3D polarization system
A 3D polarization system is a method for displaying stereoscopic 3D images. With this method, the images of the stereo image pair are emitted in oppositely polarized light . There are correspondingly offset polarization filters in front of the projection lenses and in the viewer's 3D glasses.
A metal-coated canvas is required to maintain the polarization status of the light. A normal white canvas would scatter the light again and the channel separation would be canceled. The advantage of this projection technique is the high color fidelity of the images shown. Disadvantages are, on the one hand, the drop in light due to the filters used and the metallic canvas required for this process.
This technology is now used not only for projection, but also for LCDs, e.g. B. applied to screens and 3D televisions. The basic principles are similar.
technology
A front projection is usually used in the cinema, i.e. the viewer and projector are on the same side of the screen. It is projected onto a silver-coated or aluminum-coated screen that is able to reflect the polarized light. With a rear projection, the screen stands between the viewer and the projector and has to let the polarized light through. The different images reach the screen through two different projectors or lenses. It is also possible to use only one projector if the images are projected alternately left-right through a filter with changing polarization (called Z-screen in RealD projections). In the case of 4K projectors, both partial images (in 2K ) can be separated again in one image by a beam splitter lying one above the other (Sony 4k 3D projections). Current 3D cinema projections also use polarization technology in most cases. For details of the technology used in IMAX cinemas, see IMAX 3D .
The required glasses also consist of two polarization filters , which, depending on the polarization method, only let through the “appropriately” polarized light of the corresponding view, so that each eye only receives “its” image. In the first heyday of 3D film in the 1950s, most 3D cinema films were projected in black and white and color using this method.
With polarization processes, a distinction must be made between different processes and their orientations. There are two groups of polarization methods: linear and circular. The two methods are not compatible with each other. Another decisive factor within the process is how the polarizing filters are aligned. With linearly polarized light, the head must be kept straight while viewing the image. If you hold your head at an angle, the 90 ° angle required to separate the channels changes between the foils in front of the projection lenses and the filters in the glasses. As a result, there is no longer any channel separation, "ghost images" appear. This no longer applies to modern methods such as Real-D , as they use circularly polarized light - you can move your head freely without any disadvantages.
Another problem is the inconsistent use of filters by different manufacturers of glasses and projectors. The filters in the glasses must match the filters in the projector, otherwise the channels will be mixed up.
Linear
In the linear polarization method, the light is linearly polarized, i. H. the light oscillates in a plane specified by the filter. The filters for the left and right view must be aligned at right angles to each other to allow the two views to be separated. The angle at which the polarization is aligned is irrelevant as long as the polarization direction of the projection filter matches that of the glasses. The de facto standard for linear projection is a "V" position of the filters; That is, the left swings in −45 ° (\) and right in + 45 ° (/). Other orientations also exist, such as. B. an A-position and the combination 0 ° / 90 ° or 90 ° / 0 °. The disadvantage of the linear polarization is that when the head is tilted to the right or left, one perceives crosstalk (ghosting) between the two views. This method is used in monitors from iZ3D and in IMAX cinemas.
circular
The circular polarization is a little more difficult to understand. A circular polarization occurs when there is a phase difference between the two mutually perpendicular linearly polarized components (into which each polarization can be broken down) of equal amplitude. The spatial and temporal amplitude progression can be imagined like a screw with a left or right thread. It is usually generated by two successive filtering processes from unpolarized light. First, the unpolarized light is linearly polarized and then rotated with the aid of a lambda / 4 delay filter. The arrangement of the λ / 4 filter decides whether the rotation is to the left or to the right. The λ / 4 filter must be aligned exactly 45 ° to the linear polarization in order to generate a clean circular oscillation. The circular filters are usually already available as pre-welded filters, which are then referred to as left- or right-handed circular filters.
The separation of the two circular views is done with matching circular polarized glasses. The left-turning light is blocked by the right-turning filter (and vice versa). The alignment of the linear component of the circular filters also plays a role. The linear components of the projection films must match the spectacle films. If this is not the case, a color shift from black to dark blue to purple or from white to yellowish can be seen in the case of strong contrasts. The alignment of the linear component can be the same for left and right, because this component is irrelevant for the separation of the two views. The usual linear alignments for RealD glasses are 0 ° / 0 °, the original Polaroid circular glasses have 90 ° / 0 °. Circular glasses for 3D monitors can have four different linear orientations. The reason for this is the pre-polarization of the LCD panels. Some 3D monitors have a RealD orientation (i.e. 0 ° / 0 °), e.g. B. Miracube 32 ″ and 46 ″, Hyundai 32 ″ and 46 ″ and JVC 46 ″. Other monitors have a 45 ° / 45 ° orientation, e.g. B. Zalman 22 ″ and 24 ″, Miracube 24 ″, Hyundai 22 ″ and 24 ″. The Acer 3D laptop has a different linear alignment: -45 ° / -45 ° (= 135 °).
The 3D-capable monitors have different polarization line by line - i.e. all even lines are circularly polarized to the left, all uneven lines are polarized to the right. For an S3D display, the image information of the left and right view must be nested in one another ( interleaving ). Due to the process, the vertical image resolution is halved. Advantage of the circular polarization: You can tilt your head while looking at it without noticing crosstalk ( ghosting ). At the moment, most cinemas that use the polarization process use circular polarization, with plastic disposable glasses from RealD. When using circular polarizing filters, due to the double filters (linear polarization + λ / 4), it should be noted that these filters only work if they are positioned correctly in the beam path, i.e. This means that the light emerging from the projector must first pass through the linear polarizing filter. Conversely, in the case of glasses, the λ / 4 filter must first be positioned in the beam path (the linear component thus points towards the eyes). If the filters are installed upside down, the filtering will no longer work.