Industrial polarization camera whitepaper

Industrial Polarized Camera

Daheng Imaging is dipping their toes in the industrial polarized camera segment by implementing the Sony-IMX250MZR monochrome image sensor. Industrial polarization camera systems are able to capture information, which a normal line or area industrial camera is not able to obtain. A polarization camera conducts a two-dimensional analysis. It measures both the degree and direction of the polarization of the light. As a result the final image has almost none reflections from external lightsources. A polarized camera is ideal for applications like the inspection of glossy products and transparent products.
 

Standard machine vision camera versus polarized camera

The following examples show on the left hand side an image captured with a standard machine vision camera and on the right hand side an image captured with our polarized camera.

Glass inspection (scratch and stain)
polarized camera
 
These examples show dents and dust on a homogenous glass plane. We can easily find scratches and stains (fingerprint and dust) due to differences in the degree of polarization

Tablet filling inspection
 
Again, the left image is captured with a standard industrial camera. The right image is captured with our polarization camera. Thanks to the difference in the degree of polarization between the tablet and the aluminum  package, it is easy to identify whether the tablets are filled in or not  

Distortion inspection
polarizated camera image

Left image= standard industrial camera. Right image = polarization camera.
With the information of direction of polarization, we can identify both distortions and the direction of distortion of the plane

Removal reflection
polarization camera image 
Left image= standard vision camera. Right image = polarized camera.
The information of polarization can be used to remove reflections. IMX250MZR  /  MYR  has  a  four-directional polarizer which can simultaneously remove reflection in multi planes.

How does a polarization camera work?

The image belows explains how a polarization filter works. A light (yellow bulb) send out light beams (yellow arrow). Every light beam consist out of light waves with different angles (the green and red line). A polarization filter (the black stripes) block light waves that have a certain angle (in the image the red line) and the light waves that have the right angle (green line) will pass. So only the light wave with the right angle (green light) will hit the lens and sensor of the camera. When a polarized light wave (the remaining green line after the polarization filter) hits a surface of a non-metallic or transparent object the direct reflected wave stays polarized, while the rest of the polarized light get unpolarized in all directions. The polarized reflected light can cause undesirable glare from reflective surfaces, making it harder or impossible for your machine vision system to detect certain elements of an object. With the use of a polarizing filter this effect can be controlled. The polarized light will be blocked by the polarized filter of the camera while the unpolarized light will pass. As a result the glare and reflections are reduced, making it easier for you vision system to detect defects.
polarization-camera-how-it-works
The principal of a polarization filter

First example application for polarization camera

The first example application where polarization cameras can be used is the capturing of single-colored objects, like tomatoes. With the use of a normal vision camera, it is hard in some applications to segment the different products (like tomatoes) of each other, or detect surface defects. Polarized camera systems on the other hand make this process easier, where the additional dimension of analyses can detect difference in the surface of single-colored objects. Polarized cameras could for example be used on conveyor belts which scan single-colored objects, like tomatoes in a production line.

Second example application for polarized camera

The second example application where a polarized camera could be utilized is capturing images of transparent objects. For normal vision camera systems it is in some cases difficult to detect the characteristics of transparent objects from an image that was captured. Namely, because when light transits through a translucent object, its polarization changes. Present camera systems need their polarization parameters adjusted in order to detect the product properly. However, differentiation of light transmittance can be automatically captured by a polarization camera. A polarized camera system is well suited for conveyor belts in a production line which transport transparent products, detecting the presence of foreign materials and detecting stamped marks.

Specification of the Sony IMX250MZR polarization camera

Our polarization camera with the Sony IMX250MZR image sensor uses a 3,45µm pixelsize and has a total resolution of 5.07 MegaPixel. The polarization camera has a polarizer filter which is a four-directional polarizer placed on the photodiode of the image sensor. This means that this polarized camera is able to capture a four directional image in one shot. And calculates the angle and direction of polarization based on the intensity of every direction of polarization. The filter incorporates a 2x2 pattern, where every pixel has a filter with a different angle. Whilst using polarized light the only portion of light perpendicular to the angle of the filter will pass.
Polarization filter angles
Summarized, the polarization filter characteristics are:
-    When the filter blocks light at a 90º angle, it passes 0º through the filter.
-    When it blocks light at a 45º angle it passes 135º through the filter.
-    When it blocks light at an 135º angle it passes through 45º the filter.
-    Finally, when it blocks lights at an 0º angle it passes light through a 90º angle.
The model of our polarization camera is MER-502-79U3M POL. It has a USB3 interface, a 2448(H)x2048(V) resolution, 79 fps, 2/3”, Global Shutter, and a 3,45µm x 3,45µm cell size.
Polarization-rotations
 

Conventional polarization camera sensors compared to the IMX250MZR

When speaking of conventional polarization camera sensors you see that the polarizer is welded on top of the chip lens layer. The Sony IMX250MZR sensor on the other hand is placed under the on chip lens layer. As a result are the incident angle and extinction ratio improved because of the shorter distance between the photodiode and polarizer. Often vision integrators also tend to use a polarization filter mounted on the lens. This filter can only block polarized light in one angle, while our polarization camera with the Sony IMX250MZR can block and detect light in 4 angles simultaneously.

GeT Cameras and Polarization Cameras?

Daheng Imaging will release a polarization camera with the Sony IMX250MZR image sensor soon. Samples are already available. If you think your applications requires a polarized camera, please contact us. We are more then happy to help you with selection the right polarization camera for you.

 

  Max Reijngoudt         Comments (0)

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