ProSensus is built on the foundation of over 25 years of experience in developing specialized algorithms for advanced industrial machine vision applications using color, thermal, 3D, and polarized images. ProSensus has a global install base of real-time inspection solutions across a variety of industries including the consumer-packaged goods, synthetic rubber, and food industries.
Polarization
There are three fundamental properties of light. The first two properties, intensity and wavelength, are used by standard colour and monochrome camera sensors. The third property, that is not detected by standard sensors, is polarization. Polarization describes the direction in which a light-wave is oscillating. Unpolarized light oscillates in random directions while linearly polarized light is reduced to a single direction of oscillation.1
Polarization imaging enables the detection and filtering of light at multiple polarization states.2 There are several use cases where the ability to discern this polarization information enables a level of image feature detection and analysis that is otherwise not possible with conventional color or monochrome imaging.
Common Applications
Two general applications for polarization imaging are glare reduction and material stress detection.
Glare Reduction
Figure from Lucid Vision Labs3
When light reflects off a horizontal surface, the resulting glare is polarized and filtering the light at the correct polarization state enables the reduction of glare with minimal impact on the rest of the image.
Material Stress Detection
Figure from Lucid Vision Labs3
Machine vision using a back-lit polarization imaging configuration can be used to detect variation in stress in transparent objects. Mechanically stressed plastics do not refract light consistently throughout the material, and the varying polarization states of the refracted light indicate varying stresses.
ProSensus Experience: Iron Ore Pellet Size and Pellet Moisture
The Challenge
ProSensus recently used a polarization camera for an application in the iron ore industry. The objective was to provide online estimates of pellet size distribution and moisture. The presence of moisture in the pellets results in a reflective surface, hence the need for a polarization camera to reduce the impact of glare on image quality and subsequent image analysis.
The Solution
Hardware
The configuration used for this imaging application consisted of a polarization camera and a ring light inside a protective enclosure. As illustrated, this configuration successfully reduced the amount of reflection in the images which improved image quality for analysis.
Feature Extraction Algorithms
Size Distribution
To quantify size distribution, a central, well-lit sample region was identified in the polarized images. Local contrast enhancement improved image contrast and image morphology was used to segment potential pellet centers which show up as brighter regions in the image. After locating the pellet centers, a watershed transform was used to detect entire pellets. Finally, the area of these identified pellets was used to estimate pellet size distribution.
Below, 3 polarized image samples are shown alongside its respective enhanced image following feature extraction and the calculated pellet size distribution.
Moisture
To quantify pellet moisture, the amount of reflection on the pellet surfaces was estimated by subtracting images of varying degrees of polarization. The average intensity of this “difference image” was trended over time and found to be correlated with averaged plant pellet moisture measurements.
Contact Us
ProSensus customizes our machine vision solutions to combine advanced analytics with the right hardware to maximize insight on your product or process. Contact us to get started with an on-site trial, off-site trial, or analysis of your existing images.
References
- Introduction to Polarization. Edmund Optics. https://www.edmundoptics.com/knowledge-center/application-notes/optics/introduction-to-polarization/ Accessed February 22, 2022.
- Polarization-Based Imaging: Basics and Benefits. Photonics Media. https://www.photonics.com/Articles/Polarization-Based_Imaging_Basics_and_Benefits/a60734 Accessed February 22, 2022.
- Beyond Conventional Imaging: Sony’s Polarized Sensor. Lucid Vision Labs. https://thinklucid.com/tech-briefs/polarization-explained-sony-polarized-sensor/?gclid=Cj0KCQiA3rKQBhCNARIsACUEW_YOhz0Zsv21Y3uz7wvAFRClRIokGi1ek08XWWXwVA18Y0WvmSqx_7MaAmAXEALw_wcB Accessed February 22, 2022.
