Definition and Classification of Polarizers

What is a polarizer ? A polarizer also called a polarizing filter, or simply PL filter, is a type of color filter. Its main function is to selectively allow light vibrating in a specific direction to pass through. In color and black-and-white photography, it is often used to eliminate or reduce strong reflections from non-metallic surfaces, thereby eliminating or reducing glare. Sometimes, it can even make colors more saturated than what the human eye directly sees. It can also be used to improve the tone and color of the sky. This is the most commonly used enhancement filter in the photography world. Given the powerful effects of polarizing filters, let's take a closer look at some of their functions and provide a brief overview.

A polarizing filter mainly consists of two optical glass plates sandwiching a tiny polarizing crystal (such as mica) with directional properties. Another manufacturing method involves coating the interlayer between the two optical glass plates with a polyethylene film or a crystalline substance like polyvinyl cyanide. This polymer coating creates an extremely fine, grid-like structure, acting like a dense fence that allows only light vibrating in the same direction as the gaps to pass through. The two glass plates are then independently mounted in a rotating ring; rotating one of the lenses eliminates polarized reflected light from the surface of the photographed object.This coating will gradually age and fail, and its lifespan will be shortened by moisture, impact, and vibration. Ultimately, polymer-coated polymer (PL) products are consumables, not durable goods. Under normal circumstances, they should last five or six years without problems.

In high school, we learned that light is an electromagnetic wave formed by the alternating vibrations of electric and magnetic fields perpendicular to its direction of propagation. Such waves, whose vibration direction is perpendicular to propagation, are called transverse waves. Sound waves propagate through air or other media by compressing and rarefying the medium in a back-and-forth motion; their vibration direction aligns with propagation, and such waves are termed longitudinal waves.

A characteristic of transverse waves is that their vibration is polarized. In a plane perpendicular to the direction of propagation, they can vibrate in either direction. We generally take the direction of vibration of the electric field of a light wave as the direction of light vibration. If a beam of light vibrates in the same direction, we call it polarized light, or more precisely, completely polarized light. Ordinary natural light vibrates uniformly in all directions and is unpolarized light. However, glare formed by reflection from a smooth, non-metallic surface at a certain angle (called the Brewster angle, which is related to the refractive index of the material) is polarized light. Deviating from this angle, some unpolarized light will be mixed with the polarized light. We call this partially polarized light. Partial polarization has a degree. The greater the deviation angle, the less polarized light there is, eventually becoming unpolarized light. Polarized light can sometimes be detrimental to photography. Reflected light from glass surfaces makes it impossible to photograph things inside glass shop windows; reflected light from water surfaces makes it impossible to photograph fish in the water; reflected light from leaf surfaces makes leaves appear white, and so on. The blue sky under a clear sky scatters polarized light at a 90-degree angle to the sun, making the sky appear less deep and dark. This is where a polarizing filter comes in handy.

Polarizing filters can be broadly classified into two categories: linear polarizing filters (LPL) and circular polarizing filters (CPL). LPL linear polarizing filters were primarily used in older manual focus cameras. However, with technological advancements, these older cameras have been replaced by newer generations of photography, leading to new developments in polarizing filters, such as CPL circular polarizing filters. CPL circular polarizing filters add a 1/4 wavelength thin film. This film possesses a special property: it can induce a π/2 phase shift in the polarization vector in one direction (let's say x), while having no effect on the polarization vector perpendicular to it (let's say y). Therefore, polarized light can pass through the 1/4 waveplate along the angle bisector of x and y, resulting in a beam of light composed of two polarized rays with perpendicular polarization directions and a π/2 phase difference—the so-called circularly polarized light. This type of circular polarization is more suitable for modern autofocus and automatic exposure cameras, and therefore, CPL polarizing filters are now widely used in interchangeable lens cameras with autofocus functionality.

A polarizing filter consists of two parts: a lens body and a rotating rear frame connected to it. The lens body is made of an extremely fine crystal glass grating. When rotated, the grating of the polarizing filter blocks polarized light rays that are not parallel to it. Therefore, the polarizing filter can control and select the reflected light that is parallel to it, and this reflected light is polarized light.

Sometimes, polarizing filters can be directly mounted on the camera. Slowly rotate the adjustment ring of the polarizing filter and carefully observe the polarized light source in the subject through the viewfinder until it disappears or weakens to the desired effect. When using a polarizing filter, because it requires rotating the filter, it is not advisable to use a lens hood. The camera should be placed in the shade as much as possible. Additionally, the camera lens must be protected when using a polarizing filter.