The difference between PL mirror and CPL mirror

Differences and uses between Pl polarizer and cpl polarizer

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Polarizer: it can weaken the reflected light on the surface of an object. Exterior photography can make the blue sky bluer, with sharp contrast and saturated colors. Black-and-white and color photography are acceptable. When shooting, you need to add 2 levels of exposure, which is a necessary filter for photographers.

Ordinary polarizers are called linear polarizers (PL mirrors). Install the polarizer in front of the lens and carefully rotate the polarizer to minimize or even eliminate harmful glare, so that the photos you take have no glare. If you photograph the blue sky, it will appear bluer and darker. The polarizer not only filters out polarized light, but also filters out the unpolarized light that vibrates in the same direction as the polarized light. Therefore, after using a polarizer, it is generally necessary to increase the exposure of more than one step.

However, in AF (Auto Focus) and AE (Auto Exposure) cameras, part of the light will be reflected to the photometric element. If these rays are reflected by a semi-transparent and semi-reflective mirror, and this semi-transparent and semi-reflective mirror is formed by coating with non-metallic materials, then the rays they reflect are polarized light. In this camera, if an ordinary linear polarizer is used, the polarized light passing through the lens will be reflected normally when it hits the semi-transparent mirror at an appropriate angle. If the angle is not appropriate, there will be no light reflection. So AE may be inaccurate and AF may be invalid. In addition, some cameras have LCD screens in their viewfinder. The liquid crystal displays of some cameras are illuminated by light passing through the lens. If the lens uses a linear polarizer, the LCD will fail.

How to make the polarizer not only filter out harmful polarized light, but also make the light entering the lens through the polarizer become unpolarized light? In this case, a circular polarizer (CPL mirror) will be used. Circular polarizer can be regarded as two parts: the former part is an ordinary linear polarizer, which can filter out polarized light in a certain vibration direction. The latter part can make the vibration direction of polarized light passing through it rotate on a circle. This special polarized light is called circularly polarized light, hence the name circular polarizer. For most optical elements, there is no difference between circularly polarized light and unpolarized light, so circular polarizer can be used in any camera.