A CPL filter (Circular Polarizing filter) is a filter that you place over the lens of the dashcam. A CPL filter works like sunglasses for the dashcam; it protects the lens from harmful radiation and prevents annoying reflections in the image. Especially when the sun is low, the use of a CPL filter can make a big difference to the image quality. A CPL filter usually makes the image darker, so it is better to remove the CPL filter in the evening. Note that not all dashcams support the use of a CPL filter, so always check the specifications of the dashcam first.
