New-Tech Europe Magazine | December 2018

colored images, because they do not affect the color of the image that is being examined. In industrial image processing, polarizers play an important role in the inspection of objects and surfaces. They reveal defects and structures that would otherwise not be visible and allow shapes to be identified more easily: Shape recognition when contrast levels are low – In low-contrast environments or where there are several dark objects, it can be difficult to distinguish object boundaries and shapes. Polarizing filters improve contrast and present the object more accurately, which makes it possible to identify forms and borders Scratch recognition – The increased contrast provided by the polarizer highlights surface scratches and helps identify defects in quality control scenarios Eliminating reflections – When reflected light blocks viewing of the image, a polarizing filter can remove reflections and produce a perfect image of the object. Simultaneously, glare is eliminated to reveal parts of the image previously obscured Distinguishing different directions of light – Polarizers can help to identify the normal direction of light and differentiate between vertical and horizontal light. This feature is helpful to determine the source of light. Therefore, it is possible to eliminate both reflected light, and identify other properties of the light source Types of Polarizers in Image Processing Polarization can be applied in different ways to vision systems to improve image quality, and to allow more accurate image analysis. One approach is the use of a standard image sensor and a polarizer. Traditional polarizing

Figure 1: Reflections of Unpolarized Light

to be filtered, another polarizer or polarizing film can be placed in front of the light source. When the polarization planes of the lens and the light source are in parallel with each other, this combination results in the brightest possible image with the elimination of the non-polarized ambient light. Usually, this solution can be set up easily, quickly, and can be applied to existing systems. Depending on the environment, the lifespan of the filter may be reduced by external factors. Normally, polarizers that are fitted to the sensor (i.e., on-sensor filters), are made of glass and consist of four different filters that can align with the sensor at 0°, 45°, 90°, and 135°. The angle of the polarized light and

filters similar to those used in photography, are placed in front of the lens or on the light source, and they can be rotated to change the polarizing direction. If a complete image from multiple polarization directions is required, several sequential pictures with different polarization angles must be taken, but consequently slows down the vision application. However, in conventional machine vision applications, only one image with the best suitable polarization angle is required. A rotatable filter is mounted in a frame and affixed onto the lens. The rotation of the filter permits the optimum setting with the greatest contrast and no observable reflections. In addition, if ambient light needs

Figure 2: Unpolarized and Polarized Light Waves

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