Optical Inspection of Reflective Surfaces Using Segmented Illumination
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Solution Overview
Problem
Existing methods for optically inspecting test specimens with reflective surfaces are not optimal, as they require extensive image recordings and prior knowledge about the surface properties, and do not provide complete characterization, especially for unknown surfaces, due to limitations in stripe direction and width, and are unsuitable for highly lustrous surfaces.
Innovation Solution
The method determines the scattering characteristic of a surface point by analyzing which light sources a camera pixel can 'see' via the test specimen surface, using the concept of a light origin region, which represents the spatial distribution of light contributions, allowing for comprehensive characterization with minimal prior knowledge and reduced image recordings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple illumination patterns with different periods are used to determine optimal stripe width, then measurement accuracy is improved, but the number of image recordings increases
Solution Approach 1:
The illumination device is segmented into multiple independently controllable light sources arranged in a matrix pattern. Each light source can be individually activated to create different illumination patterns, allowing systematic variation of stripe periods without requiring complete reconfiguration of the illumination system. This segmentation enables efficient acquisition of multiple patterns with reduced overhead.
Solution Approach 2:
The method employs periodic illumination patterns with varying stripe periods systematically. By using sinusoidal intensity profiles with different spatial frequencies (periods), the system can determine optimal stripe width through periodic variation. The periodic nature of the patterns allows for efficient comparison and optimization while maintaining a structured approach to data acquisition.
2Measurement precision
If the surface is displaced relative to stripe patterns in multiple images, then complete surface characterization is improved, but the complexity of the inspection process increases
Solution Approach 1:
The method uses the surface's own scattering properties to generate the necessary variation in recorded patterns. By analyzing how different light sources illuminate the surface from various angles and how the surface scatters this light, the system extracts scattering characteristics without requiring external displacement mechanisms. The surface essentially serves itself by its inherent optical properties.
Solution Approach 2:
Instead of displacing the surface in physical space to achieve complete characterization, the method transitions to another dimension by varying the illumination angles through multiple light sources. This dimensional shift from spatial displacement to angular illumination variation simplifies the inspection process while maintaining comprehensive surface characterization capability.
3Measurement precision
If narrow stripes are used in illumination patterns, then resolution and measurement accuracy are improved, but the stripes blur more easily on diffusely scattering surfaces
Solution Approach 1:
The illumination device provides locally optimized illumination by activating specific light sources based on the local scattering properties of different surface regions. For diffusely scattering areas, wider effective stripe patterns are used, while for specular regions, narrower patterns can be employed. This local adaptation of stripe width to local surface properties maintains resolution where possible while ensuring reliable stripe separation where needed.
Solution Approach 2:
The system dynamically changes illumination parameters, particularly stripe period and effective stripe width, based on the detected scattering characteristics of the surface. By adjusting these parameters adaptively, the method optimizes the balance between resolution (favored by narrow stripes) and reliable stripe separation (favored by wider stripes), allowing narrow stripes to be used effectively even on diffusely scattering surfaces when combined with appropriate pattern design.
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 enables a simple, reproducible, and rapid characterization of scattering properties and type classification of test specimens, reducing the need for extensive image recordings and prior knowledge, while being effective for both diffusely and highly reflective surfaces.
Implementation Method 1
positioning the test specimen relative to the illumination device and the camera, such that light from the light sources is reflected via the surface to the camera
Implementation Method 2
recording a series of images of the surface with in each case one of the illumination patterns
Data Source
AI summary
An apparatus for optically inspecting a test specimen has an at least partly reflective surface including a camera having a number of pixels and an illumination device having a multiplicity of spatially distributed light sources. A workpiece receptacle serves for positioning the test specimen relative to the illumination device and the camera, such that light from the light sources is reflected by the surface to the camera. An evaluation and control unit generates a series of different illumination patterns on the surface, wherein in the course of the series different light sources are switched on. An individual light origin region is determined on the basis of the images recorded by the camera for at least one pixel, whereby the region represents a spatial distribution of individual light contributions generated by the light sources via the surface on the at least one pixel.