enhances the color reproduction and resolution abilities of single-chip image sensors to match those of three-chip sensors while maintaining a simple design, effectively reducing false colors and improving image quality.
Implementation Method 1
an optical low-pass filter with a cutoff frequency set to a value 0.25 times a sampling frequency of the image sensor and having a -6dB level or a -12dB level
Implementation Method 2
color filters that separate an imaging light passed through the imaging optical system into predetermined color components
Implementation Method 3
an image sensor that photoelectrically converts the imaging light passed through the color filters to generate pixel signals
Data Source
AI summary
Frequency characteristics of an optical low-pass filter (2) are set in such a way that a first false color passing rate indicative of the rate of frequency components passing through a frequency component region not lower than the Nyquist frequency fa for the lowest sampling frequency fs among the sampling frequencies in the longitudinal, the lateral, and the oblique directions for each color in an image sensor (5), i.e. a frequency component region lower than one half of the Nyquist frequency fs of the sampling frequency fs of the image sensor (5), is not higher than a specified value. An output image signal is created from a pixel signal created by the image sensor (5) so that N pixel signals (N is real number of 2 or above) created by the image sensor (5) correspond to one output image signal.