This application note explores the use of polarized cameras to remove unwanted screen glares and reflections on reflective surfaces in applications.
A polarized camera is a tool that has an individual polarization filter for each pixel. 4 pixels form a block, and each block has 4 different directions of the polarization filter (0˚, 45˚, 135˚ and 90˚).
All polarization angles are sorted, grouped and shown simultaneously in one image. This means that 4 images with different direction lighting will be shown concurrently in one image. This multi-directional polarized image is able to cater for different directions of sunlight, and one of the angles of polarization is able to remove the unwanted glare and reflection.
In many countries, especially Singapore, it is illegal for motorists to be using their mobile phones or not wearing seatbelts while driving. However, catching people who may disobey this law can be quite challenging if the police do not have the right tools.
For example, the glare from surrounding lights may make it hard for policemen to identify if the driver is violating seatbelt or mobile phone traffic laws while driving. Moreover, the environmental lighting may cause a reflection to occur on the vehicle’s windshield, affecting the image captured.
The common solution one would think of would be to use multiple cameras or filters. While this may be a good solution to combat the glare, it might be more costly to purchase and maintain the hardware, making it inefficient in the long run.
Hence, how do we combat this problem?
In order to remove the glare from reflective surfaces, users can use a 4-direction polarized camera.
For this application, the SODA VISION Tech Team used a Basler acA2440-75umPOL with an IP67 protective housing. The protective housing is used for cameras in outdoor applications, such as traffic surveillance, to protect the camera from dirt, mechanical damage, and harsh weather. Equipped with a Sony IMX250MZR CMOS sensor, the camera is able to produce 4 different images in 4 different lighting directions.
Figure 3: Image of a man in a car in 4 different polarization angles. Taken with Basler acA2440-75umPOL camera.
The camera is able to filter out unpolarized light, or light from the sun or fluorescent lamps that are vibrating in different directions, and focus on the light vibrating in a specific direction. In other words, the polarizer allows for light that is parallel to the polarizer to go through it, while light that is perpendicular to the polarizer is cut off. This means that one of the angles of polarization is able to remove the unwanted glare and reflection.
Figure 5: Diagram of unpolarized light transmitted through a polarizing filter. Image source: https://www.sony-semicon.co.jp/e/products/IS/industry/product/polarization.html
Some may argue that getting a polarized filter may be easier than getting a polarized camera. However, polarized cameras are more cost-efficient, as one is able to get multiple images with 4 polarizing angles. On the other hand, adding a polarized filter on a normal area scan camera will only allow the user to capture 1 image at 1 polarizing angle. Moreover, users can save time, effort and costs when they use polarized cameras instead of maintaining polarizing filters.
Other than traffic surveillance, this setup is also suitable for other applications such as tablet filling inspection and glass inspection.