What is the meaning of pl-cir in the filter?

PL-CIR is another kind of filter called polarizer.

Polarizer, also called polarizer, referred to as PL mirror, is a kind of color filter. The excellent function of polarizer is that it can selectively let the light that vibrates in a certain direction pass through, and it is often used to eliminate or weaken the strong reflection of non-metallic surface in color and black-and-white photography, thus eliminating or reducing the light spot. For example, in landscape and scenery photography, it is often used to show the texture of objects with strong reflection, highlight the scenery behind the glass, darken the sky and show blue sky and white clouds.

According to different mechanisms of filtering polarized light, polarizers can be divided into circular polarizer (CPL) and linear polarizer (LPL).

According to the different mechanisms of filtering polarized light, polarizers can be divided into two types: circular polarizer (CPL) and linear polarizer (LPL), which have the same function. LPL is mainly used for old-fashioned manual focus cameras. Later, CPL was added with a film with the wavelength of 1/4. This kind of thin film has a special property, which can produce π/2 phase shift on the polarized electric vector in one direction (assuming X), but has no effect on the electric vector perpendicular to it (assuming Y). Therefore, the above polarized light can pass through the 1/4 wave plate along the bisector of X and Y, so the emergent light is a beam of polarized light with vertical polarization direction and π/2 phase difference, which is called circularly polarized light. This design makes it more suitable for new autofocus and automatic exposure cameras. So cameras that use AF lens (interchangeable camera lens with autofocus function) all use CPL as polarizer. In some light conditions, linear polarizer may mislead the built-in photometer to measure light, so DC and most autofocus cameras use circular polarizer.