Production-line vision systems now can use cameras with polarized image sensors that remove glare from objects for more efficient product analysis.
By Keith Wetzel, director of New Product Development, Imperx, Inc.
Editor’s Note: This article is adapted from the white paper, “Polarization cameras take the stress out of materials analysis.” Download the full paper for more information about monochrome, color and polarized image-processing systems; calculating stokes parameters; illustrations of the kind of images polarization cameras can generate; and resources for more information about polarization applications.
Machine vision and image-processing system designers have multiple choices for which cameras, lighting, computers and I/O peripherals to deploy. They can select from multiple monochrome, color, infrared (IR), ultraviolet (UV) and multispectral cameras, or combinations of these to perform industrial inspection tasks. Polarization cameras might be one of the most overlooked of these methods. These are cameras that don’t require polarization filters to analyze products and can remove any reflected glare on production-line objects.
Industrial automation systems often use machine-vision systems to inspect, measure, sort and analyze products. Such systems can be broadly defined in two classes: active or passive.
Active machine vision systems might use structured laser light or pattern projection systems to illuminate an object. Passive systems use simpler forms of illumination such as LED front lights, dome lights or backlights to illuminate the object being inspected.
In both types of systems, a CCD (charge-coupled device) or CMOS-based (complementary metal-oxide semiconductor) camera is used to capture the reflected image data.
Most light sources such as the sun and artificial lighting are unpolarized and consist of electromagnetic waves that oscillate perpendicular to the direction of dissemination. A light wave in which vibrations perpendicular to the direction of travel are confined to one plane is said to be polarized.
One of the most important characteristics of all polarizers is their extinction ratio. This is the ratio of the power of a plane-polarized beam transmitted through a polarizer compared with the transmitted power when the polarizer filter's axis is perpendicular to the beam's polarization plane.
Polarizing cameras can be used in various applications. This includes material stress analysis, where light is first passed through a polarizer to produce linearly polarized light. This light is then passed through a birefringent (double refraction) material such as plastic, polystyrene and polycarbonate; stress in the material causes the light to be broken up into two polarized components.
Components that are parallel and perpendicular to the direction of the stress in the material will lag each other in phase. After these polarized components are captured by the camera, only the part of each of the components in the transmission plane will emerge, resulting in two coplanar components with a phase difference.
The phase difference is proportional to the stress and results in constructive and destructive interference resulting in an interference pattern of varying colors that can be used to determine the stress at various points in the material (see Figure 1).
Such polarization cameras also can be used to reduce the specular glare associated with light being reflected from nonmetallic surfaces, such as smooth surfaces or surfaces covered with grease, oil or liquid.
To reduce this glare, a polarizing filter can be placed over the light source. In the past, it was necessary to place a separate filter over a CCD or CMOS camera to reduce this glare. Now, because the polarizing filter is incorporated into the camera, separate filters aren’t required.
Figure 2 shows the reflections from a sealed cap illuminated by ambient light and a white LED light source. This results in specular glare that renders the cap unreadable. After installing polarizing film over the light source and either using a polarizing filter on a standard monochrome camera or, by using a polarizing camera, greatly reduces any specular reflections.
Other applications that can benefit from the power of polarization cameras include remote sensing, haze removal and surveillance applications. With the introduction of solid-state cameras that incorporate on-chip polarizing sensors, systems developers no longer need to use external polarizing filters on solid-state cameras.
Interestingly, while sensors polarize visible light, further developments might lead to imagers that perform the same function in other parts of the electromagnetic spectrum, such as IR. These more specialized imagers will find more niche applications in such areas as security and surveillance.
Imperx Inc., based in Boca Raton, Florida, is a participating Encompass™ Product Partner in the Rockwell Automation PartnerNetwork™ program. Imperx designs and manufactures high-performance cameras, process video recorders and frame grabbers.
The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.