Diffuser-Free Speckle Reduction in UV Inspection Systems
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Solution Overview
Problem
Current inspection techniques using coherent light sources, such as lasers, suffer from speckle noise due to coherence and surface aberrations, leading to degraded detection accuracy and false alarms, especially in the deep UV region where conventional speckle reduction methods are costly or inefficient.
Innovation Solution
A diffuser-free speckle reduction system utilizing a beam splitter to split input light pulses into multiple pulses oriented at different angles, with an acousto-optic device introducing frequency shifts and delays, allowing for effective speckle reduction without the use of diffusers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If coherent light sources (lasers) are used for illumination, then brightness and power are sufficient for high-speed inspection, but speckle noise is generated degrading detection accuracy
Solution Approach 1:
The patent segments the coherent light beam into multiple independent beams using a beam splitter. Each beam illuminates the sample from a slightly different angle, and the resulting speckle patterns are statistically independent. By combining these multiple speckle-reduced images, the overall speckle noise is reduced while maintaining the brightness advantage of coherent light sources.
Solution Approach 2:
The patent employs periodic modulation of the light source using an acousto-optic modulator, switching between multiple light paths in a periodic manner. This allows sequential acquisition of images from different illumination angles, with the modulation frequency synchronized to the camera frame rate, effectively reducing speckle through temporal averaging.
2Measurement precision
If diffusers are used for speckle reduction, then speckle noise is reduced, but the system becomes more complex and costly
Solution Approach 1:
The patent introduces a beam splitter as an intermediary optical element that divides the coherent light into multiple paths without requiring diffusers. This intermediary device enables speckle reduction through angular diversity while maintaining system simplicity and avoiding the complexity and cost associated with diffuser-based solutions.
3Measurement precision
If multiple light pulses are used at different angles, then speckle is reduced, but the optical system becomes more complex
Solution Approach 1:
The beam splitter and acousto-optic modulator serve multiple functions simultaneously: they divide the light into multiple paths, introduce angular diversity, provide temporal gating, and enable sequential imaging. This multi-functionality reduces the need for additional specialized components, thereby limiting the increase in optical system complexity while achieving effective speckle reduction.
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 system achieves significant speckle reduction by directing multiple light pulses at different angles, providing a 'virtual' illumination numerical aperture that enhances detection accuracy and reduces false alarms, while maintaining high precision and uniformity in ultra-violet region inspections.
Implementation Method 1
a beam splitter for splitting an input light pulse from the light source to multiple light pulses that are oriented in relation to each other when exiting the beam splitter
Implementation Method 2
The beam splitter may be an acousto-optic device that is arranged to split the input light beam to provide multiple light beams while introducing a frequency shift between the multiple light pulses
Implementation Method 3
at least one optical element for directing the multiple pulses to impinge on an inspected object at different angles
Implementation Method 4
The delay module may be arranged to introduce a delay that is longer that a width of the input light pulse
Implementation Method 5
The coherence of the laser, together with the roughness and aberrations of the surfaces as well as the patterned article along the light path, creates an artifact known as speckle, which is a noisy pattern over the image of the article
Data Source
AI summary
A method and an inspection system that exhibiting speckle reduction characteristics includes a light source arranged to generate input light pulses, and diffuser-free speckle reduction optics that include a beam splitter, for splitting an input light pulse from the light source into multiple light pulses that are oriented at angles in relation to each other when exiting the beam splitter, and at least one optical element for directing the multiple light pulses to impinge on an inspected object at different angles.