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Stereoscopic displays: Polarization filter process

2023-02-17

Overview

The so-called polarization filter process is one of these newer, color-capable processes, first used in the 1937 commercial "ZUM GREIFEN NAH!" using an implementation developed by Zeiss-Ikon and based on passive glasses with opposing polarization filters (see Fig. 8 ) for both eyes, which cancel each other out in a similar way to the anaglyph process 18 / 22 / 24 .

Functioning

Simply put, the light for the left eye is polarized in one way (i.e. vertically linear or left-hand circular) and the light for the right eye is polarized in another, complementary way (i.e. horizontally linear or right-hand circular), with the polarizing filters used in the glasses and display each being translucent only to light polarized in one way 25 (see Fig. 9 ) and opaque to light polarized in a complementary way 25 (see Fig. 10 ), thereby achieving the necessary channel separation between left and right eyes 5 / 18 / 22 / 24 .

The term polarization describes the direction of oscillation of an electromagnetic transverse wave, i.e. a wave that oscillates perpendicular to its direction of spread, relative to a defined axis 26 (see Fig. 11 parts 1, 2). The visible light relevant for this application also belongs to the transverse waves 26 . Their counterpart are the so-called longitudinal waves, which oscillate along their direction of propagation and thus cannot be polarized 26 (see Fig. 11 part 3).

Linear polarization is achieved by passing unpolarized light, whose waves oscillate randomly distributed in all directions, through a linear polarization filter (also polarizer), which transmits only those waves oscillating parallel to its optical axis, which is here equivalent to polarization direction 26 (see Fig. 12 ). With this filtering, in the ideal view, the initial intensity I0 of the unpolarized light is halved to the target intensity I1 = I0 • 0.5 of the polarized light 25 . In reality, these losses are even larger, which is why very luminous displays must be used to achieve sufficient usable brightness of the display system 5 / 21 / 22 .

Linear polarization filters usually consist of long molecular chains (e.g. carbon-hydrogen chains 27 ), which can be easily excited by electromagnetic waves oscillating parallel to the possible oscillation direction of the molecules and thus perpendicular to the optical axis of the filter, and hence absorb these waves, i.e. do not allow them to pass 26 / 27 . Waves oscillating perpendicular to the possible oscillation direction of the molecular chains and thus parallel to the optical axis, on the other hand, hardly excite molecular oscillations and are hence not absorbed, which is why they can pass through the filter 26 / 27 .

However, the use of linear polarization requires an exact alignment of the head, as otherwise the changed angle may affect the polarization 25 and thus cause ghost images due to insufficient channel separation 5 / 21 . For example, at 45 degrees rotation along the polarization axis used, both partial images are equally bright on both eyes and the display is thereby completely unusable, because the optical axes of the partial images are rotated 90 degrees against each other with linear polarization 5 / 18 / 21 / 22 / 23 and following Malus's law (see Fig. 13 ) at 45 degrees rotation according to I2 = I1 - cos2(45) = I 1 - 0.5 a target intensity I2 of both partial images of 50% of the initial intensity I 1 occurs 28 .

Although circular polarization is more complicated to implement, it has gained popularity in the polarization filter process because it avoids exactly this problem 5/18/22 . In addition to a linear polarization filter, the light is passed through a so-called retarder (also lambda/4 plate), which converts linearly polarized light into circularly polarized light whose polarization direction follows a periodically repeating left- or right-rotated spiral shape 18/23 (see Fig. 14 ).

This process takes place both in the display and in the glasses, each in reverse order. Specifically, on the display side, unpolarized light is first transformed into linearly polarized and then circularly polarized by a lambda/4 plate, while on the glasses side it is first transformed back into linearly polarized and then into unpolarized light 18 . However, again some light intensity is lost during these transformations, which is why circular polarization requires even more luminous displays than linear polarization 5 / 21 .

For projections, one projector is usually used for each eye 5 / 21 / 22 , with the individual images being superimposed on the screen, which is possible thanks to their differentiability by polarization 18 / 24 , since higher brightness levels can be achieved in this way, rapid changes in polarization are difficult to implement with a single projector and these might cause the problems of so-called strobing known from the shutter method, which will be explained in more detail later 5 / 6 .

In addition, a metallic or metal-coated screen is required because conventional screens eliminate the polarization of light when it is reflected 18 / 21 / 22 / 24 . This is due to the fact that polarized light waves excite the electrons in metals to oscillate in the direction of polarization, which in turn emit waves with the same frequency and maximum intensity perpendicular to the direction of their axis of motion, i.e. the direction of polarization, and thus preserve polarization 18 . In non-conductors like paper or fabrics, on the other hand, little to no electrons are excited because they are not freely movable, and thus depolarization of the light occurs because the reflected waves oscillate in random directions again 18 .

On the other hand, a special integrated polarizer is required for screens, whereby the polarization direction usually alternates pixel by pixel in a process called interleaving, thus halving the effectively usable resolution in one axis, because the images for both eyes have to be displayed simultaneously 5 / 6 / 22 (see Fig. 15 ). The horizontal axis is usually chosen for this purpose, since the common aspect ratio of 16 to 9 has more pixels in the horizontal axis than in the vertical axis, and the loss of resolution is therefore not as noticeable.

Benefits and drawbacks

In comparison with the anaglyph process, the fundamental advantage of stereoscopic reproduction of color content in good quality arises 5 / 24 , while the glasses, which are to be considered consumables, continue to make this variant of stereoscopy profitable for cinemas due to their passive construction and their correspondingly low unit prices. This is why this variant has become the most commercially successful option today 5 / 21 .

However, a disadvantage is the, in the case of projection due to the special screen and the two projectors, very complex and expensive 5 / 21 / 22 or, in the case of a display due to the interleaving, lossy 5 / 6 / 22 reproduction system with, in each case, high losses in luminosity due to the numerous polarization filters 5 / 21 / 28 . When using linear polarization, the usable viewing angle range of the viewer is also limited 5 / 22 , but this hardly plays a role today due to the dominance of implementations based on circular polarization.

Appendix

Fig. 8: Polarization filter glasses

Polarization filter glasses
Fig. 8: Polarization filter glasses 40

Fig. 9: Parallel polarization filters

Parallel polarization filters
Fig. 9: Parallel polarization filters 41

Fig. 10: Orthogonal polarization filters

Orthogonal polarization filters
Fig. 10: Orthogonal polarization filters 42

Fig. 11: Comparison of tranversal and longitudinal waves

Comparison of tranversal and longitudinal waves
Fig. 11: Comparison of tranversal and longitudinal waves 43

Fig. 12: Structure and function of linear polarization filters

Structure and function of linear polarization filters
Fig. 12: Structure and function of linear polarization filters 44

Fig. 13: Malus's law

Malus's laws
Fig. 13: Malus's law 45

Fig. 14: Creation of circular polarization

Creation of circular polarization
Fig. 14: Creation of circular polarization 46

Fig. 15: Horizontal interleaving

Horizontal interleaving
Fig. 15: Horizontal interleaving 47

Sources

Text

  1. Heuer, Thomas: ZUM GREIFEN NAH! Der erste 3D-Film mit Polarisationstechnik, in: Institut für immersive Medien (ifim): Atmosphären: Gestimmte Räume und sinnliche Wahrnehmung. Jahrbuch immersiver Medien 2013, Marburg 2013, p. 162 – 166.
  2. Joachim Herz Stiftung: Polarisation von Licht – Fortführung, https://www.leifiphysik.de/optik/polarisation/grundwissen/polarisation-von-licht-fortfuehrung , 14.02.2023.
  3. Joachim Herz Stiftung: Polarisation von Licht – Einführung, https://www.leifiphysik.de/optik/polarisation/grundwissen/polarisation-von-licht-einfuehrung , 14.02.2023.
  4. Eckert, Bodo, Stetzenbach, Werner, und Jodl, Hans-Jörg: Polarisationsfilter, in: Eckert, Bodo, Stetzenbach, Werner, und Jodl, Hans-Jörg: Low Cost – High Tech. Freihandversuche Physik: Praktische Anregungen für einen zeitgemäßen Unterricht, 3. edition, Berlin 2018, p. 120 ff..
  5. Joachim Herz Stiftung: Gesetz von MALUS, https://www.leifiphysik.de/optik/polarisation/grundwissen/gesetz-von-malus , 14.02.2023.

Appendix

  1. Based on: Midori iro: REALD.JPG, https://commons.wikimedia.org/wiki/File:REALD.JPG , 14.02.2023.
  2. Based on: Joachim Herz Stiftung: Polarisationsfilter parallel zueinander, https://www.leifiphysik.de/sites/default/files/images/d251c540a7898b40ae249cbcb1659c40/992parallel_ausgerichtete_polfilter_neu1.svg , 14.02.2023.
  3. Based on: Joachim Herz Stiftung: Polarisationsfilter senkrecht zueinander, https://www.leifiphysik.de/sites/default/files/images/bdb5151f1a9249a08398eeeafa091e58/992senkrecht_ausgerichtete_polfilter_neu1.svg , 14.02.2023.
  4. Based on: Stefan-Xp: Wellen.svg, https://commons.wikimedia.org/wiki/File:Wellen.svg , 14.02.2023.
  5. Based on: Joachim Herz Stiftung: Polarisation von Licht mit Hilfe eines Polarisationsfilters, https://www.leifiphysik.de/sites/default/files/images/78af394bf03623f119c153918fa0d520/992polarisation_neu1.svg , 14.02.2023, and: Joachim Herz Stiftung: Aufbau eines Polarisationsfilters, https://www.leifiphysik.de/sites/default/files/images/c82bec34b9c36cf0a5dc0a88129692ba/992aufbau_polfilter.svg , 14.02.2023.
  6. Based on: Joachim Herz Stiftung: Gesetz von Malus, https://www.leifiphysik.de/sites/default/files/images/831a39868a0cd8e93b45a24bdbe25712/992gesetz_von_malus_neu1.svg , 14.02.2023.
  7. Based on: Dave3457: Circular.Polarization.Circularly.Polarized.Light Circular.PolarizerCreating.Left.Handed.Helix.View.svg, https://commons.wikimedia.org/wiki/File:Circular.Polarization.Circularly.Polarized.Light_Circular.Polarizer_Creating.Left.Handed.Helix.View.svg , 14.02.2023.
  8. Based on: Locafox: Passive-3d-tv-technology.jpg, https://commons.wikimedia.org/wiki/File:Passive-3d-tv-technology.jpg , 14.02.2023.

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