Calibrating Field Uniformity in Camera-Based Imaging Devices
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Solution Overview
Problem
Camera-based imaging devices face issues with non-uniform illumination, leading to poor image quality due to diffuse and specular hotspots, which existing calibration methods fail to adequately address, especially with the use of low-cost LED lamps that produce significant glare.
Innovation Solution
The method involves creating and storing specular and diffuse reflectance maps during calibration, allowing for pixel value adjustments during imaging operations to correct for field non-uniformities, including those from the sensor, optics, and ambient light, using a single calibration step that does not require bulky diffuse lamps or polarizing filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If low-cost LED lamps with focusing mechanisms (Fresnel lenses or back-reflectors) are used to provide adequate illumination, then illumination intensity is improved, but illumination uniformity deteriorates due to non-uniform cones with up to 50% non-uniformity
Solution Approach 1:
The patent applies preliminary action by performing a calibration scan before actual imaging to create a reflectance map that characterizes the illumination non-uniformity. This pre-characterization allows the system to compensate for the non-uniform cone produced by low-cost LED focusing mechanisms, enabling the use of inexpensive components while maintaining image quality through software-based correction.
2Measurement precision
If zonal correction calibration is applied to the sensor array, then sensor non-uniformities (offset, gain, shading, lens falloff) are corrected, but illumination non-uniformities (diffuse and specular hotspots) remain uncorrected
Solution Approach 1:
The patent merges sensor calibration with illumination characterization into a single comprehensive calibration process. By scanning a uniform reflectance target and combining the results into a reflectance map that captures both sensor properties and illumination non-uniformity, the system simultaneously corrects for both sensor array variations and lighting conditions, eliminating the need for separate calibration steps.
Solution Approach 2:
The patent introduces a uniform reflectance target as an intermediary object during calibration. This target serves as a mediator that allows the system to separate and characterize the combined effects of sensor non-uniformity and illumination non-uniformity. The reflectance map generated from scanning this intermediary target contains information about both the sensor's response variations and the lighting conditions, enabling comprehensive correction.
3Manufacturing precision
If multiple diffuse lamps are used to physically prevent specular reflections, then image quality is improved, but device complexity and cost increase due to bulky lamps and expensive filters
Solution Approach 1:
The patent replaces the mechanical/optical approach of using multiple diffuse lamps and polarizing filters with a software-based solution. Instead of physically modifying the lighting system to eliminate specular reflections, the system captures the reflections and corrects them through image processing using the reflectance map, thereby eliminating the need for bulky lamps and expensive filters while maintaining image quality.
Solution Approach 2:
The patent creates a digital copy (reflectance map) of the illumination and sensor characteristics through calibration scanning. This digital representation allows the system to simulate and correct for various non-uniformities without physically implementing complex optical systems. The reflectance map serves as a virtual model that enables software-based correction of issues that would otherwise require complex hardware solutions.
4Ease of operation
If ambient light is not accounted for during calibration, then calibration simplicity is maintained, but image accuracy deteriorates due to uncorrected field non-uniformities from ambient light
Solution Approach 1:
The patent applies preliminary action by performing calibration in the actual operating environment with ambient light present. This allows the system to characterize and store the ambient light conditions as part of the reflectance map, enabling automatic compensation during imaging without requiring separate ambient light measurements or complex additional calibration steps.
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 approach improves image quality by eliminating diffuse and specular hotspots, enabling the use of inexpensive LED lamps and reducing noise, while allowing for user-configurable calibrations for different media types, resulting in more accurate and consistent image capture.
Implementation Method 1
The camera is aimed at a document and the LED lamp illuminates the document for the camera
Implementation Method 2
The focused light reflected off a document is both diffuse and specular
Implementation Method 3
The focused light reflected off a document is both diffuse and specular
Implementation Method 4
The reflected light is collected on an array of photosensitive elements (e.g., a charge coupled device (CCD)), which convert the reflected light into electronic signals representing the image
Data Source
AI summary
Systems and methods systems and methods for calibrating field uniformity are disclosed. An exemplary method includes scanning the imaging area including a first media to obtain optical data for a specular reflectance map. The method also includes scanning the imaging area including a second media to obtain optical data for a diffuse reflective map. The method also includes storing the specular reflectance map and the diffuse reflective map for adjusting actual pixel values during an imaging operation.