Spatial Light Modulator Dithering for Diffraction Artifact Reduction
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Solution Overview
Problem
Digital image projection systems with small pixel pitches suffer from visible diffraction artifacts due to interactions between pixel elements and illuminating light, particularly across shallow gradients, which are exacerbated by dithering techniques used to create gray scale shades between limited directly producible levels.
Innovation Solution
A method is implemented where a digital controller coupled to a spatial light modulator processes pixel image data to determine desired gray scales, compares them to achievable levels, and applies modified dithering techniques by restricting high-frequency dithering to low-intensity bit planes, reducing diffraction artifacts below perceivable levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If dithering techniques are used to create gray scale shades between limited directly producible levels, then gray scale precision is improved, but visible diffraction artifacts are generated
Solution Approach 1:
The patent applies different dithering strategies to different bit planes based on their intensity characteristics. Low-intensity bit planes use high-frequency dithering patterns, while high-intensity bit planes use low-frequency or no dithering. This local differentiation resolves the contradiction by applying dithering only where it provides benefit (low-intensity regions) without generating harmful diffraction artifacts in high-intensity regions.
Solution Approach 2:
The patent dynamically changes the dithering frequency parameter based on the intensity level of each bit plane. By adjusting the dithering frequency according to local intensity conditions, the system achieves precise gray scale control in low-intensity regions while avoiding diffraction artifacts in high-intensity regions where dithering is unnecessary or harmful.
2Measurement precision
If high-frequency dithering is applied across all bit planes, then gray scale accuracy is improved, but diffraction artifacts become visible across the entire image
Solution Approach 1:
The patent implements spatially selective dithering by analyzing the intensity characteristics of each bit plane and applying high-frequency dithering only to low-intensity bit planes. High-intensity bit planes receive low-frequency dithering or none at all. This local quality approach ensures gray scale accuracy where needed while eliminating visible diffraction artifacts from the overall image.
Solution Approach 2:
Instead of applying dithering uniformly across all bit planes, the patent applies partial dithering action only to the extent necessary for low-intensity bit planes. This partial action approach avoids the excessive application of high-frequency dithering to high-intensity regions, thereby preventing the generation of visible diffraction artifacts while maintaining gray scale accuracy where it matters most.
3Measurement precision
If dithering is used to produce intermediate gray shades, then image quality is improved, but interference patterns are created that alter the visible gray shade
Solution Approach 1:
The patent changes the dithering frequency parameter based on the intensity level of each bit plane. By adapting the dithering frequency to local intensity conditions, the system produces accurate intermediate gray shades in low-intensity regions without creating interference patterns that would alter the visible gray shade in high-intensity regions.
Solution Approach 2:
The patent applies different dithering characteristics to different bit planes according to their intensity properties. Low-intensity bit planes receive high-frequency dithering for precise gray shade control, while high-intensity bit planes use minimal or low-frequency dithering to avoid interference patterns. This local quality differentiation resolves the contradiction between gray shade precision and interference pattern generation.
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 approach effectively reduces or eliminates visible diffraction artifacts in the displayed image by limiting dithering to low-intensity bit planes, ensuring that the resulting gray scale shades align closely with intended values, thereby improving image quality.
Implementation Method 1
SLMs can be DMDs that receive digital image data and reflect (or do not reflect) the illumination light, forming an image corresponding to the digital image data
Implementation Method 2
A DMD with a small pixel pitch of less than about 7 microns operates as a diffraction grating for the wavelengths of illuminating light being used. This interaction can produce visible artifacts in the displayed image
Implementation Method 3
When the pixel pitch is reduced to these new small sizes, the dithering techniques can also operate as interference patterns for the wavelength of visible light. The gray shade that is visible is then either brighter than expected (due to constructive interference) or darker than expected (due to destructive interference)
Data Source
AI summary
For an example method of reducing diffraction artifacts in a displayed image from a digital image projection system, a digital controller is coupled to a spatial light modulator. The spatial light modulator includes a two dimensional array of pixels. The method includes receiving signals containing pixel image data and processing the pixel image data to determine a desired gray scale for a pixel. The desired gray scale is compared to achievable gray scales of the digital image projection system. An upper bounding achievable gray scale is determined, and a lower bounding achievable gray scale is determined. Also, a dithering percentage for the desired gray scale is determined. The method chooses one of the upper bounding achievable gray scale and the lower bounding achievable gray scale. Also, a modified lower bounding gray scale is chosen to reduce the dithering percentage.