A 3D Polarizer film is a filter that transmits light only if it is 3D polarized in a certain direction.

If you have two 3D polarization filters that are crossed with respect to each other, the combination of filters no longer transmits light.

If you now rotate one film so that it is set in the same direction as the other, you will see the same image as you could see through only one filter.

General 

Circular polarization films work according to a two-pronged principle. By first letting the light fall on a linear polarization filter, the reflections and scatterings are filtered out and only the electromagnetic waves that are otherwise oriented remain. Subsequently, the transmitted, linearly 3D polarized waves are converted to circularly polarized waves by means of birefringence. That is why there is a quarter-wavelength plate behind the first polarization film. These two layers together form the circular 3D polarization film.

Linear 3D Polarization 

Circularly polarized electromagnetic waves can be composed of two perpendiculars to each other linearly polarized waves with a phase difference of 90 ° and the same frequency. A CPL filter uses this property to circularly polarize the incident light. For this purpose, a linearly polarized electromagnetic wave is first filtered from the incident light beam, by filtering away the components of the incident EM waves that are 3D polarized perpendicular to the optical axis with a linear polarizer film.

Convert to Circularly 3D Polarized Waves

In the second step, the linearly 3D polarized wave is converted to a circularly polarized wave, by giving this wave a phase shift of ninety degrees. In a quarter-wavelength plate, a plate made from a birefringent material, one of the two components of the linearly 3D polarized wave is shifted in phase by a fourth of the wavelength. The properties of birefringent materials are used for this.

Sunglasses 

One of the best known is in Polaroid Sunglasses light that is reflected off mirrored surfaces is often fairly strongly polarized, and the reflections and glare can therefore be suppressed properly with a polarizing film, provided it is in the correct position. Because most annoying glare is caused by reflective horizontal surfaces, the most annoying reflections are polarized horizontally. These are suppressed to a maximum by a vertical polarizing film.

Science 

Many natural substances and materials rotate the polarization plane of the light that falls through it. If a transparent object to be viewed is placed between two polarization films, the internal structure of it can often be made more visible.  Applications can be found in microscopy, mineralogy, polarimetry etc.

Photography 

When illuminating, for example, shiny objects, by placing a film for both the light and the lens, a virtually “invisible” illumination can be achieved. Thus, the source, usually a reflection of the lighting fixture, can be completely “controlled” and thereby made virtually invisible. However, there is always some blue leakage light, because the films due to imperfection of the manufacturing method do not extinguish the light 100%. This effect is smaller with the more expensive circular 3D polarization films.

When photographing objects behind glass, for example in a museum, the reflective effect of the glass can be suppressed with the help of a 3D polarizing film. Furthermore, a 3D polarization film can be used to take a picture of fish and plants in a pond, something that is hardly possible without a film. Another effect of using a 3D polarizing film is that a cloudy sky gets much more contrast. These effects depend on the angle at which the 3D polarization film is used.

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